5-i 

CHAPTER 5. ENGINEERING ANALYSIS

TABLE OF CONTENTS

5.1 INTRODUCTION ............................................................................................... 5-1

5.2 STRUCTURING THE ENGINEERING ANALYSIS ........................................ 5-1

5.2.1 Summary of Design Line Coverage ......................................................... 5-8

5.2.2 Scaling Relationships in Transformer Manufacturing ........................... 5-10

5.3 TECHNICAL DESIGN INPUTS FOR SOFTWARE MODEL ........................ 5-14

5.3.1 A and B Loss Valuation Inputs .............................................................. 5-15

5.3.2 Core Material Options............................................................................ 5-17

5.3.3 Core Configurations ............................................................................... 5-18

5.3.3.1 Standard Core Configurations .................................................... 5-19

5.3.3.2 Symmetric Core Configurations ................................................ 5-21

5.3.4 Less-Flammable Liquid-Immersed Transformers ................................. 5-22

5.3.5 Design Line 1 Representative Unit ........................................................ 5-22

5.3.6 Design Line 2 Representative Unit ........................................................ 5-23

5.3.7 Design Line 3 Representative Unit ........................................................ 5-24

5.3.8 Design Line 4 Representative Unit ........................................................ 5-25

5.3.9 Design Line 5 Representative Unit ........................................................ 5-26

5.3.10 Design Line 6 Representative Unit ........................................................ 5-27

5.3.11 Design Line 7 Representative Unit ........................................................ 5-28

5.3.12 Design Line 8 Representative Unit ........................................................ 5-30

5.3.13 Design Line 9 Representative Unit ........................................................ 5-31

5.3.14 Design Line 10 Representative Unit ...................................................... 5-32

5.3.15 Design Line 11 Representative Unit ...................................................... 5-33

5.3.16 Design Line 12 Representative Unit ...................................................... 5-34

5.3.17 Design Line 13A Representative Unit ................................................... 5-35

5.3.18 Design Line 13B Representative Unit ................................................... 5-36

5.3.19 Newly Optimized Designs and Previously Optimized Designs ............ 5-37

5.4 MATERIAL AND LABOR INPUTS ................................................................ 5-37

5.4.1 Material Prices ....................................................................................... 5-39

5.4.2 Material Price Inputs to the Design Software – Liquid-Immersed ........ 5-41

5.4.3 Material Price Inputs to the Design Software – Dry-Type .................... 5-44

5.4.4 Labor Costs ............................................................................................ 5-48

5.4.4.1 Liquid-Immersed Labor Hours .................................................. 5-49

5.4.4.2 Dry-Type Labor Hours .............................................................. 5-52

5.5 EFFICIENCY LEVELS..................................................................................... 5-54

5.5.1 Criteria for Developing Efficiency Levels ............................................. 5-54

5.5.2 Efficiency Levels Selected ..................................................................... 5-54

5.5.3 BusLead and Bus Loss Correction ......................................................... 5-58

5.6 RESULTS OF THE ANALYSIS ON EACH DESIGN LINE .......................... 5-58

5.6.1 Traditional Core Designs for the Reference Case .................................. 5-58

5.6.2 Symmetric Core Designs ....................................................................... 5-86

5.7 THREE EXAMPLE DESIGNS AND COST BREAKDOWNS ....................... 5-86
5-ii

5.7.1 Design Details Report for Transformer from Design Line 1 ................. 5-87

5.7.2 Design Details Report for Transformer from Design Line 7 ................. 5-91

5.7.3 Design Details Report for Transformer from Design Line 12 ............... 5-95

LIST OF TABLES

Table 5.1 Equipment Classes and Number of kVA Ratings ........................................... 5-2

Table 5.2 Engineering Design Lines (DLs) and Representative Units for Analysis ...... 5-3

Table 5.3 Liquid-Immersed Design Lines and Representative Units ............................. 5-8

Table 5.4 Dry-Type, Low-Voltage Design Lines and Representative Units .................. 5-9

Table 5.5 Dry-Type, Medium-Voltage, Three-Phase Design Lines ............................. 5-10

Table 5.6 Common Scaling Relationships in Transformers ......................................... 5-11

Table 5.7 Scaling Exponents By Equipment Class ....................................................... 5-13

Table 5.8 A and B Grid Combinations Used by Software to Generate Design Database 5-

16

Table 5.9 A and B Fan Combinations Used by Software to Generate Design Database . 5-

16

Table 5.10 Core Steel Grades, Thicknesses and Associated Losses ............................. 5-18

Table 5.11 Core Configurations Used in Each Design Line ......................................... 5-19

Table 5.12 Design Option Combinations for the Design Line 1 Representative Unit . 5-23

Table 5.13 Design Option Combinations for the Design Line 2 Representative Unit . 5-24

Table 5.14 Design Option Combinations for the Design Line 3 Representative Unit . 5-25

Table 5.15 Design Option Combinations for the Design Line 4 Representative Unit . 5-26

Table 5.16 Design Option Combinations for the Design Line 5 Representative Unit . 5-27

Table 5.17 Design Option Combinations for the Design Line 6 Representative Unit . 5-28

Table 5.18 Design Option Combinations for the Design Line 7 Representative Unit . 5-29

Table 5.19 Limits on Flux Density for Design Line 7 Representative Unit ................. 5-30

Table 5.20 Design Option Combinations for the Design Line 8 Representative Unit . 5-31

Table 5.21 Design Option Combinations for the Design Line 9 Representative Unit . 5-32

Table 5.22 Design Option Combinations for the Design Line 10 Representative Unit 5-33

Table 5.23 Design Option Combinations for the Design Line 11 Representative Unit 5-34

Table 5.24 Design Option Combinations for the Design Line 12 Representative Unit 5-35

Table 5.25 Design Option Combinations for the Design Line 13A Representative Unit. 5-

36

Table 5.26 Typical Manufacturer’s Material Prices for Liquid-Immersed Design Lines 5-

42

Table 5.27 Summary Table of Fixed Material Costs for Liquid-Immersed Units........ 5-43

Table 5.28 Manufacturer’s Material Prices for Dry-Type Design Lines ...................... 5-44

Table 5.29 Summary Table of Fixed Material Costs for Dry-Type Units .................... 5-48

Table 5.30 Labor Markups for Liquid-Immersed and Dry-Type Manufacturers ......... 5-49

Table 5.31 Summary of Labor Times for Liquid-Immersed Units ............................... 5-52

Table 5.32 Summary of Baselines and Efficiency Levels for Distribution Transformer 

Representative Units ................................................................................... 5-55

Table 5.33 Summary of 2010 Incremental Manufacturer Selling Prices Over the Baseline 

for Distribution Transformer Representative Units .................................... 5-56
5-iii

Table 5.34 Summary of 2011 Incremental Manufacturer Selling Prices Over the Baseline 

for Distribution Transformer Representative Units .................................... 5-57

Table 5.35 Bill of Materials for Transformer from Design Line 1 ............................... 5-90

Table 5.36 Bill of Materials for Transformer from Design Line 7 ............................... 5-94

Table 5.37 Bill of Materials for Transformer from Design Line 12 ............................. 5-99

LIST OF FIGURES

Figure 5.2.1 Efficiency Levels within an Equipment Class (Logarithmic) ................... 5-12

Figure 5.3.1 A and B Combination Software Inputs Used in the Engineering Analysis .. 5-

17

Figure 5.3.2 Graphic of Single-Phase Core Configurations ......................................... 5-20

Figure 5.3.3 Graphic of Three-phase Core Configurations .......................................... 5-20

Figure 5.3.4 Cruciform Core Cross-Section ................................................................. 5-21

Figure 5.4.1 Method of Cost Account for Distribution Transformers Rulemaking ...... 5-38

Figure 5.6.1 Engineering Analysis Results, Design Line 1, 2011 ................................ 5-59

Figure 5.6.2 Engineering Analysis Results, Design Line 1, 2010 ................................. 5-60

Figure 5.6.3 Engineering Analysis Results, Design Line 2, 2011 ................................ 5-61

Figure 5.6.4 Engineering Analysis Results, Design Line 2, 2010 ................................. 5-62

Figure 5.6.5 Engineering Analysis Results, Design Line 3, 2011 ................................ 5-63

Figure 5.6.6 Engineering Analysis Results, Design Line 3, 2010 ................................. 5-64

Figure 5.6.7 Engineering Analysis Results, Design Line 4, 2011 ................................ 5-65

Figure 5.6.8 Engineering Analysis Results, Design Line 4, 2010 ................................. 5-66

Figure 5.6.9 Engineering Analysis Results, Design Line 5, 2011 ................................ 5-67

Figure 5.6.10 Engineering Analysis Results, Design Line 5, 2010 ............................... 5-68

Figure 5.6.11 Engineering Analysis Results, Design Line 6, 2011 .............................. 5-69

Figure 5.6.12 Engineering Analysis Results, Design Line 6, 2010 ............................... 5-70

Figure 5.6.13 Engineering Analysis Results, Design Line 7, 2011 .............................. 5-71

Figure 5.6.14 Engineering Analysis Results, Design Line 7, 2010 ............................... 5-72

Figure 5.6.15 Engineering Analysis Results, Design Line 8, 2011 .............................. 5-73

Figure 5.6.16 Engineering Analysis Results, Design Line 8, 2010 ............................... 5-74

Figure 5.6.17 Engineering Analysis Results, Design Line 9, 2011 .............................. 5-75

Figure 5.6.18 Engineering Analysis Results, Design Line 9, 2010 ............................... 5-76

Figure 5.6.19 Engineering Analysis Results, Design Line 10, 2011 ............................ 5-77

Figure 5.6.20 Engineering Analysis Results, Design Line 10, 2010 ............................. 5-78

Figure 5.6.21 Engineering Analysis Results, Design Line 11, 2011 ............................ 5-79

Figure 5.6.22 Engineering Analysis Results, Design Line 11, 2010 ............................. 5-80

Figure 5.6.23 Engineering Analysis Results, Design Line 12, 2011 ............................ 5-81

Figure 5.6.24 Engineering Analysis Results, Design Line 12, 2010 ............................. 5-82

Figure 5.6.25 Engineering Analysis Results, Design Line 13A, 2011 ......................... 5-83

Figure 5.6.26 Engineering Analysis Results, Design Line 13A, 2010 .......................... 5-84

Figure 5.6.27 Engineering Analysis Results, Design Line 13B, 2011 ......................... 5-85

Figure 5.6.28 Engineering Analysis Results, Design Line 13B, 2010 .......................... 5-86

Figure 5.7.1 Manufacturer Selling Price Breakdown, Transformer from Design Line 1 . 5-

91
5-iv

Figure 5.7.2 Manufacturer Selling Price Breakdown, Transformer from Design Line 7 . 5-

95

Figure 5.7.3 Manufacturer Selling Price Breakdown, Transformer from Design Line 125-

100
5-1 

CHAPTER 5. ENGINEERING ANALYSIS

5.1 INTRODUCTION

This chapter provides the technical support documentation for the engineering analysis, 

evaluating both liquid-immersed and dry-type distribution transformers. The purpose of the 

engineering analysis is to estimate the relationship between the manufacturer’s selling price

(MSP) of a transformer and its corresponding efficiency rating. This relationship serves as the 

basis for the subsequent cost-benefit calculations for individual customers, manufacturers, and 

the nation (see chapter 8, Life-Cycle Cost and Payback Period Analyses).

5.2 STRUCTURING THE ENGINEERING ANALYSIS

As discussed in the market and technology assessment (chapter 3), distribution 

transformers are classified by their insulation type (liquid-immersed or dry-type), the number of 

phases (single or three), the primary voltage (low-voltage or medium-voltage for dry-types) and 

the basic impulse insulation level (BIL) rating (for medium-voltage dry-type). Following this 

convention, the U.S. Department of Energy (DOE) developed ten equipment classes, shown in 

Table 5.1. These equipment classes were adapted from the National Electrical Manufacturers 

Association (NEMA)’s TP 1 classification system, although they do not follow the classification 

system precisely. NEMA’s TP 1 classifies medium-voltage, dry-type distribution transformers 

into two equipment classes, ≤ 60 kilovolt (kV) BIL and > 60 kV BIL. Based on input from 

manufacturers, DOE elected to increase the differentiation of medium-voltage, dry-type 

transformers, and create three equipment classes of BIL ratings: 20–45 kV BIL, 46–95 kV BIL, 

and ≥ 96kV BIL (see chapter 3, section 3.3). 

Within each of these equipment classes, DOE further classified distribution transformers 

by their kilovolt-ampere (kVA) rating. These kVA ratings are size categories, indicating the 

power handling capacity of the transformers. Due to differences in construction methods and 

material properties, efficiency levels vary by both equipment class and kVA rating. For NEMA’s 

TP 1-2002,1

Table 

5.1

there are 99 kVA ratings across all the equipment classes. For DOE’s rulemaking, 

because of the greater degree of differentiation around the BIL rating in medium-voltage, drytype transformers, there are 115 kVA ratings across all the equipment classes, as shown in 

. 

 

1 NEMA’s TP 1-2002 can be found online at: http://www.nema.org/Products/Pages/Transformers.aspx. 
5-2 

Table 5.1 Equipment Classes and Number of kVA Ratings

Distribution Transformer Equipment Class kVA 

Range

Number of kVA 

Ratings

1. Liquid-immersed, medium-voltage, single-phase 10–833 13

2. Liquid-immersed, medium-voltage, three-phase 15–2500 14

3. Dry-type, low-voltage, single-phase 15–333 9

4. Dry-type, low-voltage, three-phase 15–1000 11

5. Dry-type, medium-voltage, single-phase, 20-45 kV BIL 15–833 12

6. Dry-type, medium-voltage, three-phase, 20-45 kV BIL 15–2500 14

7. Dry-type, medium-voltage, single-phase, 46-95 kV BIL 15–833 12

8. Dry-type, medium-voltage, three-phase, 46-95 kV BIL 15–2500 14

9. Dry-type, medium-voltage, single-phase, > 96 kV BIL 75–833 8

10. Dry-type, medium-voltage, three-phase, > 96 kV BIL 225–2500 8

Total 115

DOE recognized that it would be impractical to conduct a detailed engineering analysis on 

all 115 kVA ratings, so it sought to develop an approach that simplified the analysis while 

retaining reasonable levels of accuracy. DOE consulted with industry representatives and 

transformer design engineers and developed an understanding of the construction principles for 

distribution transformers. It found that many of the units share similar designs and construction 

methods. Thus, DOE simplified the analysis by creating 14 engineering design lines, which 

group kVA ratings based on similar principles of design and construction. The 14 design lines 

subdivide the equipment classes, to improve the accuracy of the engineering analysis. These 14 

engineering design lines differentiate the transformers by insulation type (liquid-immersed or 

dry-type), number of phases (single or three), and primary insulation levels for medium-voltage, 

dry-type (three different BIL ratings). 

DOE then selected one unit from each of the engineering design lines for study in the 

engineering analysis and the life-cycle cost (LCC) analysis (see chapter 8), reducing the number 

of units for analysis from 115 to 14. It then extrapolated the results of its analysis from the unit 

studied to the other kVA ratings within that same engineering design line. DOE performed this 

extrapolation in the national impacts analysis (see chapter 10). DOE used kVA scaling to

extrapolate findings from a representative unit to the other kVA ratings within its design line. . 

An example of how DOE applied this scaling appears in section 5.2.1 of this chapter. A technical 

discussion of the derivation of kVA scaling appears in appendix 5B. 

Table 5.2 presents DOE’s 14 design lines and the representative units selected from each 

engineering design line for analysis. Descriptions of each of the design lines and the rationale 

behind the selection of the representative units follow Table 5.2. 
5-3 

Table 5.2 Engineering Design Lines (DLs) and Representative Units for Analysis

DL Type of Distribution 

Transformer

kVA 

Range

Representative Unit for this

Engineering Design Line 

1 Liquid-immersed, singlephase, rectangular tank 10–833 

50 kVA, 65°C, single-phase, 60Hz, 14400V 

primary, 240/120V secondary, rectangular 

tank, 95kV BIL

2 Liquid-immersed, singlephase, round tank 10–833 

25 kVA, 65°C, single-phase, 60Hz, 14400V 

primary, 120/240V secondary, round tank, 

125 kV BIL

3 Liquid-immersed, singlephase 10–833 500 kVA, 65°C, single-phase, 60Hz, 14400V 

primary, 277V secondary, 150kV BIL

4 Liquid-immersed, threephase 15–2500 

150 kVA, 65°C, three-phase, 60Hz, 

12470Y/7200V primary, 208Y/120V 

secondary, 95kV BIL

5 Liquid-immersed, threephase 15–2500 

1500 kVA, 65°C, three-phase, 60Hz, 

24940GrdY/14400V primary, 480Y/277V 

secondary, 125 kV BIL

6 Dry-type, low-voltage, 

single-phase 15–333 25 kVA, 150°C, single-phase, 60Hz, 480V 

primary, 120/240V secondary, 10kV BIL

7 Dry-type, low-voltage, 

three-phase 15–1000 75 kVA, 150°C, three-phase, 60Hz, 480V 

primary, 208Y/120V secondary, 10kV BIL

8 Dry-type, low-voltage, 

three-phase 15–1000 

300 kVA, 150°C, three-phase, 60Hz, 480V 

Delta primary, 208Y/120V secondary, 10kV 

BIL

9 

Dry-type, mediumvoltage, three-phase, 20-

45kV BIL

15–2500 

300 kVA, 150°C, three-phase, 60Hz, 4160V 

Delta primary, 480Y/277V secondary, 45kV 

BIL

10 

Dry-type, mediumvoltage, three-phase, 20-

45kV BIL

15–2500 1500 kVA, 150°C, three-phase, 60Hz, 4160V 

primary, 480Y/277V secondary, 45kV BIL 

11 

Dry-type, mediumvoltage, three-phase, 46-

95kV BIL 

15–2500 300 kVA, 150°C, three-phase, 60Hz, 12470V 

primary, 480Y/277V secondary, 95kV BIL 

12 

Dry-type, mediumvoltage, three-phase, 46-

95kV BIL 

15–2500 

1500 kVA, 150°C, three-phase, 60Hz, 

12470V primary, 480Y/277V secondary, 

95kV BIL

13A 

Dry-type, mediumvoltage, three-phase, 96-

150kV BIL

225–

2500 

300 kVA, 150°C, three-phase, 60Hz, 24940V 

primary, 480Y/277V secondary, 125kV BIL 

13B 

Dry-type, mediumvoltage, three-phase, 96-

150kV BIL 

225–

2500 

2000 kVA, 150°C, three-phase, 60Hz, 

24940V primary, 480Y/277V secondary, 

125kV BIL 
5-4 

* EC means equipment class (see chapter 3 of the TSD). DOE did not select any 

representative units from the single-phase, medium-voltage equipment classes (EC5, EC7 and 

EC9), but calculated the analytical results for EC5, EC7, and EC9 based on the results for their 

three-phase counterparts. 

DOE divided liquid-immersed transformers into five engineering design lines, based on 

their tank shape, number of phases, and kVA ratings. DOE believes that this breakdown enables 

the analysis to identify and capture a more accurate representation of the manufacturer’s selling 

price and efficiency relationship. DOE broke dry-type distribution transformers into eight 

engineering design lines, primarily according to their BIL ratings. DOE believes this level of 

disaggregation is necessary to capture important differences in the price-efficiency relationship, 

particularly as the BIL rating varies. For example, a 300 kVA, three-phase, dry-type unit could 

be classified in design lines 8, 9, or 11, or 13A, depending on whether the BIL rating is 10 kV 

(low-voltage), 20-45 kV, 46-95 kV, or 96-150 kV. 

For design lines 9 through 13B, the representative units selected for some of the dry-type 

design lines may not be the standard BILs associated with a given primary voltage. DOE selected 

a slightly higher BIL for the representative units from these design lines to ensure that any 

minimum efficiency standard would not excessively penalize customers purchasing transformers 

at higher BIL ratings within the range. For example, a 300 kVA transformer with a 4160V 

primary is called a “5kV class” transformer and would normally be built with a 30kV BIL. 

However, customers may also choose to order this transformer with 45kV BIL or 60kV BIL. If 

the minimum efficiency level were set based on a 30kV BIL, it may not be possible to achieve 

that same efficiency rating for customers ordering 60kV BIL. Thus, DOE evaluated the middle 

BIL rating (in this example, 45kV BIL), making it slightly easier to comply for a lower BIL, and 

not too difficult (or impossible) for the higher BIL. 

The remainder of this section discusses each of the 14 engineering design lines, providing 

a description and explanation of the transformers covered. 

Design Line 1. This is the basic, high-volume line for rectangular-tank, single-phase, 

liquid-immersed distribution transformers, ranging from 10 kVA to 833 kVA. Transformers in 

this design line typically have BILs ranging from 30 kV to 150 kV (this unit is 95 kV) and a tap 

configuration of four 21⁄2 percent taps—two above and two below the nominal voltage. Tap 

configurations enable transformer users to maintain full (rated) output voltage by slightly 

increasing or decreasing the number of turns in the primary in anticipation of an input voltage 

slightly above or below the rated nominal. This design line has a primary voltage less than 35 

kV, and a secondary voltage less than or equal to 600 Volts (V). 

The representative unit selected for design line 1 is a 50 kVA pad-mounted unit, as this is 

a high shipment volume rating. 

Design Line 2. This is the basic, high-volume line for round-tank (pole-mounted), singlephase, liquid-immersed distribution transformers, ranging from 10 kVA to 833 kVA. Although 

some manufacturers tend to employ the same basic core/coil design for design line 1 and design 

line 2, others may have design differences between pad-mounted and pole-mounted 
5-5 

transformers. DOE decided to analyze these two types of distribution transformers separately for 

the engineering and LCC analyses. Transformers in design line 2 typically have BILs ranging 

from 30 kV to 150 kV (this unit is 125 kV), a tap configuration of four 21⁄2 percent taps—two 

above and two below the nominal, a primary voltage less than 35 kV, and a secondary voltage 

less than or equal to 600 V. 

The representative unit selected for design line 2 is a 25 kVA pole-mounted unit, as this is 

a high-volume rating for pole-mounted transformers. 

Design Line 3. This design line groups together single-phase, round-tank, liquidimmersed distribution transformers, ranging from 10 kVA to 833 kVA. Together, design lines 1 

through 3 cover all the single-phase, liquid-immersed units. Transformers in this design line 

typically have BILs ranging from 30 kV to 150 kV (this unit is 150 kV), a tap configuration of 

four 21⁄2 percent taps—two above and two below the nominal, a primary voltage less than 35 kV, 

and a secondary voltage less than or equal to 600 V. 

The representative unit selected for design line 3 is a 500 kVA round-tank. Although high 

currents result from having a 277 V secondary at the larger kVA ratings, high current bushings 

are available, and a market does exist for these transformers. Together, results from DL 1-3 are 

used to set standards for equipment class 1, single-phase liquid-immersed units. 

 

Design Line 4. Design line 4 represents rectangular tank, three-phase, liquid-immersed 

distribution transformers, ranging from 15 kVA to 2500 kVA. Transformers in this design line 

typically have BILs ranging from 30 kV to 150 kV (this unit is 95 kV), a tap configuration of 

four 21⁄2 percent taps—two above and two below the nominal, a primary voltage less than 35 kV, 

and a secondary voltage less than or equal to 600 V. 

The representative unit selected for design line 4 is a 150 kVA transformer, as this is a 

common rating in this design line. 

Design Line 5. Design line 5 represents rectangular tank, three-phase, liquid-immersed 

distribution transformers, ranging from 750 kVA to 2500 kVA. Together, design lines 4 and 5 

are scaled to cover all the three-phase, liquid-immersed units. Transformers in this design line 

typically have BILs ranging from 95 kV to 150 kV (this unit is 125 kV), a tap configuration of 

four 21⁄2 percent taps—two above and two below the nominal, a primary voltage less than 35 kV, 

and a secondary voltage less than or equal to 600 V. 

The representative unit selected for this design line is a 1500 kVA transformer, as this is a 

common rating in this size range. Together, design line 4 and design line 5 are used to determine 

standards for the whole kVA range in equipment class 2. 

Design Line 6. Design line 6 represents single-phase, low-voltage, ventilated dry-type 

distribution transformers, ranging from 15 kVA to 333 kVA. Transformers in this design line 

typically have BIL ratings of 10 kV and a “universal” tap arrangement, meaning six 21⁄2 percent 

taps, two above and four below the nominal. DOE selected this tap arrangement based on 
5-6 

recommendations from manufacturers who produce transformers at these ratings. The primary 

and secondary voltages are both 600 V or below. 

The representative unit selected for design line 6 is a 25 kVA transformer, as this is a 

common rating in this size range, and occurs toward the low end of the kVA ratings for this 

design line (15 kVA, 25 kVA, 37.5 kVA, 50 kVA, 75 kVA, 100 kVA, 167 kVA, 250 kVA, and 

333 kVA). Design line 6 is used to generate results for equipment class 3. 

Design Line 7. Design line 7 represents three-phase, low-voltage, ventilated dry-type 

distribution transformers, ranging from 15 kVA to 1000 kVA. Transformers in this design line 

typically have BIL ratings of 10 kV and a “universal” tap arrangement, meaning six 21⁄2 percent 

taps, two above and four below the nominal. The primary and secondary voltages are both 600 V 

or below. 

The representative unit selected for design line 7 is a 75 kVA transformer, as this is a 

common rating in this size range. 

Design Line 8. Design line 8 represents three-phase, low-voltage, ventilated dry-type 

distribution transformers, ranging from 15 kVA to 2500 kVA. Transformers in this design line 

typically have BIL ratings of 10 kV and a tap arrangement of four 21⁄2 percent taps, two above 

and two below the nominal. The primary and secondary voltages are both 600 V or below. 

The representative unit selected for design line 8 is a 300 kVA transformer, as this is a 

common rating in this size range. DL8 and DL7 are used together to produce results for the 

entire kVA range of equipment class 4. 

Design Line 9. Design line 9 represents three-phase, medium-voltage, ventilated dry-type 

distribution transformers, ranging from 15 kVA to 2500 kVA. Transformers in design line 9 

typically have primary voltages less than or equal to 5 kV with a BIL rating between 20 kV and 

45 kV. The secondary voltage is less than or equal to 600 V and the tap arrangement is typically 

four 21⁄2 percent taps, two above and two below the nominal. 

The representative unit selected for design line 9 is 300 kVA, as this is a common rating 

in this size range. 

Design Line 10. Design line 10 represents three-phase, medium-voltage, ventilated drytype distribution transformers, ranging from 750 kVA to 2500 kVA. Transformers in this design 

line typically have primary voltages less than or equal to 5 kV with a BIL rating between 20 kV 

and 45 kV. The secondary voltage is less than or equal to 600 V and the tap arrangement is 

typically four 21⁄2 percent taps, two above and two below the nominal. 

The representative unit selected for this design line is a 1500 kVA transformer, as this is a 

common rating. Results from DL9 and DL10 are used together to scale to all of the kVA ratings 

in equipment class 6. Results from these two design lines are scaled to all of the kVA ratings in 

equipment class 5. 

 
5-7 

Design Line 11. Design line 11 represents three-phase, medium-voltage, ventilated drytype distribution transformers, ranging from 15 kVA to 500 kVA. This design line parallels 

design line 9, with a higher primary insulation level, 46 kV to 95 kV BIL. Because dry-type 

transformer designs and, more importantly, the efficiency of those designs, are strongly 

influenced by changes in BIL, DOE considered these higher BIL ratings separately. The typical 

tap arrangement is four 21⁄2 percent taps, two above and two below the nominal. The primary 

voltage is typically less than or equal to 15 kV and the secondary voltage is less than or equal to 

600 V. 

 The shipments for this design line are primarily in the kVA range inclusive of and 

between 225 kVA and 500 kVA; therefore, DOE selected the 300 kVA rating as the 

representative unit for analysis.

Design Line 12. Design line 12 represents three-phase, medium-voltage, ventilated drytype distribution transformers, ranging from 750 kVA to 2500 kVA. This design line parallels 

design line 10, with a higher primary insulation level, 46 kV to 95 kV BIL. The typical tap 

arrangement is four 21⁄2 percent taps, two above and two below the nominal. The primary voltage 

is typically less than or equal to 15 kV and the secondary voltage is less than or equal to 600 V. 

The representative unit selected for this design line is a 1500 kVA transformer, as it is a 

common rating in this size range and BIL rating. DL11 and DL12 are used to produce results for 

the entire kVA range of equipment class 8 and are scaled to equipment class 7.

Design Lines 13A and 13B. As a further extension on the dry-type, three-phase, mediumvoltage BIL ranges, DOE originally analyzed 96 kV to 150 kV BIL in two design lines ranging 

from 225 kVA to 2500 kVA. The 225 kVA rating is considered to be the lowest kVA rating 

where one would expect to see a unit with a BIL greater than 110 kV. The typical tap 

arrangement is four 21⁄2 percent taps, two above and two below the nominal. The primary voltage 

is typically less than or equal to 35 kV and the secondary voltage is less than or equal to 600 V. 

. The representative unit selected for design line 13A is a 300 kVA transformer. This unit 

is a common rating in this size range, and occurs toward the low end of the range covered by this 

design line. The representative unit selected for design line 13 B is a 2000 kVA transformer, 

which occurs toward the high end of the range covered by this design line. DL13A and DL13B 

are used to develop results for the full kVA range of equipment class 10, and scaled to set 

standards for equipment class 9. 

In addition to the three equipment classes for dry-type, medium-voltage, three-phase 

distribution transformers (for which there are five engineering design lines) presented in Table 

5.1, there are three equipment classes for single-phase, dry-type, medium-voltage units. As 

discussed in chapter 3, the shipment volume for single-phase, dry-type, medium-voltage 

transformers is very low as a percentage of the total dry-type shipments. Additionally, the total 

megavolt-ampere (MVA) capacity of single-phase, dry-type, medium voltage transformers is 

relatively low as a percentage of the total MVA capacity for dry-type, medium voltage 

transformers. Therefore, it does not warrant the level of effort involved in conducting analysis on 

these specific units. DOE decided instead to scale the analysis findings from three-phase units to 
5-8 

the single-phase units. During the negotiations, DOE worked with various manufacturers and 

committee members to develop a new approach. In the end, DOE decided to scale the losses 

from each three-phase transformer to calculate losses and efficiency for the equivalently sized 

single-phase transformer. Additional details and rationale appear in section 5.2.2. In this way, 

DOE was able to concentrate resources and improve the accuracy in other, higher volume and 

more important distribution transformer equipment classes. 

5.2.1 Summary of Design Line Coverage

The following four tables summarize the coverage of each of the design lines in relation to 

the various equipment classes and kVA ratings. The abbreviation DL stands for design line, and 

the row in the table where the phrase “DL X” appears indicates the kVA rating of the 

representative unit from that design line. The representative unit is the kVA rating that DOE 

analyzed in the engineering and LCC analyses. For example, DL1 stands for design line 1, 

spanning from 10 to 167 kVA liquid-type, single-phase. The label “DL 1” appears in row 50 

kVA, indicating that the 50 kVA is the representative unit for DL1. Similarly, the representative 

unit for DL2 is the 25 kVA unit. 

There are five liquid-immersed transformer design lines, three single-phase and two threephase, as shown in Table 5.3. To capture any design differences between a single-phase pole and 

a pad-mounted transformer, DOE analyzed units in both DL1 and DL2, spanning the same kVA 

ratings (10 kVA to 833 kVA). 

Table 5.3 Liquid-Immersed Design Lines and Representative Units

Table 5.4 presents the low-voltage, dry-type design lines. For single-phase units, one 

design line spans all nine kVA ratings. For the three-phase units, two design lines cover the 11 

kVA ratings in that equipment class. 

kVA Design Line kVA Design Line

10 15

15 30

25 45

37.5 75

50 112.5

75 150

100 225

167 300

250 500

333 750

500 1000

667 1500

833 2000

2500

Equipment Class 1, 

Liquid-Immersed,

Single-Phase

Equipment Class 2, 

Liquid-Immersed,

Three-Phase

DL 1

DL 2

DL3

DL 5

DL 4
5-9 

Table 5.4 Dry-Type, Low-Voltage Design Lines and Representative Units

As discussed in chapter 3, section 3.4 (National Shipment Estimate), medium-voltage, 

single-phase, dry-type units have a low shipment volume and low total MVA capacity. All three

medium-voltage, single phase, dry type equipment classes together represent less than onequarter of one percent of dry-type shipments on an MVA capacity basis, and less than one 

percent of medium-voltage dry-type shipments on an MVA capacity basis. Thus, DOE did not 

consider it appropriate to conduct a detailed analysis of any units from these three equipment 

classes.

Table 5.5 presents the equipment classes (abbreviated “EC” in this table) for the mediumvoltage, three-phase, dry-type distribution transformers and each of the design lines and 

respective representative units. Because those equipment classes have high volumes and large 

ranges of kVA ratings, DOE used two separate design lines for each, to maintain accuracy. For

DL13A and DL13B, DOE did not extrapolate the results of these units to ratings of 150kVA and 

below because there were no shipments at these ratings in the shipments analysis and it is very 

unlikely that they would be built. 

kVA Design Line kVA Design Line

15 15

25 30

37.5 45

50 75

75 112.5

100 150

167 225

250 300

333 500

750

1000

Equipment Class 3

Dry-Type, Low Voltage,

Single-Phase

Equipment Class 4

Dry-Type, Low Voltage,

Three-Phase

DL 6

DL 7

DL 8
5-10

Table 5.5 Dry-Type, Medium-Voltage, Three-Phase Design Lines

5.2.2 Scaling Relationships in Transformer Manufacturing

DOE simplified the engineering analysis by creating design lines, selecting representative 

units from these design lines, and scaling the results of the analysis on these representative units 

within their respective design lines. This section briefly introduces the scaling relationship DOE 

used to extrapolate the findings on the representative units to the other kVA ratings. A more 

detailed discussion of the derivation of scaling factors is provided in Appendix 5B. 

The scaling formulae are mathematical relationships that exist between the kVA ratings 

and the physical size, cost, and performance of transformers. The size-versus-performance 

relationships arise from fundamental equations describing a transformer's voltage and kVA 

rating. For example, when the kVA rating, voltage, and frequency are fixed, the product of the 

conductor current density, core flux density, core cross-sectional area, and total conductor crosssectional area is constant. 

To illustrate this point, consider a transformer with four fixed variables: frequency, 

magnetic flux density, current density, and BIL rating. If one enlarges (or decreases) the kVA 

rating, then the only parameters free to vary are the core cross-section and the core window area 

through which the windings pass. Thus, to increase (or decrease) the kVA rating, the dimensions 

for height, width, and depth of the core/coil assembly scale equally in all directions. Analysis of 

this scaling relationship reveals that each of the linear dimensions varies as the ratio of kVA 

ratings to the 1⁄4 power. Similarly, areas vary as the ratios of kVA ratings to the 1⁄2 power and 

volumes vary as the ratio of the kVA ratings to the 3⁄4 or 0.75 power, hence the term "0.75 scaling 

rule." Application of the 0.75 scaling rule assumes that the efficiency profile of a given 

transformer will have the same shape as the transformer being scaled. Table 5.6 depicts the most 

common scaling relationships in transformers.

kVA

EC 6

Low BIL

EC 8

Med BIL

EC 10

High BIL

20-45kV 46-95kV >96kV

15 -

30 -

45 -

75 -

112.5 -

150 -

225

300

500

750

1000

1500

2000

2500

Dry-Type, Medium Voltage, Three-Phase

DL 9

DL 10

DL 11

DL 12

DL 13B

DL 13A
5-11

Table 5.6 Common Scaling Relationships in Transformers

Parameter Being Scaled Relationship to kVA Rating

(varies with ratio of kVAx) 

Weight (kVA1/kVA0)

3/4

Cost (kVA1/kVA0)

3/4

Length (kVA1/kVA0)

1/4

Width (kVA1/kVA0)

1/4

Height (kVA1/kVA0)

1/4

Total Losses (kVA1/kVA0)

3/4

No-load Losses (kVA1/kVA0)

3/4

The following three relationships are true as the kVA rating increases or decreases, if the 

type of transformer (liquid-immersed or dry-type, single-phase or three-phase), the primary 

voltage, the core configuration, the core material, the core flux density, and the current density 

(amperes per square inch of conductor cross-section) in both the primary and secondary 

windings are all held constant:

1. The physical proportions are constant (same relative shape),

2. The eddy loss proportion is essentially constant, and 

3. The insulation space factor (voltage or BIL) is constant. 

In practical applications, it is rare to find that all of the above are constant over even 

limited ranges; however, over a range of one order of magnitude in both directions (e.g., from 50 

kVA to 5 kVA or from 50 kVA to 500 kVA), the scaling rules shown in Table 5.6 can be used to 

establish reasonable estimates of performance, dimensions, costs, and losses. In practice, these 

rules can be applied over even wider ranges to estimate general performance levels. 

Although these laws dictate that an ideal transformer will yield a scaling exponent of 0.75, 

DOE recognizes that a different exponent may produce better behaved results based on realworld engineering. For the NOPR, DOE used unique scaling exponents for each equipment class. 

For each equipment class DOE derived an exponent to scale relative kVA rating by examining 

the proposals discussed during the negotiations. Because the proposals discussed during the 

negotiations included efficiency levels across multiple designs lines, a scaling relationship was 

implied by the proposal. The exponents used for each equipment class are shown below in Table 

5.7. 

If one imagines the standard for a particular equipment class as a function on a plot of 

efficiency (y-axis) versus kVA (x-axis), then the efficiency levels in each design line are a series 

of points along an imaginary vertical line that intersects the x-axis at the design line’s kVA. If 

there is more than one design line in a given equipment class, there will be more than one series 

of points. Because exponential scaling is performed on losses and because exponential function 

will appear as straight lines on logarithmic plots, the concept is more tractable if illustrated that 

way, as is done in 5.2.1 below. Note that efficiency and loss values have a one-to-one 

correspondence with each other, so one can use whichever coordinate is easier to illustrate 

identical information. Although standards are ultimately given in terms of efficiency, DOE 
5-12

performs the scaling in loss coordinates. Also note that the following figures are given to 

illustrate the scaling concept, and have no relation to actual transformer data.

If one is to select efficiency levels for each design line, as was done by the negotiating 

committee for MVDT transformers, the task remains to scale those chosen efficiencies at certain 

kVA ratings to all of the other kVA ratings that DOE covers. Drawing a straight line2

 through 

the chosen points accomplishes that goal, but may produce a slope different from .75. 

Deriving the .75 rule requires a number of assumptions to be made, among them that the 

overall form and proportions of the transformer remain intact as it changes in size. This 

assumption may break down in a number of ways. For example, MVDT BIL ratings require 

fixed spacings between the edge of a winding and the window of a core. Proportionally, these 

fixed values will be much larger for smaller transformers than for larger units. Thus, while the 

rest of the transformer may behave closer to what the .75 rule would predict, the “fixed” portion 

will cause losses to fall more slowly with decreasing kVA. Stated alternatively, losses will grow 

more slowly with increasing kVA and imply a scaling behavior of less than .75. 

Figure 5.2.1 Efficiency Levels within an Equipment Class (Logarithmic)

 

2 A straight line in logarithmic space is an exponential in the original dimensions, which is the logical 

scaling behavior for transformers to exhibit.
5-13

Table 5.7 Scaling Exponents By Equipment Class 

Distribution Transformer Equipment Class Scaling Exponent

1. Liquid-immersed, medium-voltage, single-phase .76

2. Liquid-immersed, medium-voltage, three-phase .79

3. Dry-type, low-voltage, single-phase .75

4. Dry-type, low-voltage, three-phase .74

5. Dry-type, medium-voltage, single-phase, 20-45 kV BIL .67

6. Dry-type, medium-voltage, three-phase, 20-45 kV BIL .67

7. Dry-type, medium-voltage, single-phase, 46-95 kV BIL .67

8. Dry-type, medium-voltage, three-phase, 46-95 kV BIL .67

9. Dry-type, medium-voltage, single-phase, > 96 kV BIL .68

10. Dry-type, medium-voltage, three-phase, > 96 kV BIL .68

 

To illustrate how DOE used the scaling exponents, consider two transformers with kVA 

ratings of S0 and S1. The no-load losses (NL) and total losses (TL) of these two transformers 

would be depicted as NL0 and TL0, and NL1 and TL1. Then the relationships between the NL and 

TL of the two transformers could be shown as follows: 

Equation 5.2.1 

NL1 = NL0 × (S1 / S0)

E

where:

NL1 = no-load losses of transformer “1,” 

 NL0 = no-load losses of transformer “0,” 

S1 = kVA rating of transformer “1,” and 

S0 = kVA rating of transformer “0.” 

E = Scaling Exponent 

and

Equation 5.2.2 

TL1 = TL0 × (S1 / S0)

E

where:

TL1 = total losses of transformer “1,” and 

TL0 = total losses of transformer “0.” 

E = Scaling Exponent 

Equation 5.2.1 and Equation 5.2.2 can be manipulated algebraically to show that the load 

loss also varies to the “E” power. Starting with the concept that total losses equal no-load losses 

plus load losses, DOE can derive the relationship for load loss (LL), and show that it also scales 

to the “E” power. Specifically:
5-14

Equation 5.2.3 

LL1 = TL1 - NL1

where:

LL1 = load losses of transformer “1” 

Inserting the TL1 and NL1 terms into this equation, DOE finds: 

Equation 5.2.4 

LL1 = (TL0 × (S1 / S0)

E

) - (NL0 × (S1 / S0)

E

) 

Equation 5.2.5 

LL1 = (TL0 - NL0)× (S1 / S0)

E

Equation 5.2.6 

LL1 = (LL0) × (S1 / S0)

E

where:

LL0 = load losses of transformer “0.” 

Thus, kVA scaling can be applied to estimate the losses of a transformer, given the losses 

and kVA rating of a reference (analyzed) unit. However, in order for this rule to be applicable, 

the transformer type must be the same, and key parameters—such as the type of core material, 

core flux density, and conductor current density in the high and low voltage windings—must be 

fixed. Additionally, use of kVA scaling assumes that the efficiency profile of a given transformer 

will have the same shape as the transformer being scaled. See Appendix 5B for detailed 

discussion on the derivation of scaling factors. 

DOE used the kVA scaling to scale the analysis findings on each of the representative 

units within the 14 design lines to the 115 kVA ratings that it did not analyze. DOE applied the 

scaling rule within the design lines in the national impact analysis (chapter 10), where it 

calculated efficiency ratings for the 115 kVA ratings not analyzed. 

5.3 TECHNICAL DESIGN INPUTS FOR SOFTWARE MODEL

For all 14 representative units, the engineering analysis explored the relationship between 

the manufacturer selling prices and corresponding transformer efficiencies. For this analysis, 

DOE contracted Optimized Program Service, Inc. (OPS) in Ohio, a software company 

specializing in transformer design since 1969. Using a range of input parameters and material 

prices, the OPS software produces a design. This design has specific information about the core 
5-15

and coil, including physical characteristics, dimensions, material requirements, and mechanical 

clearances, as well as a complete electrical analysis of the final design. This optimized, practical 

transformer design, the bill of materials, and an electrical analysis report contain sufficient 

information for a manufacturer to build the unit. DOE uses the software’s output to generate an 

estimated cost of manufacturing materials and labor, which it then converts to a MSP by 

applying markups. 

The electrical analysis report estimates the performance of the transformer design 

(including efficiency) at 25 percent, 35 percent, 50 percent, 65 percent, 75 percent, 100 percent, 

125 percent, and 150 percent of nameplate load. The software output provides a clear 

understanding of the relationship between cost and efficiency because it provides detailed data 

on design variances, as well as a bill of materials, labor costs, and efficiency. The software does 

not capture retooling costs associated with changing production designs for a specific 

manufacturer. In some cases, however, DOE captured tooling costs associated with 

manufacturing mitered cores by applying adders to the steel price. 

5.3.1 A and B Loss Valuation Inputs

One of the inputs to the design software consisted of a range of what are known in the 

industry as A and B evaluation combinations (see chapter 3, section 3.6, Total Ownership Cost 

Evaluation). The combination of A and B input to the design software mimics a distribution 

transformer purchase order. The A parameter represents a customer's present value of future 

losses in the transformer core (no-load losses). The B value represents a customer's present value 

of future losses in the windings (load losses). The B parameter is never larger than A, as this 

would imply a specification for a transformer whose average load would be more than 100 

percent of the nameplate load. The A and B values take into account a range of factors that 

usually vary from customer to customer. 

 

The A and B values are expressed in terms of dollars per watt of loss. The greater the 

values of A and B, the greater the importance a customer attaches to the value of future 

transformer losses. As A and B values increase, the customer places greater importance on 

reducing the watts of core and winding losses, respectively, and so the customer chooses a more 

energy-efficient transformer.

DOE used broad ranging combinations of A and B evaluation formulae (presented in 

Table 5.8 and Table 5.9) to create a complete set of efficiency levels for each design option 

combination analyzed. The efficiency levels spanned from a low-first-cost unit to a maximum 

technologically feasible (“max-tech”) design. For the low-first-cost design, the A and B 

evaluation values were both $0/watt, indicating that the customer does not attach any financial 

value to future losses in the core or coil of the transformer. For the maximum technologically 

feasible design, the A and B evaluation values were very high, pushing the software to design at 

the highest efficiencies achievable.

DOE created its combinations of A and B evaluation formulae combining two techniques 

to ensure there were sufficient designs in the database for the analysis. The first technique was to 

create a ‘grid’ of A and B combinations. The ‘grid’ technique involved increasing the value of A 
5-16

by a step value, and then increasing the B value from zero to that value of A, using a different 

step value. Thus, if A had incremental steps of $0.25 and B had steps of $0.20, the combinations 

would work as follows: ($0.00, $0.00), ($0.25, $0.00), ($0.25, $0.20), ($0.50, $0.00), ($0.50, 

$0.20), ($0.50, $0.40), ($0.75, $0.00), and so on. Table 5.8 presents the ranges and incremental 

steps for the A and B combinations used in the three grids. 

Table 5.8 A and B Grid Combinations Used by Software to Generate Design Database 

Grid 

Number 

A values

and increments

B values

and increments

Resultant # of

(A, B) combinations 

1 $0 to $2 by 0.25 steps $0 to $2 by 0.20 steps 47

2 $2.50 to $8 by 0.50 steps $0 to $8 by 0.40 steps 157

3 $9 to $16 by 1.00 steps $3 to $8 by 0.50 steps 85

The second technique for generating A and B evaluation formulae in the engineering 

analysis is called the “fan.” DOE understands that the ratio of A to B represents an implicit 

loading for the transformer. Therefore, DOE created a set of (A, B) values in which the B is 

calculated from the A. The B term is calculated as the A times the percent load squared. In other 

words, if A equals $1 and DOE is interested in calculating the appropriate B for a 50 percent 

root-mean-square (RMS) load, then it would be $1 times (0.50)^2, or $0.25. Thus, the 

combination of ($1.00, $0.25) represents approximately a 50 percent RMS load. As with the 

“grid,” the A values increased with a step function, and B values were calculated as fractions of 

A so that the ratio of A to B encompassed the RMS loading points that were identified in DOE’s 

loading analysis (i.e., 35 percent and 50 percent). DOE calculated the B values for each A at the 

following RMS loading points: 5 percent, 10 percent, 15 percent, 20 percent, 25 percent, 30 

percent, 35 percent, 40 percent, 45 percent, 50 percent, 55 percent, and 60 percent. Table 5.9

presents the range of the two fan combinations used in the analysis. 

Table 5.9 A and B Fan Combinations Used by Software to Generate Design Database 

Fan 

Number 

A values

and increments

B values

and increments

Resultant # of

(A,B) 

combinations 

1 $0 to $2 by 0.50 

steps

5% to 60% implicit loading by 5% 

steps 47 

2 $3 to $16 by 1.00 

steps

5% to 60% implicit loading by 5% 

steps 182

When used together, these two techniques created a broad spectrum of A and B 

combinations as inputs to the OPS software. Figure 5.3.1 illustrates the coverage of designs for 

the 518 A and B combinations. DOE used each of these A and B pairs with each combination of 

core steel and winding material analyzed for each representative unit studied. 
5-17

Figure 5.3.1 A and B Combination Software Inputs Used in the Engineering Analysis

Occasionally, the design software generated the same transformer design for two different 

A and B combinations, creating duplicate designs in the engineering analysis database. DOE 

removed these duplicate designs before the engineering database was imported into the LCC 

analysis. Similarly, DOE removed any designs that yielded an efficiency value below the current 

standard level efficiency.

 

5.3.2 Core Material Options

DOE understands that there are many ways to build a transformer, even with constant 

kVA and voltage ratings. For instance, manufacturers can vary the core steels (e.g., M2, M3, 

M6), the winding materials (aluminum or copper), and core configurations (shell or core-type). 

For each of the design lines, DOE provides tables listing the design option combinations that it 

used to analyze each of the representative units. Depending on customer needs, the cost of 

materials, the capital equipment in their facility, and the skills of their labor force, manufacturers 

make decisions on how to manufacture a given transformer using different core configurations, 

core steels, and winding materials. To capture this variation in design, DOE analyzed the 14 

representative units using 8 – 14 different design option combinations of core type, core steel, 

and winding material. As discussed in the technology assessment (see chapter 3), core steel is 

produced in a range of qualities (from an efficiency perspective). M2 core steel is oriented grain 

silicon steel and has thin laminations, and consequently has very low losses. M12 core steel is 
5-18

non-oriented grain silicon steel and is rolled in thicker laminations, thus contributing to higher 

core losses. Table 5.10 lists all the steel types used in the analysis, and properties associated with 

these steels. Each steel grade provides the nominal thickness and core losses per pound of steel, 

under a specified typical magnetic flux density, measured in Tesla (T).

Table 5.10 Core Steel Grades, Thicknesses and Associated Losses

Steel 

Grade

Nominal 

Thickness 

inches

Core Loss at 60 Hz

Watts per Pound at 

magnetic flux density*

Notes / Remarks

M12 0.014 1.36 Watts/lb at 1.5 T Non-oriented grain silicon steel

M6 0.014 0.60 Watts/lb at 1.5 T

0.84 Watts/lb at 1.7 T Grain-oriented silicon steel

M5 0.012 0.51 Watts/lb at 1.5 T

0.74 Watts/lb at 1.7 T Grain-oriented silicon steel

M4 0.011 0.46 Watts/lb at 1.5 T

0.66 Watts/lb at 1.7 T Grain-oriented silicon steel

M3 0.009 0.39 Watts/lb at 1.5 T

0.60 Watts/lb at 1.7 T Grain-oriented silicon steel

M2 0.007 0.38 Watts/lb at 1.5 T

0.58 Watts/lb at 1.7 T Grain-oriented silicon steel

H-0 DR 0.009 0.34 Watts/lb at 1.5 T

0.47 Watts/lb at 1.7 T

Domain-refined, high permeability grade 

silicon steel

ZDMH 0.009 0.38 Watts/lb at 1.5 T

0.57 Watts/lb at 1.7 T 

Imported silicon steel, magnetic domainrefined by mechanical process 

SA1 0.001 

0.108 Watts/lb at 1.35

T 

0.098 Watts/lb at 1.3 T

Amorphous core steel (silicon and boron); 

flux density limitation - testing at ~ 1.3 T 

* Watts of loss per pound of core steel are only comparable at the same magnetic flux 

density (measured in Tesla).

5.3.3 Core Configurations 

In addition to selecting a core steel, the manufacturer’s selection of a core design may also 

contribute to the overall efficiency of a transformer. A transformer facility may be optimized to 

work around one or two core configurations. Table 5.11 provides a list of all the core 

configurations used for each of the 14 design lines. DOE selected these configurations, in 

combination with the range of core steels and winding materials, to represent the most common 

construction methods for these kVA ratings in the U.S. market. 
5-19

Table 5.11 Core Configurations Used in Each Design Line

Design 

Line # Phases Core Configurations Used in the Engineering Analysis

DL1 1 Wound core - distributed gap;

Shell-type

DL2 1 Wound core - distributed gap;

Shell-type or core-type

DL3 1 Wound core - distributed gap;

Shell-type or core-type

DL4 3 Wound core - distributed gap;

5-leg

DL5 3 Wound core - distributed gap; 

5-leg

DL6 1 Wound core – distributed gap; or stacked butt-lap;

Shell-type or core-type

DL7 3 Wound core - distributed gap; or stacked butt-lap, step-lap or full mitered; 

3-leg or 5-leg

DL8 3 Wound core - distributed gap; or stacked butt-lap, step-lap or full mitered; 

3-leg or 5-leg

DL9 3 Wound core - distributed gap; or stacked full mitered; 

3-leg or 5-leg

DL10 3 Wound core – distributed gap; or stacked, cruciform, mitered joint;

3-leg

DL11 3 Wound core – distributed gap; or stacked, step-lap or full mitered; 

3-leg or 5-leg

DL12 3 Wound core – distributed gap; or stacked, cruciform or step-lap mitered joint;

3-leg or 5-leg

DL13

(A & B) 3 Wound core – distributed gap; or stacked, cruciform or step-lap mitered joint;3-leg or 5-leg

5.3.3.1 Standard Core Configurations 

For the single-phase representative units, the configurations used are either core-type or 

shell-type. This applies whether the core consists of stacked or wound laminations of core steel. 

For wound cores, manufacturers generally employ a technique known as ‘distributed gap.’ This 

means that each lamination of core steel wound around the form will have a start and finish point 

(the ‘gap’), staggered with respect to the previous and the next lamination. Distributed gap core 

construction techniques are used to minimize the performance impact of the lamination joint 

gaps (reducing the exciting current) and, by locating inside the coil window, reduce the 

transformer’s operating sound level. Figure 5.3.2 illustrates the two types of single-phase core 

construction. 
5-20

Figure 5.3.2 Graphic of Single-Phase Core Configurations 

Three-phase transformers can have three-legged, four-legged, five-legged, Evans, or 

symmetric cores. In the engineering analysis, DOE considered the three-legged construction 

techniques for the three-phase dry-types and five-legged construction for the three-phase liquidimmersed transformers. Some of the dry-type designs using an amorphous core also use a fivelegged construction technique. Figure 5.3.3 below illustrates the difference between the threelegged and the five-legged core construction techniques. A three-legged core is assembled from 

stacked laminations, the joints of which can be butt-lapped or mitered. Where there is an 

economic need to reduce core losses, particularly in keeping with the use of more efficient 

grades of core steel (M2 or M3), the mitered core tends to be selected. DOE recognizes that there 

are a variety of approaches to mitered core construction: “scrapless T-mitering,” “full-mitering,” 

and “step-mitering.” DOE modeled full-mitered and step-mitered cores. 

Figure 5.3.3 Graphic of Three-phase Core Configurations 

For larger kVA ratings, design economics may cause the selection of a cruciform core 

section, where multiple lamination widths are stacked in increasing and then decreasing widths 

to create a circular core form (or “log”) around which the windings are placed. Figure 5.3.4

illustrates the cruciform core by showing a cross-section. This figure shows four different widths 

of steel being used, but there can be fewer or more widths, depending on the design. By using a 

core configuration that better follows the contours of the windings, losses are again reduced, 

resulting in a more efficient transformer. The use of the three-legged core usually depends on the 
5-21

primary winding being delta-connected. If the primary winding is wye-connected, as is 

frequently the case for pad-mounted transformers used in underground distribution, the core 

configuration needs to be four-legged or five-legged. 

Figure 5.3.4 Cruciform Core Cross-Section

The five-legged core is assembled from four wound-core loops, and is the common 

configuration for liquid-filled, three-phase distribution transformers having a wye-wye voltage 

connection. Again, this occurs for pad-mounted transformers used in underground distribution. 

The individual core loops have distributed gaps, as explained for single-phase, wound-core 

transformers.

5.3.3.2 Symmetric Core Configurations

 Transformers with symmetric core configurations use continuously wound cores with 120 

degree radial symmetry, where no one leg is magnetically distinguishable from the other two. . 

Following the preliminary analysis, DOE was unable to identify a company with commercial 

modeling software that could model symmetric core designs. DOE did speak with transformer 

manufacturers and industry experts about symmetric core designs. Through these conversations, 

DOE received information on a few symmetric core designs. These designs were insufficient to 

conduct a full-scale engineering analysis comparable to the other design types. In the preliminary 

analysis, DOE was able to approximate the cost-efficiency relationship for symmetric core 

designs based on trends in the data received from manufacturers, published literature, and 

through conversations with industry experts. . However, because the data was so limited and 

DOE did not receive any additional information from manufacturers, DOE did not consider 

symmetric core designs in this stage of the rulemaking. . 
5-22

5.3.4 Less-Flammable Liquid-Immersed Transformers

For liquid-immersed distribution transformers, DOE studied the differences between 

mineral oil cooled units and less-flammable cooled units. DOE understands that the IEEE 

standard C57.12.80 divides less-flammable liquid-immersed (LFLI) transformers into two 

groups: KNAN (which have an insulating liquid with a fire point greater than 300 degrees 

Celsius) and LNAN (which have an insulating liquid with no measurable fire point). The fire 

point for mineral oil is approximately 175 degrees Celsius, and therefore this type of transformer 

is not used inside buildings or in areas designated as hazardous. While industry has a 

specification for KNAN for a certain degree of fire protection or LNAN for users who prefer an 

extra measure of safety, DOE will continue to refer to both KNAN and LNAN using the phrase 

‘less-flammable,’ or LFLI.

DOE understands that the viscosity of the insulating liquid can have a slight impact on the 

efficiency of a transformer. When the viscosity is higher than mineral oil, transformer designers 

must make slightly larger cooling ducts to permit an easier flow of the fluid. Larger ducts result 

in larger physical size of the winding assembly and a greater mean number of turns of the 

conductor, therefore contributing to a slightly higher load loss. However, as efficiency increases, 

the transformer will run cooler, which negates part of the need for larger cooling ducts. As such, 

LFLI transformers are still able to achieve the same efficiency levels as transformers using 

mineral oil. DOE verified this fact through conversations with manufacturers and industry 

experts. In fact, DOE was informed that LFLI transformers might be capable of higher 

efficiencies than mineral oil units since their higher temperature tolerance may allow the unit to 

be downsized and run hotter than mineral oil units.

For the KNAN transformers (i.e., those with a fire point of 300 degrees or greater), DOE 

is not aware of any viscosity differences with mineral oil that might impede designs or make 

efficiency levels significantly more difficult to reach. For LNAN transformers (i.e., those with no 

fire point), DOE understands that the viscosity under usual operating conditions is slightly 

greater than that of mineral oil, which may require design engineers to increase the duct size, 

leading to a marginal impact on efficiency. However, as explained above, DOE believes this 

increased viscosity is offset by the cooler operating temperature, which could allow the 

transformer to be downsized and run hotter. This would negate any impact on efficiency. Chapter 

2 provides additional discussion of less-flammable liquid-immersed transformers.

5.3.5 Design Line 1 Representative Unit

Design line 1 (DL1) represents rectangular-tank, liquid-immersed, single-phase 

distribution transformers, ranging from 10 kVA to 833 kVA. The representative unit selected for 

this design line is a 50kVA pad-mounted unit. The following are the technical specifications that 

constitute input parameters to the OPS design software: 

KVA: 50 (liquid-immersed, rectangular-tank) 

Primary: 14400 Volts at 60 Hz 

Secondary: 240/120V 

T Rise: 65 ̊C
5-23

Ambient: 20 ̊C

Winding Configuration: Lo-Hi-Lo (Shell-Type)

Core: Wound core - distributed gap 

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 1.5–4.5 percent

BIL: 95 kV 

For DL1, DOE selected nine design option combinations, based on input from 

manufacturers and other technical experts. The core selected was shell-type, because the 

application is for a pad-mounted unit, and this shape is well suited to a rectangular tank. With the 

exception of the max-tech/high efficiency designs, DOE selected nine design option 

combinations to represent the most common construction practices for this representative unit. 

Table 5.12 Design Option Combinations for the Design Line 1 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M5 Cu – wire Al – strip Shell – DG* Wound Core

M3 Al – wire Al – strip Shell – DG Wound Core

M3 Cu – wire Al – strip Shell – DG Wound Core

M2 Al – wire Al – strip Shell – DG Wound Core

M2 Cu – wire Al – strip Shell – DG Wound Core

ZDMH Al – wire Al – strip Shell – DG Wound Core

ZDMH Cu – wire Cu – strip Shell – DG Wound Core

SA1 (Amorphous) Al – wire Al – strip Shell – DG Wound Core

SA1 (Amorphous) Cu – wire Cu – strip Shell – DG Wound Core

* DG – Distributed gap wound core construction, where the core laminations are wound 

in such a way that the gap between the start and finish of a lamination is staggered in the crosssection of the core.

DOE analyzed each of the nine design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 1,924 designs. 

5.3.6 Design Line 2 Representative Unit

Design line 2 (DL2) represents round-tank, liquid-immersed, single-phase distribution 

transformers, ranging from 10 kVA to 833 kVA. The representative unit selected for this design 

line is a 25kVA pole-mounted unit. The following are the technical specifications that constitute 

input parameters to the OPS design software: 

KVA: 25 (liquid-immersed, round-tank)

Primary: 14400 Volts at 60 Hz (125 kV BIL) 

Secondary: 120/240V 

T Rise: 65 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi-Lo (Shell-Type), Lo-Hi (Core-Type, for amorphous core) 
5-24

Core: Wound core - distributed gap 

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 1.0–4.5 percent

BIL: 125 kV 

For DL2, DOE selected eleven design option combinations, based on input from 

manufacturers and other technical experts. With the exception of the max-tech/high-efficiency 

designs, DOE selected these design option combinations to represent the most common 

construction practices for the representative unit. 

Table 5.13 Design Option Combinations for the Design Line 2 Representative Unit 

Core Material High-Voltage 

Conductor 

Low-Voltage 

Conductor Core Design Type 

M5 Cu – wire Al – strip Shell – DG Wound Core

M4 Al – wire Al – strip Shell – DG Wound Core

M4 Cu – wire Al – strip Shell – DG Wound Core

M3 Al – wire Al – strip Shell – DG Wound Core

M3 Cu – wire Al – strip Shell – DG Wound Core

M2 Al – wire Al – strip Shell – DG Wound Core

M2 Cu – wire Al – strip Shell – DG Wound Core

ZDMH Al – wire Al – strip Shell – DG Wound Core

ZDMH Cu – wire Cu – strip Shell – DG Wound Core

SA1 (Amorphous) Al – wire Al – strip Core – DG Wound Core

SA1 (Amorphous) Cu – wire Cu – strip Core – DG Wound Core

DOE analyzed each of the eleven design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 2,301designs. 

5.3.7 Design Line 3 Representative Unit

Design line 3 (DL3) represents round-tank, liquid-immersed, single-phase distribution 

transformers, ranging from 10 kVA to 833 kVA. The representative unit selected for this design 

line is a 500kVA round-tank transformer. The following are the technical specifications which 

constitute input parameters to the OPS design software: 

KVA: 500 (liquid-immersed, round-tank) 

Primary: 14400 Volts at 60 HZ (150kV BIL) 

Secondary: 277 Volts

T Rise: 65 ̊C 

Ambient: 20 ̊C 

Winding Configuration: Lo-Hi (Shell-Type and Core-Type)

Core: Wound core - distributed gap 

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 1.5–7.0 percent

BIL: 150 kV 
5-25

For DL3, DOE selected twelve design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 

the max-tech/high-efficiency designs, DOE chose design option combinations to represent the 

most common construction practice for this representative unit. 

Table 5.14 Design Option Combinations for the Design Line 3 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M5 Cu – wire Al – strip Shell – DG Wound Core

M4 Al – wire Al – strip Shell – DG Wound Core

M4 Cu – wire Al – strip Shell – DG Wound Core

M3 Al – wire Al – strip Shell – DG Wound Core

M3 Cu – wire Al – strip Shell – DG Wound Core

M2 Al – wire Al – strip Shell – DG Wound Core

M2 Cu – wire Al – strip Shell – DG Wound Core

ZDMH Al – wire Al – strip Shell – DG Wound Core

ZDMH Cu – wire Cu – strip Shell – DG Wound Core

SA1 (Amorphous) Al – wire Al – strip Core – DG Wound Core

SA1 (Amorphous) Cu – wire Cu – strip Shell – DG Wound Core

SA1 (Amorphous) Cu – wire Cu – strip Core – DG Wound Core

DOE analyzed each of the twelve design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 2,740 designs. 

5.3.8 Design Line 4 Representative Unit

Design line 4 (DL4) represents rectangular tank, liquid-immersed, three-phase distribution 

transformers, ranging from 15 kVA to 2500 kVA. The representative unit selected for this design 

line is a 150kVA transformer. The following are the technical specifications that constitute input 

parameters to the OPS design software:

KVA: 150 (liquid-immersed, pad mount) 

Primary: 12470Y/7200 Volts at 60 Hz (95kV BIL) 

Secondary: 208Y/120 Volts 

T Rise: 65 ̊C

Ambient: 20 ̊C

Terminal Configuration: ANSI/IEEE C57.12.26, Loop Feed 

Winding Configuration: Lo-Hi

Core: Wound core - distributed gap, 5-leg

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 1.2-6.0 percent

BIL: 95 kV 
5-26

For DL4, DOE selected nine design option combinations of core steel and winding types 

based on input from manufacturers and other technical experts. With the exception of the maxtech/high-efficiency designs, DOE selected these design option combinations to represent the 

most common construction practice for the representative unit. 

Table 5.15 Design Option Combinations for the Design Line 4 Representative Unit

Core Material High-Voltage 

Conductor 

Low-Voltage 

Conductor Core Design Type 

M5 Cu – wire Al – strip 5-Leg DG Core

M3 Al – wire Al – strip 5-Leg DG Core

M3 Cu – wire Al – strip 5-Leg DG Core

M2 Al – wire Al – strip 5-Leg DG Core

M2 Cu – wire Al – strip 5-Leg DG Core

ZDMH Al – wire Al – strip 5-Leg DG Core

ZDMH Cu – wire Cu – strip 5-Leg DG Core

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – strip 5-Leg DG Core

DOE analyzed each of the nine design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 1,977 designs. 

5.3.9 Design Line 5 Representative Unit

Design line 5 (DL5) represents rectangular tank, liquid-immersed, three-phase distribution 

transformers, ranging from 15 kVA to 2500 kVA. The representative unit selected for this design 

line is a 1500kVA transformer. The following are the technical specifications that constitute 

input parameters to the OPS design software: 

KVA: 1500 (liquid-immersed, pad mount) 

Primary: 24940GrdY/14400 Volts (125kV BIL) 

Secondary: 480Y/277 Volts 

T Rise: 65 ̊C

Ambient: 20 ̊C

Terminal Configuration: ANSI/IEEE C57.12.26, Loop Feed 

Winding Configuration: Lo-Hi

Core: Wound core - distributed gap, 5-leg

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 5-7.5 percent

BIL: 125 kV 

For DL5, DOE selected nine design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practices for the representative unit. 
5-27

Table 5.16 Design Option Combinations for the Design Line 5 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M4 Cu – wire Al – strip 5-Leg DG Core

M3 Al – wire Al – strip 5-Leg DG Core

M3 Cu – wire Al – strip 5-Leg DG Core

M2 Al – wire Al – strip 5-Leg DG Core

M2 Cu – wire Al – strip 5-Leg DG Core

ZDMH Al – wire Al – strip 5-Leg DG Core

ZDMH Cu – wire Cu – strip 5-Leg DG Core

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – strip 5-Leg DG Core

DOE analyzed each of the nine design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 1,294 designs. 

5.3.10 Design Line 6 Representative Unit

Design line 6 (DL6) represents ventilated dry-type, single-phase, low-voltage distribution 

transformers, ranging from 15 kVA to 333 kVA. The representative unit selected for this design 

line is a 25 kVA transformer. The following are the technical specifications that constitute input 

parameters to the OPS design software:

KVA: 25 (dry-type)

Phases: Single

Primary: 480 Volts at 60 Hz (10 kV BIL) 

Secondary: 120/240 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi (for Core-Type and Shell-Type)

Core: Stacked, butt-lap; Wound core - distributed gap

Taps: Six 21⁄2 percent, two above and four below the nominal 

Impedance Range: 1.5-6.0 percent

For DL6, DOE selected ten design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit.
5-28

Table 5.17 Design Option Combinations for the Design Line 6 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M6 Al – wire Al – wire Stacked Core Butt-lap

M5 Al – wire Al – strip Stacked Core Butt-lap

M4 Al – wire Al – wire Stacked Core Butt-lap

M3 Al – wire Al – strip Stacked Core Butt-lap

M3 Cu – wire Al – wire Stacked Core Butt-lap

M3 Cu – wire Al – wire Stacked Shell Butt-lap

H-0 DR* Al – wire Al – strip Stacked Core Butt-lap

H-0 DR Cu – wire Cu – wire Stacked Core Butt-lap

SA1 (Amorphous) Al – wire Al – strip Core – DG Wound Core

SA1 (Amorphous) Cu – wire Cu – wire Core – DG Wound Core

* H-0 DR is a domain-refined, high permeability core steel.

DOE analyzed each of the ten design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 3,091 designs. 

5.3.11 Design Line 7 Representative Unit

Design line 7 (DL7) represents ventilated dry-type, three-phase, low-voltage distribution 

transformers, ranging from 15 kVA to 1000 kVA. The representative unit selected for this design 

line is a 75 kVA transformer. The following are the technical specifications that constitute input 

parameters to the OPS design software:

KVA: 75 (dry-type)

Phases: Three

Primary: 480 Volts at 60 Hz (10 kV BIL) 

Secondary: 208Y/120 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi

Core: Stacked, butt-lap; Stacked, mitered; Wound core - distributed gap

Taps: Six 21⁄2 percent, two above and four below the nominal 

Impedance Range: 1.5–6.0 percent 

For DL7, DOE selected fifteen design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit. 
5-29

Table 5.18 Design Option Combinations for the Design Line 7 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M12 Al – wire Al – wire 3-Leg Stacked Butt-lap

M12 Cu – wire Al – wire 3-Leg Stacked Butt-lap

M6 Al – wire Al – wire 3-Leg Stacked Butt-lap

M6 Al – wire Al – wire 3-Leg Stacked Full Miter**

M4 Al – wire Al – wire 3-Leg Stacked Butt-lap

M4 Al – wire Al – wire 3-Leg Step-Lap Miter

M4 Cu – wire Al – wire 3-Leg Stacked Full Miter

M3 Al – wire Al – wire 3-Leg Stacked Full Miter

M3 Al – wire Al – wire 3-Leg Step-Lap Miter

M3 Cu – wire Cu – wire 3-Leg Stacked Butt-lap

H-0 DR* Al – wire Al – strip 3-Leg Stacked Full Miter

H-0 DR Al – wire Al – strip 3-Leg Step-Lap Miter

H-0 DR Cu – wire Cu – wire 3-Leg Stacked Full Miter

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – wire 5-Leg DG Core

* H-0 DR is a domain-refined, high permeability core steel.

** Full miters are not step-miters, but are mitered joints for all three legs. These cores are 

stacked three by three.

DOE analyzed each of the fifteen design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 1990 designs. 

For the NOPR, DOE filtered out a number of transformer designs with high flux density 

from DL7. Based on comments in negotiations, DOE removed designs over a certain flux density 

to maintain consistency with designs submitted by manufacturers. There is a variety of reasons 

that manufacturers would choose to limit flux density (e.g. vibration and noise). Designs using 

conventional steels begin to experience these issues at flux densities over 1.3 Tesla; while those 

with domain-refined high permeability core steels see issues starting at 1.5 Tesla. Designs that 

use amorphous metal have naturally lower flux densities, and therefore do not often experience 

these issues. DOE set the flux density limitations described in Table 5.19. 
5-30

Table 5.19 Limits on Flux Density for Design Line 7 Representative Unit

Core 

Material

High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type Flux Density Limit

M12 Al – wire Al – wire 3-Leg Stacked Butt-lap 1.3

M12 Cu – wire Al – wire 3-Leg Stacked Butt-lap 1.3

M6 Al – wire Al – wire 3-Leg Stacked Butt-lap 1.3

M6 Al – wire Al – wire 3-Leg Stacked Full 

Miter** 1.3 

M4 Al – wire Al – wire 3-Leg Stacked Butt-lap 1.3

M4 Al – wire Al – wire 3-Leg Step-Lap Miter 1.3

M4 Cu – wire Al – wire 3-Leg Stacked Full Miter 1.3

M3 Al – wire Al – wire 3-Leg Stacked Full Miter 1.3

M3 Al – wire Al – wire 3-Leg Step-Lap Miter 1.3

M3 Cu – wire Cu – wire 3-Leg Stacked Butt-lap 1.3

H-0 DR* Al – wire Al – strip 3-Leg Stacked Full Miter 1.5

H-0 DR Al – wire Al – strip 3-Leg Step-Lap Miter 1.5

H-0 DR Cu – wire Cu – wire 3-Leg Stacked Full Miter 1.5

SA1 

(Amorphous) Al – wire Al – strip 5-Leg DG Core No limit set

SA1 

(Amorphous) Cu – wire Cu – wire 5-Leg DG Core No limit set

 

5.3.12 Design Line 8 Representative Unit

Design line 8 (DL8) represents ventilated dry-type, three-phase, low-voltage distribution 

transformers, ranging from 15 kVA to 1000 kVA. The representative unit selected for this design 

line is a 300 kVA transformer. The following are the technical specifications that constitute input 

parameters to the OPS design software:

KVA: 300 (dry-type)

Phases: Three

Primary: 480V at 60 Hz (10 kV BIL) Delta Connected

Secondary: 208Y/120 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi

Core: Stacked, butt-lap; Stacked, mitered; Wound core - distributed gap

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 3.0–7.0 percent

For DL8, DOE selected fourteen design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit. 
5-31

Table 5.20 Design Option Combinations for the Design Line 8 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M6 Al – wire Al – strip 3-Leg Stacked Butt-lap

M6 Al – wire Al – strip 3-leg Stacked Full Miter

M6 Cu – wire Cu – strip 3-Leg Stacked Full Miter**

M5 Al – wire Al – strip 3-Leg Stacked Butt-lap

M5 Al – wire Al – strip 3-Leg Stacked Full Miter

M4 Cu – wire Al – strip 3-Leg Stacked Full Miter

M3 Al – wire Al – strip 3-Leg Stacked Full Miter

M3 Al – wire Al – strip 3-Leg Step-Lap Miter

M3 Cu – wire Al – strip 3-Leg Stacked Full Miter

H-0 DR* Al – wire Al – strip 3-Leg Stacked Full Miter

H-0 DR Al – wire Al – strip 5-Leg Step-Lap Miter

H-0 DR Cu – wire Cu – strip 3-Leg Stacked Full Miter

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – strip 5-Leg DG Core

* H-0 DR is a domain-refined, high permeability core steel.

** Full miters are not step-miters, but are mitered joints for all three legs. These cores are 

stacked three by three.

DOE analyzed each of the fourteen design option combinations using the matrix of A and 

B values described in Table 5.8 and Table 5.9, creating 4,443 designs. 

5.3.13 Design Line 9 Representative Unit

Design line 9 (DL9) represents ventilated dry-type, three-phase, medium-voltage 

distribution transformers with a 20-45kV BIL, ranging from 15 kVA to 2500 kVA. The 

representative unit selected for this design line is a 300 kVA transformer. The following are the 

technical specifications that constitute input parameters to the OPS design software:

KVA: 300 (dry-type)

Phases: Three

Primary: 4160V at 60 Hz (45 kV BIL) Delta Connected

Secondary: 480Y/277 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi

Core: Stacked, mitered; Wound core - distributed gap

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 3.0–7.0 percent

For DL9, DOE selected thirteen design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 
5-32

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit. 

Table 5.21 Design Option Combinations for the Design Line 9 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M6 Cu – wire Cu – wire 3-Leg Stacked Full Miter**

M5 Al – wire Al – wire 3-Leg Stacked Full Miter

M4 Al – wire Al – wire 3-Leg Step-Lap Miter

M3 Al – wire Al – strip 3-Leg Stacked Full Miter

M3 Al – wire Al – strip 3-Leg Step-Lap Miter

M3 Al – wire Al – strip 5-Leg DG Core

M3 Cu – wire Al – strip 3-Leg Stacked Full Miter

H-0 DR* Al – wire Al – strip 3-Leg Stacked Full Miter

H-0 DR Al – wire Al – strip 3-Leg Step-Lap Miter

H-0 DR Cu – wire Cu – strip 3-Leg Stacked Full Miter

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – strip 3-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – strip 5-Leg DG Core

* H-0 DR is a domain-refined, high permeability core steel.

** Full miters are not step-miters, but are mitered joints for all three legs. These cores are 

stacked three by three.

DOE analyzed each of the thirteen design option combinations using the matrix of A and 

B values described in Table 5.8 and Table 5.9, creating 5,600 designs. 

5.3.14 Design Line 10 Representative Unit

Design line 10 (DL10) represents dry-type, three-phase, medium-voltage distribution 

transformers with a 20-45kV BIL, ranging from 15 kVA to 2500 kVA. The representative unit 

selected for this design line is a 1500 kVA transformer. The following are the technical 

specifications that constitute input parameters to the OPS design software:

KVA: 1500 (dry-type)

Phases: Three

Primary: 4160V at 60 Hz (45 kV BIL) 

Secondary: 480Y/277 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi

Core: Stacked, cruciform, mitered joint, 3-leg; Wound core - distributed gap

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 5.0-8.0 percent
5-33

For DL10, DOE selected eleven design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit. 

Table 5.22 Design Option Combinations for the Design Line 10 Representative Unit 

Core Material High-Voltage 

Conductor 

Low-Voltage 

Conductor Core Design Type 

M5 Cu – wire Al – strip 3-Leg Mitered Cruciform

M4 Al – wire Al – strip 3-Leg Step-Lap Miter

M4 Cu – wire Al – strip 3-Leg Mitered Cruciform

M3 Al – wire Al – strip 3-Leg Mitered Cruciform

M3 Al – wire Al – strip 3-Leg Step-Lap Miter

M3 Cu – wire Cu – strip 3-Leg Mitered Cruciform

H-0 DR* Al – wire Al – strip 3-Leg Mitered Cruciform

H-0 DR Al – wire Al – strip 3-Leg Step-Lap Miter

H-0 DR Cu – wire Cu – strip 3-Leg Mitered Cruciform

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – strip 3-Leg DG Core

* H-0 DR is a domain-refined, high permeability core steel.

DOE analyzed each of the eleven design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 2,501 designs. 

5.3.15 Design Line 11 Representative Unit

Design line 11 (DL11) represents dry-type, three-phase, medium-voltage distribution 

transformers with a 46-95kV BIL, ranging from 15 kVA to 2500 kVA. The representative unit 

selected for this design line is a 300 kVA transformer. The following are the technical 

specifications that constitute input parameters to the OPS design software:

KVA: 300 (dry-type)

Phases: Three

Primary: 12470 Volts at 60 Hz (95 kV BIL) 

Secondary: 480Y/277 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi

Core: Stacked, mitered joint, 3-leg; Wound core - distributed gap, 5-leg

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 3.0-7.0 percent 

For DL11, DOE selected eleven design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 
5-34

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit. 

Table 5.23 Design Option Combinations for the Design Line 11 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M6 Cu – wire Cu – strip 3-Leg Stacked Full Miter**

M4 Al – wire Al – strip 3-Leg Step-Lap Miter

M4 Cu – wire Al – strip 3-Leg Stacked Full Miter

M3 Al – wire Al – strip 3-Leg Stacked Full Miter

M3 Al – wire Al – strip 3-Leg Step-Lap Miter

M3 Cu – wire Cu – strip 3-Leg Stacked Full Miter

H-0 DR* Al – wire Al – strip 3-Leg Stacked Full Miter

H-0 DR Al – wire Al – strip 3-Leg Step-Lap Miter

H-0 DR Cu – wire Cu – strip 3-Leg Stacked Full Miter

SA1 (Amorphous) Al – wire Al – strip 3-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – strip 5-Leg DG Core

* H-0 DR is a domain-refined, high permeability core steel.

** Full miters are not step-miters, but are mitered joints for all three legs. These cores are 

stacked three by three.

DOE analyzed each of the eleven design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 1,896 designs. 

5.3.16 Design Line 12 Representative Unit

Design line 12 (DL12) represents dry-type, three-phase, medium-voltage distribution 

transformers with a 46-95kV BIL, ranging from 15 kVA to 2500 kVA. The representative unit 

selected for this design line is a 1500 kVA transformer. The following are the technical 

specifications that constitute input parameters to the OPS design software:

KVA: 1500 (dry-type) 

Phases: Three

Primary: 12470 Volts at 60 Hz (95 kV BIL) 

Secondary: 480Y/277 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi

Core: Stacked, cruciform, mitered joint, 3-leg; Wound core - distributed gap, 5-leg

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 5.0–8.0 percent

For DL12, DOE selected eleven design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 
5-35

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit. 

Table 5.24 Design Option Combinations for the Design Line 12 Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M5 Al – wire Al – strip 3-Leg Mitered Cruciform

M4 Al – wire Al – strip 3-Leg Step-Lap Miter

M4 Cu – wire Al – strip 3-Leg Mitered Cruciform

M3 Al – wire Al – strip 3-Leg Mitered Cruciform

M3 Al – wire Al – strip 3-Leg Step-Lap Miter

M3 Cu – wire Cu – strip 3-Leg Mitered Cruciform

H-0 DR* Al – wire Al – strip 3-Leg Mitered Cruciform

H-0 DR Al – wire Al – strip 3-Leg Step-Lap Miter

H-0 DR Cu – wire Cu – strip 3-Leg Mitered Cruciform

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

SA1 (Amorphous) Cu – wire Cu – strip 5-Leg DG Core

* H-0 DR is a domain-refined, high permeability core steel.

DOE analyzed each of the eleven design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 3,393 designs. 

5.3.17 Design Line 13A Representative Unit

Design line 13A (DL13A) represents dry-type, three-phase, medium-voltage distribution 

transformers with a ≥96kV BIL, ranging from 225 kVA to 2500 kVA. The representative unit 

selected for this design line is a 300 kVA transformer. The following are the technical 

specifications that constitute input parameters to the OPS design software:

KVA: 300 (dry-type)

Phases: Three

Primary: 24940 Volts at 60 Hz (125 kV BIL) 

Secondary: 480Y/277 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi

Core: Stacked, cruciform, mitered joint, 3-leg; Wound core - distributed gap, 5-leg

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 3.0–7.0 percent 

For DL13A, DOE selected seven design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit. 
5-36

Table 5.25 Design Option Combinations for the Design Line 13A Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M5 Al – wire Al – strip 3-Leg Mitered Cruciform

M4 Al – wire Al – strip 3-Leg Mitered Cruciform

M4 Al – wire Al – strip 3-Leg Step-Lap Miter

M3 Al – wire Al – strip 3-Leg Mitered Cruciform

M3 Al – wire Al – strip 3-Leg Step-Lap Miter

H-0 DR* Al – wire Al – strip 3-Leg Mitered Cruciform

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

* H-0 DR is a domain-refined, high permeability core steel.

DOE analyzed each of the seven design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 831 designs. 

5.3.18 Design Line 13B Representative Unit

Design line 13B (DL13B) also represents dry-type, three-phase, medium-voltage 

distribution transformers with a ≥96kV BIL, ranging from 225 kVA to 2500 kVA. The 

representative unit selected for this design line is a 2000 kVA transformer. The following are the 

technical specifications that constitute input parameters to the OPS design software:

KVA: 2000 (dry-type)

Phases: Three

Primary: 24940 Volts at 60 Hz (125 kV BIL) 

Secondary: 480Y/277 Volts 

T Rise: 150 ̊C

Ambient: 20 ̊C

Winding Configuration: Lo-Hi

Core: Stacked, cruciform, mitered joint, 3-leg; Wound core - distributed gap, 5-leg

Taps: Four 21⁄2 percent, two above and two below the nominal 

Impedance Range: 5.0–8.0 percent

For DL13B, DOE selected eight design option combinations of core steel and winding 

material, based on input from manufacturers and other technical experts. With the exception of 

the max-tech/high-efficiency designs, DOE selected these design option combinations to 

represent the most common construction practice for the representative unit. 
5-37

Table 5.3.20 Design Option Combinations for the Design Line 13B Representative Unit 

Core Material High-Voltage 

Conductor

Low-Voltage 

Conductor Core Design Type 

M5 Al – wire Al – strip 3-Leg Mitered Cruciform

M4 Al – wire Al – strip 3-Leg Mitered Cruciform

M4 Al – wire Al – strip 3-Leg Step-Lap Miter

M3 Al – wire Al – strip 3-Leg Mitered Cruciform

M3 Al – wire Al – strip 3-Leg Step-Lap Miter

H-0 DR* Al – wire Al – strip 3-Leg Mitered Cruciform

H-0 DR Al – wire Al – strip 3-Leg Step-Lap Miter

SA1 (Amorphous) Al – wire Al – strip 5-Leg DG Core

* H-0 DR is a domain-refined, high permeability core steel.

DOE analyzed each of the eight design option combinations using the matrix of A and B 

values described in Table 5.8 and Table 5.9, creating 1,881 designs. 

5.3.19 Newly Optimized Designs and Previously Optimized Designs

For the NOPR, DOE utilized a combination of newly optimized design runs and designs 

that were optimized during the preliminary analysis for distribution transformers. For each 

design option combination chosen, DOE generates designs based on 518 A and B factor 

combinations. These A and B factor combinations cover the spectrum of typical load loss and 

no-load loss valuations, generating a unique design across a range of efficiencies.

DOE understands that typically a design would be optimized based on the current material 

prices. Optimizing a design based on historical material prices may result in a differently 

optimized design, such as a design that utilizes relatively more conductor than core. However, 

DOE believes that it adequately covered the spectrum of possible designs for each design option 

combination used in the preliminary analysis based on the large sample of A and B factor 

combinations considered for each design option combination. As such, DOE believes that these 

designs are still valid when updated material prices are applied to them.

DOE updated the cost of these previous design runs by applying updated prices to the 

design’s bill of materials. Effectively, DOE calculated the present cost of developing the same 

design that was used in the preliminary analysis. DOE also updated labor prices and applied the 

markups consistently with any newly optimized designs generated for the NOPR analysis. DOE 

carried over the NOPR analysis to the Final Rule unchanged.

5.4 MATERIAL AND LABOR INPUTS

DOE uses a standard method of cost accounting with minor changes to determine the 

costs associated with manufacturing. This methodology is illustrated in 5.4, where production 

costs and non-production costs are combined to determine the manufacturer’s selling price of the 

equipment. 
5-38

Figure 5.4.1 Method of Cost Account for Distribution Transformers Rulemaking

Together, the full production cost and the non-production cost equal the manufacturer’s 

selling price of the equipment. Full production cost is a combination of direct labor, direct 

materials, and overhead. The overhead contributing to full production cost includes indirect 

labor, indirect material, maintenance, depreciation, taxes, and insurance related to company 

assets. Non-production cost includes the cost of selling, general and administrative items (market 

research, advertising, sales representatives, logistics), research and development (R&D), interest 

payments, warranty and risk provisions, shipping, and profit factor. Because profit factor is 

included in the non-production cost, the sum of production and non-production costs is an 

estimate of the manufacturer’s selling price.

DOE used several estimates of the costs listed in Figure 5.4.1 from DOE’s previous 

rulemaking on distribution transformers, published in October 2007. The estimates from this 

rulemaking relied on U.S. Industry Census Data Reports, manufacturer interviews, and Securities 

and Exchange Commission (SEC) 10-K reports for several manufacturers. It then refined these 

estimates through meetings and dialogue with transformer manufacturers in 2010. The following 

markups resulted: 

• Scrap and handling factor: 2.5 percent markup. This markup applies to variable materials 

(e.g., core steel, windings, insulation). It accounts for the handling of material (loading 

into assembly or winding equipment) and the scrap material that cannot be used in the 

production of a finished transformer (e.g., lengths of wire too short to wind, trimmed core 

steel).

• Amorphous scrap factor: 1.5 percent markup. This markup accounts for breakage of 

prefabricated amorphous cores and any scrap associated with assembling the windings on 

the core. Since amorphous cores are assumed to be prefabricated, the regular scrap and 

handling factor is reduced. 

Full Cost of Product

Full Production Cost

Direct 

Labor

Direct 

Material Overhead

Indirect Labor

Indirect 

Material

Maintenance

Depreciation

Taxes

Insurance 

related to 

assets

Non-Production Cost

Selling, 

General 

& Admin.

Mkt. research

Advertising

Point of Sale 

promotion

Cust. service

Sales person 

salary + travel 

+ commission

Logistics

R & D

Costs 

associated with 

efforts to find 

new or 

improved 

products or 

production 

processes

Code 

Compliance

Modernization

Interest

Costs of 

borrowing 

funds 

Warranty 

& Risk Shipping

Costs of 

freight for 

final delivery

Profit
5-39

• Mitered scrap factor: 4.0 percent markup. An additional scrap markup applies to steel 

used in full-mitered cores. This markup represents material cut from the notch in the 

yoke.

• Factory overhead: 12.5 percent markup. Factory overhead includes all the indirect costs 

associated with production, indirect materials and energy use (e.g., annealing furnace), 

taxes, and insurance. DOE only applied factory overhead to the direct material production 

costs.

• Shipping: $0.28 per pound for each transformer. The shipping costs include the freight 

from a manufacturer’s facility to the customer. This shipping cost does not include any 

freight charges for the customer to subsequently move the transformer to its end-use 

location. DOE applied the shipping charge prior to applying the profit markup based on 

feedback from manufacturer interviews in 2011. 

• Non-production: 25 percent markup. This markup reflects costs including selling, 

general and administrative, R&D, interest payments, warranty and risk provisions, and 

profit factor. DOE applied the non-production markup to the sum of direct material, 

direct labor, and factory overhead. 

The following example shows how DOE applied the markups to the materials, and how it 

determined the manufacturer selling price. Consider a 300kVA 45kV BIL three-phase, dry-type 

transformer designed with a $1.50 A and a $0.30 B. This design has $4,795 of materials, 

including M6 core steel, copper primary and secondary windings, and all the transformer 

hardware. There are approximately 27 hours of labor involved in manufacturing this design, 

resulting in a labor cost of $1,361. The factory overhead on this design is $599, as it is only 

applied to the material cost (i.e., 12.5 percent of $4,795). The shipping cost is $504, based on a 

weight of 1,792 pounds. The non-production cost is $1,706, since the 25 percent is applied to the 

material, labor, factory overhead, and shipping costs (i.e., 25 percent of $4,795 + $1,391 + $599 

+ $504). Thus, in total, DOE estimates this 300kVA three-phase transformer to have a 

manufacturer selling price of $9,033. 

5.4.1 Material Prices

DOE used prices of core steel, conductor, mineral oil, insulation, and other materials as an 

input to the transformer design software used for the engineering analysis. As the price of one 

material increases or decreases relative to the other materials, the software will modify its design 

and increase or decrease the amount of that material while balancing other design parameters, 

creating a cost-optimized transformer. Material pricing is also critical because the manufacturer’s 

selling prices calculated in the engineering analysis are based on a bill of materials that includes, 

for example, specifications for pounds of steel, pounds of conductor, gallons of mineral oil, and 

tank dimensions. Therefore, as material prices increase, so will the manufacturer’s selling price. 
5-40

Furthermore, as discussed in chapter 3, energy-efficient transformers tend to incorporate more 

materials (e.g., pounds of core steel, pounds of conductor), making the impact of more expensive 

materials even more significant at higher efficiencies.

DOE contracted OPS to develop material price estimates for the engineering analysis. 

OPS used data from their own records as well as data provided by transformer manufacturers and 

material suppliers and wholesalers. Although not all transformer manufacturers pay the same 

amount per pound for electrical-grade steels, due to varied contract negotiations, these prices are 

intended to be representative of a standard quantity order for a medium- to large-scale U.S. 

transformer manufacturer.

DOE supplemented that price data by aggregating information obtained during interviews 

with manufacturers of distribution transformers. After the preliminary analysis, DOE received 

feedback that volatility in the conductor commodities markets made it difficult to consider any 

price accurate for any length of time because the rate of price movement was high relative to 

typical purchase contract terms.

DOE received feedback that it could develop conductor prices by establishing a 

processing “adder,” or a cost required to form the finished good from the underlying commodity. 

The adder would change much more slowly than the price of the processed commodity and 

would allow DOE to derive conductor prices by summing the processing adder and the price of 

the underlying commodity obtained from exchanges such as COMEX and London Metal 

Exchange (LME). DOE intends to continue deriving conductor prices in this manner in future 

updates to the analysis. For the NOPR and Final Rule, DOE used this approach. 

Prior to the NOPR, DOE conducted the engineering analysis using material prices over a 

five-year time period from 2006-2010, all in constant 2010$. Using the material prices from this 

time period, DOE considered a current (2010) material price, a minimum price (based on 2006 

prices), and a maximum price (based on 2008 prices) for its analysis. This was done to account 

for variation in pricing for the different materials, which could have a significant impact on the 

total cost of the distribution transformer. All transformer designs that were newly optimized used 

the current 2010 material price, which DOE used as one of its reference cases. The maximum 

and minimum prices were then applied to these same designs to generate a manufacturer selling 

price for each of those scenarios. The results of the current 2010 material prices are presented 

here in chapter 5, while the results of the minimum and maximum material prices are presented 

in Appendix 5C. 

For the NOPR, based on discussions in negotiations, DOE decided to implement a second 

reference case, using 2011 material prices. The 2011 steel prices were developed using 

manufacturer feedback gathered in interviews and during the negotiation process. Relative to the 

2010 prices, 2011 steel prices were lower, particularly for M2 grade steel and worse. Results 

from the 2011 price trend are also presented here in chapter 5. 

DOE noted that the price of the most critical material input to a distribution transformer, 

electrical core steel, had varied significantly for some M-grades over the five-year time horizon. 

For this reason, DOE researched the grain-oriented electrical steel market to gain a better 
5-41

understanding of the main players and some of the factors influencing these price fluctuations 

(see Appendix 3A). 

In the LCC analysis (chapter 8), DOE presents results on its sensitivity analyses 

conducted on various LCC inputs, which included material prices. In chapter 8, the 2008 + 25% 

material price scenario is referred to as the “high” price scenario, the 2010 and 2011 price 

scenarios are called “medium” price scenarios, and the 2006 – 25% material price scenario is 

referred to as the “low” price scenario. DOE originally chose to utilize the current 2010 material 

price in the reference case after receiving feedback from transformer manufacturers and suppliers 

of core steel indicating that current prices would be a better price indicator than a five-year 

average price. In the NOPR analysis, DOE added 2011 prices as a second reference case based 

on negotiator feedback. These material prices can be found in the material price tables presented 

in this section. The resulting manufacturer selling prices are provided in the LCC and 

engineering spreadsheets.

5.4.2 Material Price Inputs to the Design Software – Liquid-Immersed

Table 5.26 presents the material prices for a typical manufacturer of liquid-immersed

transformers. All designs were optimized on a set of 2010 prices. After optimization, slight 

adjustments were made to 2010 prices, and a 2011 price scenario was developed. Both of these 

reference scenarios are presented along with a minimum and maximum price scenario. 
5-42

Table 5.26 Typical Manufacturer’s Material Prices for Liquid-Immersed Design Lines

Item and Description

2010 

Price

2011 

Price

Min 

Price 

(2006 - 

25%)

Max 

Price 

(2008 + 

25%)

M6 core steel 1.33 1.04 0.94 2.19 

M5 core steel 1.38 1.10 0.99 2.24 

M4 core steel 1.45 1.20 1.03 2.30 

M3 core steel 1.88 1.30 1.06 2.60 

M3 Lite Carlite core steel 1.95 1.95 1.47 2.44 

M2 core steel 2.00 1.40 1.32 2.79 

M2 Lite Carlite core steel 2.10 2.10 1.58 2.63 

ZDMH (mechanically-scribed core steel) 2.05 1.90 1.41 3.22 

SA1 (amorphous) - finished core, volume 

production 2.38 2.20 1.72 3.64 

Copper wire, formvar, round #10-20 4.87 4.87 3.33 5.97 

Copper wire, enameled, round #7-10 4.84 4.84 3.31 5.93 

Copper wire, enameled, rectangular sizes 4.97 4.97 3.41 6.09 

Aluminum wire, formvar, round #9-17 3.07 3.07 2.30 3.91 

Aluminum wire, formvar, round #7-10 2.57 2.57 1.93 3.28 

Copper strip, thickness range 0.02-0.045 4.97 4.97 3.41 6.09 

Copper strip, thickness range 0.030-0.060 4.97 4.97 3.41 6.09 

Aluminum strip, thickness range 0.02-

0.045 2.08 2.08 1.56 2.67 

Aluminum strip, thickness range 0.045-

0.080 2.08 2.08 1.56 2.67 

Kraft insulating paper with diamond 

adhesive 1.52 1.52 1.17 1.93 

Mineral oil 3.35 3.35 1.94 3.84 

Tank Steel 0.38 0.38 0.32 0.60 

The price used for a prefabricated amorphous core is based on prices of finished cores 

from North American manufacturers. In the previous rulemaking for distribution transformers, 

DOE analyzed the cost of importing finished cores from overseas. Since that time, several North 

American core manufacturers have begun producing amorphous cores. For the preliminary 

analysis and NOPR, DOE considered the price of a prefabricated amorphous core bought from a 

North American core manufacturer.

In addition to the aforementioned materials that vary during the design optimization 

process (e.g., core steel, windings, insulation), there are other direct materials inputs that are 

fixed costs and generally do not influence the design or vary with efficiency rating. These 

include direct materials, such as the high- and low-voltage bushings and the core clamps. DOE 

also prepared estimates of the tank fabrication cost, based on the optimized transformer design 

(the software considers this variable) and the labor necessary to build the tank. Table 5.27

summarizes all the estimated fixed material costs and estimates of the tank costs for each of the 

five liquid-immersed design lines.
5-43

For DL1, a 50kVA single-phase pad-mounted unit, the high-voltage bushings are two 

universal bushing wells, 15 kV, 95 BIL, 14400V, costing $14 each. The low-voltage bushings 

are three threaded copper studs, 240/120V, 50 kVA, costing $30 for the set. Internal hardware 

costs include a core clamp, nameplate, and other miscellaneous hardware costing $41.65. The 

finished tank size (and associated cost) varies by design, but the average cost is approximately 

$143. 

For DL2, a 25kVA single-phase pole-mounted unit, the high-voltage terminal is a single, 

wet-process porcelain bushing assembly, 15 kV, 125 BIL, costing $6. The low-voltage terminals 

are three molded polymer bushings, 120/240V, 25 kVA, costing $8 for the set. Internal hardware 

costs include a core clamp, nameplate, and other miscellaneous hardware, costing $19.15. The 

finished tank sizes (height and diameter) vary by design, but the average cost is approximately 

$73. 

For DL3, a 500kVA single-phase unit, the high-voltage connector is a single, wet-process 

porcelain bushing, 25 kV, 125 BIL, costing $6. The low-voltage bushings are two four-hole “J” 

Spade 500kVA, 277V, costing $60 for the set. The internal hardware includes a core clamp 

($30), nameplate ($0.65), and miscellaneous hardware ($20), totaling $50.65. The design 

software optimized the tank cost with each design, including radiators (external cooling) for this 

kVA rating. The resultant finished round tank has a diameter of 33" to 52", with an average cost 

of approximately $629 (including radiators). 

For DL4, a 150kVA three-phase, pad-mounted unit, the high-voltage bushings are three 

externally clamped, universal high-voltage bushing wells, 8.3/14.4 kV, 95 BIL, costing $7 each. 

The low-voltage bushings are three copper studs at $8 each. The internal hardware includes core 

clamps ($30), nameplate ($0.65), and miscellaneous hardware ($45), totaling $75.65. The 

optimized finished tank sizes measure 50 inches high and vary in width and depth. The finished 

rectangular, welded tank has an average cost of approximately $389. 

For DL5, a 1500kVA three-phase, pad-mounted unit, the high-voltage bushings are three 

externally clamped, universal high-voltage bushing wells, 15.2/26.3 kV, 125kV BIL, costing $20 

each. The low-voltage bushings are four externally clamped bushings, each having six-hole 

spade, costing $160 for the set. The internal hardware includes core clamps ($60), nameplate 

($0.65), and miscellaneous hardware ($45), totaling $105.65. The optimized finished tank sizes 

measure 70 inches high and vary in width and depth. The finished rectangular, welded tank, 

including radiators as specified by the design software, has an average cost of approximately 

$1,016. 

Table 5.27 Summary Table of Fixed Material Costs for Liquid-Immersed Units

Item DL1 DL2 DL3 DL4 DL5

High voltage bushings $28 $6 $6 $21 $60

Low voltage bushings $30 $8 $60 $24 $160

Core clamp, nameplate, and misc. 

hardware

$41.65 $19.15 $50.65 $75.65 $105.65

Transformer tank average cost* ~$143 ~$73 ~$629 ~$389 ~$1,016
5-44

* Transformer tank steel is used in the design optimization software and varies with the 

efficiency (and size) of each design. DL3 and DL5 include calculated costs of radiators, which 

are scaled for each design based on the required cooling surface area.

5.4.3 Material Price Inputs to the Design Software – Dry-Type 

Table 5.28 presents the material prices for a typical dry-type transformer manufacturer 

indicating the reference cases (2010 and 2011 prices), minimum (2006), and maximum (2008) 

prices (all in constant 2010$). 

Table 5.28 Manufacturer’s Material Prices for Dry-Type Design Lines

Item and Description

2010 

Price

2011 

Price

Min 

Price 

(2006 - 

25%)

Max 

Price 

(2008 + 

25%)

M36 core steel (26 gauge) 0.60 0.66 0.46 0.84 

M19 core steel (26 gauge) 0.83 0.91 0.56 1.19 

M12 core steel 0.95 0.78 0.85 1.60 

M6 core steel 1.33 1.04 0.94 2.19 

M5 core steel 1.38 1.10 0.99 2.24 

M4 core steel 1.45 1.20 1.03 2.30 

M3 core steel 1.88 1.30 1.06 2.60 

M2 core steel 2.00 1.40 1.32 2.79 

H-0 DR core steel (laser-scribed) 2.06 1.70 1.41 3.23 

SA1 (amorphous) - finished core, volume 

production 2.38 2.20 1.72 3.64 

Copper wire, rectangular 0.1 x 0.2, Nomex 

wrapped 4.52 4.52 3.07 5.53 

Aluminum wire, rectangular 0.1 x 0.2, 

Nomex wrapped 2.97 2.97 2.23 3.78 

Copper strip, thickness range 0.02-0.045 4.97 4.97 3.41 6.09 

Aluminum strip, thickness range 0.02-0.045 2.08 2.08 1.56 2.67 

Nomex insulation (per pound) 24.50 24.50 13.72 29.03 

Cequin insulation (per pound) 5.53 5.53 3.84 6.09 

Impregnation (per gallon) 22.55 22.55 17.16 27.31 

Winding Combs (per pound) 12.34 12.34 6.08 15.41 

Enclosure Steel (per pound) 0.38 0.38 0.32 0.60 

As stated in section 5.3, the OPS software does not take into account retooling costs 

associated with changing production designs. Therefore, to partially capture these differential 

costs in the design lines that had both buttlap and mitered designs, DOE used adders in DL7 and 

DL8. The adders specified an extra 10 cents per pound of core steel for full-mitered designs. 

More detailed costing of the retooling costs for mitering equipment will be covered in the 

manufacturer impact analysis (MIA). (See chapter 12.) 
5-45

Similar to the liquid-immersed designs, there are fixed (and some partially variable) 

hardware costs associated with dry-type distribution transformers. These are discussed 

individually and then summarized in Table 5.29. 

For DL6, a 25 kVA single-phase, low-voltage, dry-type transformer, the low-voltage and 

high-voltage terminal set costs $4. The mounting frame that attaches the core/coil assembly to 

the transformer enclosure costs approximately $9.25. The fiberglass dog-bone duct-spacers used 

for this design line cost $0.24 per foot. DOE estimated the miscellaneous hardware costs at 

$4.50. The ventilated enclosure – a 16-gauge steel enclosure, base, and mounting feet – varies 

with the size of the core-coil assembly for the 25kVA unit, and costs approximately $50. 

For DL7, a 75 kVA three-phase, low-voltage, dry-type transformer, the fixed hardware 

costs are $9 per phase for the high-voltage terminal board with connection points. DOE 

estimated the secondary (low-voltage) bus-bar to be seven feet at $1.50 per foot, or $10.50. The 

mounting frame that attaches the core/coil assembly to the transformer enclosure costs 

approximately $19. The fiberglass dog-bone duct-spacers used for this design line cost $0.32 per 

foot. DOE estimated the miscellaneous hardware costs at $7. The ventilated enclosure – a 14-

gauge steel enclosure, base, and mounting feet – varies with the size of the core-coil assembly 

for the 75kVA unit, and costs approximately $90. 

For DL8, a 300 kVA three-phase, low-voltage, dry-type transformer, the high-voltage 

terminal board costs $27. DOE estimated the secondary (low-voltage) bus-bar to be nine feet at 

$2.50 per foot, or $22.50. The mounting frame that attaches the core/coil assembly to the 

transformer enclosure costs approximately $36. The fiberglass dog-bone duct-spacers used for 

this design line cost $0.42 per foot. DOE estimated the miscellaneous hardware costs at $12. The 

ventilated enclosure – a 14-gauge steel enclosure, base, and mounting feet – varies with the size 

of the core-coil assembly for the 300kVA unit, and costs approximately $100. 

For DL9, a 300 kVA three-phase, medium-voltage, dry-type transformer at 45 kV BIL, 

the low-voltage and high-voltage terminal set costs $75. DOE estimated the secondary (lowvoltage) bus-bar to be eight feet at $10 per foot, or $80. The mounting frame that attaches the 

core/coil assembly to the transformer enclosure costs approximately $36. The fiberglass dogbone duct-spacers used for this design line cost $0.42 per foot. DOE estimated the miscellaneous 

hardware costs at $25. The ventilated enclosure – a 14-gauge steel enclosure, base, and mounting 

feet – varies with the size of the core-coil assembly for the 300 kVA unit, and costs 

approximately $135. 

 

For DL10, a 1500 kVA three-phase, medium-voltage, dry-type transformer at 45 kV BIL, 

the low-voltage and high-voltage terminal set costs $120. DOE estimated the low-voltage busbar to be 14 feet at $10 per foot, or $140. The mounting frame that attaches the core/coil 

assembly to the transformer enclosure costs approximately $120. DOE accounted for the cost of 

additional bracing in the amorphous design since the amorphous design uses a wound core rather 

than a round, cruciform core like the other designs. This extra bracing is needed for the 

amorphous design due to the size of DL10 (1500 kVA). The weight of the added bracing was 

calculated as 7 percent of the core and coil weight, and was multiplied by the price for enclosure 

steel to derive a cost. The bracing weighs 600 pounds on average and costs approximately $230. 
5-46

The fiberglass dog-bone duct-spacers used for this design line cost $0.52 per foot. DOE 

estimated the miscellaneous hardware costs at $42. The ventilated enclosure – a 14-gauge steel 

enclosure, base, and mounting feet – varies with the size of the core-coil assembly for the 1500 

kVA unit, and costs approximately $400. 

For DL11, a 300 kVA three-phase, medium-voltage, dry-type at 95 kV BIL, the lowvoltage and high-voltage terminal set costs $100. The high-voltage terminal boards cost $27. 

DOE estimated the low-voltage bus-bar is estimated to be 10 feet at $8 per foot, or $80. The 

mounting frame that attaches the core/coil assembly to the transformer enclosure costs $42. The 

fiberglass dog-bone duct-spacers used for this design line cost $0.42 per foot. DOE estimated the 

miscellaneous hardware costs at $32. The ventilated enclosure – a 14-gauge steel enclosure, 

base, and mounting feet – varies with the size of the core-coil assembly for the 300 kVA unit, 

and costs approximately $200. 

For DL12, a 1500 kVA three-phase, medium-voltage, dry-type at 95 kV BIL, the lowvoltage and high-voltage terminal set costs $135. The high-voltage terminal boards cost $27. 

DOE estimated the low-voltage bus-bar is estimated to be 16 feet at $12 per foot, or $192. The 

mounting frame that attaches the core/coil assembly to the transformer enclosure costs $125. 

DOE accounted for the cost of additional bracing in the amorphous design since the amorphous 

design uses a wound core rather than a round, cruciform core like the other designs. This extra 

bracing is needed for the amorphous design due to the size of DL12 (1500 kVA). The weight of 

the added bracing was calculated as 7 percent of the core and coil weight, and was multiplied by 

the price for enclosure steel to derive a cost. The added bracing weighs 700 pounds on average 

and costs approximately $270. The fiberglass dog-bone duct-spacers used for this design line 

cost $0.56 per foot. DOE estimated the miscellaneous hardware costs at $54. The ventilated 

enclosure – a 14-gauge steel enclosure, base, and mounting feet – varies with the size of the 

core-coil assembly for the 1500 kVA unit, and costs approximately $450. 

For DL13A, a 300 kVA three-phase, medium-voltage, dry-type at 125 kV BIL, the lowvoltage and high-voltage terminal set costs $115. The high-voltage terminal boards cost $27. 

DOE estimated the low-voltage bus-bar is estimated to be 10 feet at $10 per foot, or $100. The 

mounting frame that attaches the core/coil assembly to the transformer enclosure costs $50. The 

fiberglass dog-bone duct-spacers used for this design line cost $0.42 per foot. DOE estimated the 

miscellaneous hardware costs at $36. The ventilated enclosure – a 14-gauge steel enclosure, 

base, and mounting feet – varies with the size of the core-coil assembly for the 300 kVA unit, 

and costs approximately $200. 

For DL13B, a 2000 kVA three-phase, medium-voltage, dry-type at 125 kV BIL, the lowvoltage and high-voltage terminal set costs $150. The high-voltage terminal boards cost $27. 

DOE estimated the low-voltage bus-bar is estimated to be 18 feet at $15 per foot, or $270. The 

mounting frame that attaches the core/coil assembly to the transformer enclosure costs $175. 

DOE accounted for the cost of additional bracing in the amorphous design since the amorphous 

design uses a wound core rather than a round, cruciform core like the other designs. This extra 

bracing is needed for the amorphous design due to the size of DL13B (2000 kVA). The weight of 

the added bracing was calculated as 7 percent of the core and coil weight, and was multiplied by 

the price for enclosure steel to derive a cost. The added bracing weighs 850 pounds on average 
5-47

and costs approximately $330. The fiberglass dog-bone duct-spacers used for this design line 

cost $0.60 per foot. DOE estimated the miscellaneous hardware costs at $60. The ventilated 

enclosure – a 14-gauge steel enclosure, base, and mounting feet – varies with the size of the 

core-coil assembly for the 300 kVA unit, and costs approximately $450. 
5-48

Table 5.29 Summary Table of Fixed Material Costs for Dry-Type Units

Item DL6 DL7 DL8 DL9 DL10 DL11 DL12 DL13

A 

DL13

B 

LV and HV 

terminals (set) $4 n/a n/a $75 $120 $100 $135 $115 $150 

HV terminal 

board(s) n/a $27 $27 $27 $27 $27 $27 $27 $27 

LV bus-bar n/a $10.

50

$22.5

0 $80 $140 $80 $192 $100 $270 

Core/coil mounting 

frame

$9.2

5 $19 $36 $36 $120 $42 $125 $50 $175 

Additional Bracing n/a n/a n/a n/a ~$23

0 n/a ~$270 n/a ~$330 

Nameplate $0.6

5

$0.6

5 $0.65 $0.65 $0.65 $0.65 $0.65 $0.65 $0.65 

Dog-bone duct 

spacer (ft.)

$0.2

4

$0.3

2 $0.42 $0.42 $0.52 $0.42 $0.56 $0.42 $0.60 

Winding combs (lb.) n/a n/a n/a n/a n/a $10.0

0

$10.0

0 $10.00 $10.00 

Misc. hardware $4.5

0 $7 $12 $25 $42 $32 $54 $36 $60 

Enclosure (12, 14 

gauge)

~$5

0

~$9

0

~$10

0

~$13

5

~$40

0

~$20

0

~$45

0 ~$200 ~$450 

LV = low voltage 

HV = high voltage 

5.4.4 Labor Costs 

Labor costs are a critical aspect of the cost of manufacturing a distribution transformer. 

DOE used the same hourly labor cost for both liquid and dry-type distribution transformers. It 

developed the hourly cost of labor using a similar approach to the development of the cost of 

materials; however, it used different markups. DOE developed the markups shown in Table 5.30

after reviewing publicly available information, speaking with transformer manufacturers during 

2011, and consulting with industry experts familiar with transformer manufacturing in the U.S.
5-49

Table 5.30 Labor Markups for Liquid-Immersed and Dry-Type Manufacturers

Item description Markup percentage Rate per hour

Labor cost per hour* $ 16.80 

Indirect Production** 33% $ 22.35 

Overhead*** 30% $ 29.05 

Fringe† 24% $ 36.03 

Assembly Labor Up-time†† 43% $ 51.52

Fully-Burdened Cost of 

Labor 25% $ 64.40 

* Cost per hour is from U.S. Census Bureau, 2007 Economic Census - Detailed Statistics, 

published October 2009. Data for NAICS code 3353111 "Power and distribution transformers, 

except parts" Production workers hours and wages. 

** Indirect production labor (e.g., production managers, quality control) as a percent of 

direct labor on a cost basis. Navigant Consulting, Inc. (NCI) estimate. 

*** Overhead includes commissions, dismissal pay, bonuses, vacation, sick leave, and 

social security contributions. NCI estimate.

† Fringe includes pension contributions, group insurance premiums, workers 

compensation. Source: U.S. Census Bureau, 2007 Economic Census - Detailed Statistics, 

published October 2009. Data for NAICS code 3353111 "Power and distribution transformers, 

except parts" Total fringe benefits as a percent of total compensation for all employees (not just 

production workers). 

†† Assembly labor up-time is a factor applied to account for the time that workers are not 

assembling units and/or reworking unsatisfactory units. The markup of 43 percent represents a 

70 percent utilization (multiplying by 100/70). NCI estimate.

5.4.4.1 Liquid-Immersed Labor Hours

There are several labor steps involved in manufacturing a liquid-immersed transformer. 

DOE prepared estimates of the amount of labor involved, some varying with the transformer 

design and others fixed on a per-unit basis. These steps are described below, and the amount of 

time dedicated to each is given in Table 5.31. 

• Cutting, Forming, and Annealing – This task involves cutting the core steel to lengths on 

a distributed-gap core cutting machine, forming the resulting “donut” of core steel into a 

rectangular shape in a hydraulic press, and then annealing the core in a high temperature 

annealing furnace. DOE calculated the labor involved in these activities based on the 

weight of core (pounds) multiplied by a constant, which varies with the lamination 

thickness of the core steel. For DL1, DL2, and DL4, on M6 designs the constant is 0.08, 

M5 is 0.09, M4 is 0.10, M3 (with or without Lite Carlite) and ZDMH are 0.125, and M2

(with or without Lite Carlite) is 0.16. For DL3 and DL5, on M6 designs the constant is 

0.05, M5 is 0.06, M4 is 0.07, M3 (with or without Lite Carlite) and ZDMH are 0.09, and 

M2 (with or without Lite Carlite) is 0.11. For the prefabricated core — SA1 (amorphous 

material)—DOE set the labor for cutting, forming, and annealing to zero. 
5-50

• Primary Winding – This task entails winding the primary conductor of the transformer. It 

includes set-up time as well as winding time. The labor hours vary with the number of 

turns (per phase) for the primary winding. For DL1, DL2, and DL4, the winding time is 

0.0001 hours per turn. For these smaller kVA ratings (and smaller cores), this rate is very 

low because some of the larger, liquid-immersed manufacturers wind multiple coils 

simultaneously on the same winding machine. This manufacturing approach improves 

throughput and productivity at the facility. The rate of 0.0001 hours per turn equates to 

approximately one-third of a second per turn. On DL3 and DL5, due to the larger coil 

size associated with these units, the winding time is 0.002 hours per turn (approximately 

7.2 seconds per turn). 

• Secondary Winding – This task involves winding the secondary conductor of the 

transformer. It includes set-up time as well as winding time. On a distribution (stepdown) transformer, the number of secondary turns is always less than the primary. For 

the liquid-immersed units, which are taking a relatively high primary voltage and 

dropping to below 600V, the turns ratio can be as large as 100:1. For this reason, the 

hours per turn of the secondary are considerably higher than the primary, because there 

are fewer turns over which to amortize the set-up time as well as a slower winding rate 

for the secondary, which has larger cross-sectional area than the primary. For DL1, DL2, 

and DL4, the hours per turn of the secondary are 0.015 (54 seconds per turn); for DL3 

and DL5, the hours per turn are 0.02 (72 seconds per turn). 

• Lead Dressing – Once a wound coil is taken off the winding machine, work must be 

performed on the leads to prepare them for the next manufacturing step. Enamel is 

removed to enable good electrical connection and insulating tubing is slipped over the 

cable. This is a fixed amount of labor, and does not vary with efficiency or design. Lead 

dressing time ranges from 0.1 to 1 hour. 

• Coil Varnishing and Baking – Once they are complete, the coils are vacuum-dipped in 

varnish and baked in an oven to cure the varnish and enhance the integrity of the coil. 

This task varies slightly with kVA rating, but does not vary with efficiency. The 

estimated times range from 0.07 to 0.25 hours. 

• Core Assembly (“Lacing”) – This task involves assembling and banding the annealed 

wound core laminations around varnished windings. The annealed bundle of core steel is 

disassembled from the inside out by grabbing approximately 1/4 inch bundles, then 

reassembling the core steel around the coils. Once all the laminations are reassembled, 

the core material is clamped to maintain the structure. The activity involves feeding a 

banding strip around the core material and using a locking clamp to compress and contain 

the core material. The labor rate varies with stack height and lamination thickness for 

each design. The average time for core assembly ranges from approximately 0.2 hours 

(for DL2) to 4.9 hours (for DL5). 

• Tanking and Impregnating – This task involves inserting and fastening the core/coil 

assembly into the tank. Then, a vacuum is pulled and oil is introduced to the tank. On 
5-51

round tanks, the vacuum and oil step is done through a lid attached to the top of the unit. 

On the rectangular and pad-mounted tanks, the vacuum is pulled in a chamber, which 

takes a little longer per unit. Finally, tap changers and bushings are mounted, and bolted 

connections made. The time for this activity does not vary with design or efficiency, but 

it does vary by kVA rating and tank shape. The estimates of labor time for the five liquidimmersed design lines range from 0.1 to 1.8 hours. 

• Inspection – This activity involves verifying that the transformer is assembled properly 

and is up to a manufacturer's quality specification. This task includes inspecting the lead 

dressing, lead tie-up, and other quality certification specifications. The time for this 

activity does not vary with design or efficiency, but it does vary by kVA rating, from 

0.05 hours for the smallest units to 0.20 hours for the largest units. 

• Preliminary Test – This step involves conducting a test to ensure that the core/coil meets 

the specified turns ratio, polarity, core loss, etc. . The time for this activity does not vary 

with design or efficiency, but it does vary by kVA rating from 0.05 to 0.15 hours. 

• Final Test – This activity involves testing of the final, assembled unit, with the core/coil 

assembly immersed in oil. This test verifies that the unit meets the guaranteed values, 

including core and coil losses, impedance, and dielectric tests. The time for this activity 

does not vary with design or efficiency, but it does vary by kVA rating from 0.1 to 0.25 

hours. 

• Pallet Loading – This activity involves preparing the transformer for shipping to the 

customer. This includes loading the finished transformer onto a pallet, banding the 

transformer to the pallet, wrapping, and all other necessary steps for shipping. The time 

for this activity does not vary with design or efficiency, but it does vary by kVA rating 

from 0.15 hours for the smallest units to 3 hours for the largest units. 

• Marking and Miscellaneous – This task involves preparing any extra markings around the 

bushings or on the surface of the transformer and other miscellaneous labor associated 

with preparing the finished transformer for the customer. The time for this activity does 

not vary with design or efficiency, but it does vary by kVA rating from 0.08 to 0.35 

hours. 

Table 5.31 summarizes the estimates of labor time that DOE used for the five liquidimmersed units.
5-52

Table 5.31 Summary of Labor Times for Liquid-Immersed Units

Labor Activity DL1

hrs.

DL2

hrs.

DL3

hrs.

DL4

hrs.

DL5

hrs.

Cutting, Forming, & 

Annealing ~1.00 ~0.75 ~4.00 ~3.00 ~8.50 

Primary Winding (hrs/turn) 0.0001 0.0001 0.002 0.0001 0.002

Secondary Winding 

(hrs/turn) 0.015 0.015 0.020 0.015 0.020 

Lead Dressing 0.50 0.1 0.35 0.75 1.00

Baking Coils 0.10 0.07 0.15 0.17 0.25

Core Assembly ~0.40 ~0.20 ~1.20 ~1.00 ~4.90

Tanking and Impregnating 0.30 0.11 0.65 0.50 1.80

Inspection 0.10 0.05 0.10 0.15 0.20

Preliminary Test 0.10 0.05 0.10 0.10 0.15

Final Test 0.15 0.1 0.15 0.20 0.25

Pallet Loading 0.5 0.15 0.75 0.50 3.00

Marking and Misc. 0.35 0.08 0.35 0.35 0.75

5.4.4.2 Dry-Type Labor Hours 

Likewise, there are several labor steps involved in manufacturing a dry-type transformer. 

For the preliminary analysis, DOE prepared estimates of the amount of labor involved, some 

varying with the transformer design and others fixed on a per-unit basis. . For the NOPR analysis

DOE modified its approach based on comments in negotiations, and calculated a core labor 

estimate based on the weight of the transformer. . In addition, DOE prepared a constant labor 

hour value for all other labor steps involved. . This value was held constant for all designs within 

a design line and was prepared based on data and feedback from manufacturers in negotiations. 

These steps are described below. 

• Core Stacking – This task involves stacking (assembling) the cut steel laminations into a 

distribution transformer core. The amount of labor for this task varies by kVA rating, 

stack height, and whether the core is grain-oriented or non-oriented. Thus, the labor for 

core stacking varies with the efficiency of the transformer. 

• Primary Winding – This task encompasses winding the primary conductor of the 

transformer. It includes set-up time as well as winding time. 

• Secondary Winding – This task involves winding the secondary conductor of the 

transformer. It includes set-up time as well as winding time. The winding time of the 

secondary is considerably higher than that of the primary, because there are fewer turns 

over which to amortize the set-up time as well as a slower winding rate for the secondary, 

which has larger cross sectional area. 

• Lead Dressing – Once a wound coil is taken off the winding machine, work must be 

performed on the leads to prepare them for the next manufacturing step. Enamel is 
5-53

removed to enable good electrical connection and insulating tubing is slipped over the 

cable. 

• Assembly – This task involves installing the wound coils onto the partially assembled 

core, and then lacing the top (yoke) laminations to complete the core. It also includes 

setting all the core clamps and completing the core/coil assembly. DOE assumed the 

assembly time varies by kVA rating, but does not vary by design within a kVA rating. 

• Inspection – This activity involves verifying that the transformer is assembled properly 

and is up to a manufacturer's quality specification. It includes inspecting the lead 

dressing, lead tie up, and other quality certification specifications. 

• Preliminary Test – This step involves conducting a test to ensure that the core/coil meets 

the specified turns ratio, polarity, core loss, etc. . 

• Final Test – This activity involves testing the final, assembled unit, with the core/coil 

assembly immersed in oil. This test verifies that the unit meets the guaranteed values, 

including core and coil losses, impedance, and dielectric tests. 

• Enclosure Manufacturing – The labor estimate for this task encompasses all activity 

associated with the cutting, forming, assembly, priming, painting, and preparation of the 

enclosure. 

• Packing – This activity involves preparing the transformer for shipping to the customer. 

This includes loading the finished transformer onto a pallet, banding the transformer to 

the pallet, wrapping, and all other necessary steps for shipping. 

• Marking and Miscellaneous – This task involves preparing any extra markings on the 

terminal board or on the surface of the transformer, and other miscellaneous labor 

associated with preparing the finished transformer for the customer. 

During negotiations, DOE learned that mitering, particularly step-lap mitering, results in 

a higher cost per pound of core steel than butt-lapping. . In response, DOE incorporated a 

processing adder for mitered designs for both low- and medium-voltage dry-type designs. . In the 

medium-voltage case, DOE incorporated a processing adder of 10 cents per pound for step-lap 

mitering. . In the low-voltage case, DOE incorporated a processing cost of 10 cents per pound for 

ordinary mitering and 20 cents per pound for step-lap mitering. . Different processing adders 

were used for low-voltage and medium-voltage to account for the fact that the base case design 

option is different. . In low-voltage units, DOE modeled butt-lapped designs at the baseline 

efficiency level whereas ordinary mitering was modeled at the baseline for medium-voltage. . 

These changes were applied to all dry-type design lines. . 
5-54

5.5 EFFICIENCY LEVELS

DOE analyzed designs over a range of efficiency values for each representative unit. 

Within the efficiency range, DOE developed designs that approximate a continuous function of 

efficiency. However, DOE analyzes the incremental impacts of increased efficiency by 

comparing discrete efficiency benchmarks to a constant baseline efficiency. The baseline 

efficiency evaluated for each representative unit is the existing standard level efficiency for 

distribution transformers established in DOE’s previous rulemaking. The incrementally higher 

efficiency levels are meant to characterize the cost-efficiency relationship above the baseline. 

These efficiency levels are ultimately used by DOE if it decides to amend the existing energy 

conservation standards.

5.5.1 Criteria for Developing Efficiency Levels

After the preliminary analysis, DOE developed efficiency levels for each design line. . To 

accomplish this, DOE first found the range of efficiencies possible for each design line, ranging 

from the baseline to max-tech, and selected ELs as evenly spaced as possible for each design 

line. While selecting the ELs, Doe also considered the efficiency potential of non-amorphous 

core steels and other benchmarks such as NEMA premium levels. As much as possible, DOE 

incorporated these benchmarks into their selections. 

5.5.2 Efficiency Levels Selected

Table 5.32 presents the efficiency levels (ELs) identified for each design line in the 

engineering analysis. 

Table 5.33and Table 5.34 present the incremental MSP for each of the least-costly design 

options at each efficiency level using 2010 prices and 2011 prices respectively. 
5-55

Table 5.32 Summary of Baselines and Efficiency Levels for Distribution Transformer 

Representative Units

DL

Representative Unit

Baseline EL1 EL2 EL3 EL4 EL5 EL6 EL7

Eff.

[%]

Eff.

[%]

Eff.

[%]

Eff.

[%]

Eff.

[%]

Eff.

[%]

Eff.

[%]

Eff.

[%]

1 

50 kVA, 65°C, single-phase, 

60Hz, 14400V primary, 240/120V 

secondary, rectangular tank

99.08 99.16 99.22 99.25 99.31 99.42 99.50 99.50 

2 

25 kVA, 65°C, single-phase, 

60Hz, 14400V primary, 120/240V 

secondary, round tank

98.91 99.00 99.07 99.11 99.18 99.31 99.41 99.47

3 

500 kVA, 65°C, single-phase, 

60Hz, 14400V primary, 277V 

secondary

99.42 99.48 99.51 99.54 99.57 99.61 99.69 99.73

4 

150 kVA, 65°C, three-phase, 

60Hz, 12470Y/7200V primary, 

208Y/120V secondary

99.08 99.16 99.22 99.25 99.31 99.42 99.50 99.60

5 

1500 kVA, 65°C, three-phase, 

60Hz, 24940GrdY/14400V 

primary, 480Y/277V secondary

99.42 99.48 99.51 99.54 99.57 99.61 99.69 99.69

6 

25 kVA, 150°C, single-phase, 

60Hz, 480V primary, 120/240V 

secondary, 10kV BIL

98.00 98.23 98.47 98.60 98.80 98.93 99.17 99.44

7 

75 kVA, 150°C, three-phase, 

60Hz, 480V primary, 208Y/120V 

secondary, 10kV BIL

98.00 98.23 98.47 98.60 98.80 98.93 99.17 99.44

8 

300 kVA, 150°C, three-phase, 

60Hz, 480V Delta primary, 

208Y/120V secondary, 10kV BIL

98.60 98.80 99.02 99.14 99.25 99.32 99.44 99.58

9 

300 kVA, 150°C, three-phase, 

60Hz, 4160V Delta primary, 

480Y/277V secondary, 45kV BIL

98.82 98.93 99.04 99.15 99.22 99.39 99.55 99.55 

10

1500 kVA, 150°C, three-phase, 

60Hz, 4160V primary, 

480Y/277V secondary, 45kV BIL

99.22 99.29 99.37 99.45 99.51 99.58 99.63 99.67 

11

300 kVA, 150°C, three-phase, 

60Hz, 12470V primary, 

480Y/277V secondary, 95kV BIL

98.67 98.81 98.94 99.06 99.13 99.32 99.50 99.50 

12

1500 kVA, 150°C, three-phase, 

60Hz, 12470V primary, 

480Y/277V secondary, 95kV BIL

99.12 99.21 99.30 99.39 99.46 99.53 99.59 99.63

13A

300kVA, 150 ̊C, three-phase, 

60Hz, 24940V primary, 

480Y/277V secondary, 125kV 

BIL

98.63 98.69 98.84 98.97 99.04 99.25 99.45 99.45 

13B

2000kVA, 150 ̊C, three-phase, 

60Hz, 24940V primary, 

480Y/277V secondary, 125kV 

BIL

99.15 99.19 99.28 99.38 99.45 99.52 99.58 99.62 
5-56

Table 5.33 Summary of 2010 Incremental Manufacturer Selling Prices Over the Baseline 

for Distribution Transformer Representative Units

DL

Representative Unit

Baseline EL1 EL2 EL3 EL4 EL5 EL6 EL7

$ $ $ $ $ $ $ $ 

1 

50 kVA, 65°C, single-phase, 

60Hz, 14400V primary, 

240/120V secondary, rectangular 

tank

- 199 666 811 339 823 1168 N/A

2 

25 kVA, 65°C, single-phase, 

60Hz, 14400V primary, 

120/240V secondary, round tank

- 216 397 215 247 487 708 1185

3 

500 kVA, 65°C, single-phase, 

60Hz, 14400V primary, 277V 

secondary

- 849 935 2481 1606 2131 4139 7002

4 

150 kVA, 65°C, three-phase, 

60Hz, 12470Y/7200V primary, 

208Y/120V secondary

- 574 1328 2211 1413 1164 1893 4681

5 

1500 kVA, 65°C, three-phase,

60Hz, 24940GrdY/14400V 

primary, 480Y/277V secondary

- 3472 3881 5886 6585 9350 25704 N/A

6 

25 kVA, 150°C, single-phase, 

60Hz, 480V primary, 120/240V 

secondary, 10kV BIL

- 20 153 235 361 404 582 1144

7 

75 kVA, 150°C, three-phase, 

60Hz, 480V primary, 208Y/120V 

secondary, 10kV BIL

- 1503 701 929 565 745 950 2157

8 

300 kVA, 150°C, three-phase, 

60Hz, 480V Delta primary, 

208Y/120V secondary, 10kV BIL

- (844) (107) 1030 1937 3282 2371 7372

9 

300 kVA, 150°C, three-phase, 

60Hz, 4160V Delta primary, 

480Y/277V secondary, 45kV BIL

- (422) (406) 577 1392 1933 4508 N/A

10

1500 kVA, 150°C, three-phase, 

60Hz, 4160V primary, 

480Y/277V secondary, 45kV BIL

- 290 2565 7501 13638 17260 22868 N/A

11

300 kVA, 150°C, three-phase, 

60Hz, 12470V primary, 

480Y/277V secondary, 95kV BIL

- (322) 1245 2149 2142 4003 7952 N/A

12

1500 kVA, 150°C, three-phase, 

60Hz, 12470V primary, 

480Y/277V secondary, 95kV BIL

- 1426 2878 7726 9313 12199 18482 2634

6 

13A

300kVA, 150 ̊C, three-phase, 

60Hz, 24940V primary, 

480Y/277V secondary, 125kV 

BIL

- 381 1409 4592 4780 7090 13256 N/A

13B

2000kVA, 150 ̊C, three-phase, 

60Hz, 24940V primary, 

480Y/277V secondary, 125kV 

- (162) 5848 12562 19966 27890 N/A N/A
5-57

BIL

Table 5.34 Summary of 2011 Incremental Manufacturer Selling Prices Over the Baseline 

for Distribution Transformer Representative Units

DL

Representative Unit

Baseline EL1 EL2 EL3 EL4 EL5 EL6 EL7

$ $ $ $ $ $ $ $ 

1 

50 kVA, 65°C, single-phase, 60Hz, 

14400V primary, 240/120V 

secondary, rectangular tank

- 170 651 794 472 923 1253 N/A

2 

25 kVA, 65°C, single-phase, 60Hz, 

14400V primary, 120/240V 

secondary, round tank

- 209 378 272 302 531 749 1232

3 

500 kVA, 65°C, single-phase, 

60Hz, 14400V primary, 277V 

secondary

- 841 1581 3104 2280 2764 4675 7394

4 

150 kVA, 65°C, three-phase, 

60Hz, 12470Y/7200V primary, 

208Y/120V secondary

- 460 1282 2218 1375 1090 1793 4573

5 

1500 kVA, 65°C, three-phase, 

60Hz, 24940GrdY/14400V 

primary, 480Y/277V secondary

- 4037 4394 6341 7045 9703 26145 N/A

6 

25 kVA, 150°C, single-phase, 

60Hz, 480V primary, 120/240V 

secondary, 10kV BIL

- 25 126 214 362 403 572 1117

7 

75 kVA, 150°C, three-phase, 

60Hz, 480V primary, 208Y/120V 

secondary, 10kV BIL

- 1428 568 690 421 645 886 2025

8 

300 kVA, 150°C, three-phase, 

60Hz, 480V Delta primary, 

208Y/120V secondary, 10kV BIL

- (768) (69) 619 1652 3062 2490 7359

9 

300 kVA, 150°C, three-phase, 

60Hz, 4160V Delta primary, 

480Y/277V secondary, 45kV BIL

- (170) (234) 750 1586 2393 4837 N/A

10

1500 kVA, 150°C, three-phase, 

60Hz, 4160V primary, 480Y/277V 

secondary, 45kV BIL

- (327) 1538 6326 13392 16858 22374 N/A

11

300 kVA, 150°C, three-phase, 

60Hz, 12470V primary, 

480Y/277V secondary, 95kV BIL

- (88) 1176 2592 2723 4517 8378 N/A

12

1500 kVA, 150°C, three-phase, 

60Hz, 12470V primary, 

480Y/277V secondary, 95kV BIL

- 468 2915 7820 10430 13254 19295 26749

13

A 

300kVA, 150 ̊C, three-phase, 

60Hz, 24940V primary, 

480Y/277V secondary, 125kV BIL

- (344) 392 (731) 488 7113 13187 N/A

13B

2000kVA, 150 ̊C, three-phase, 

60Hz, 24940V primary, 

480Y/277V secondary, 125kV BIL

- 1073 4420 11564 22162 29706 N/A N/A
5-58

5.5.3 BusLead and Bus Loss Correction 

DOE received comment during negotiations that substation-style designs common to the 

medium-voltage, dry-type transformer market are larger than the designs that DOE had 

previously modeled and experience correspondingly larger bud and lead losses, which can force 

a unit to employ larger, more efficient cores and coils to overcome the added loss. 

DOE worked with manufacturers to explore the magnitude of this effect and made small 

upward adjustments to bus and lead losses of all medium-voltage, dry-type design lines. For each 

design line, DOE added a constant loss value to account for lead and bus losses. This change 

resulted in slightly lower efficiencies (generally close to .02%) and had the effect of nudging the 

entire design cloud slightly to the left. Because the cost/efficiency curve is upward-sloping, this 

has the effect of marginally increasing the lowest MSPs for a given efficiency even though no 

direct cost was added to each unit.

5.6 RESULTS OF THE ANALYSIS ON EACH DESIGN LINE

This section provides a visual representation of the results of the engineering analysis. 

The scatter plots in this section show the relationship between the manufacturer’s selling price 

and efficiency for each of the 14 design lines. Each dot on the plots represents one unique design 

created by the software at a given manufacturer’s selling price and efficiency level. The

placement of each dot (and the uniqueness of each design) is dictated by the design option 

combinations (core steel and windings), core shape, A/B combination, and the variable design 

parameters generated by the design software. 

5.6.1 Traditional Core Designs for the Reference Case

The designs in this section represent the traditional core designs that DOE analyzed in the 

life-cycle cost and national impact analyses. In addition to the results provided in this section, 

DOE prepared scatter plots depicting the engineering analysis results for the 14 representative 

units, including watts of core and coil loss and the weight by efficiency (see Appendix 5A). For 

each of the 14 representative units DOE presents the results with the 2010 and 2011 steel price 

scenarios. 

Figure 5.6.1 and Figure 5.6.2 present plots of the manufacturer selling prices and 

efficiency levels for the full database of designs for the representative unit from DL1, a 50kVA 

single-phase, liquid-immersed, pad-mounted distribution transformer. The efficiency levels 

shown in this plot represent transformers at 50 percent of nameplate load and are corrected for 

temperature. The following observations can be made about these scatter plots: 

• The current standard efficiency level of 99.08 percent is most cost-effectively met by 

designs using M3 core steel (2011 prices) or ZDMH core steel (2010 prices). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.25 percent, and can reach efficiencies of 99.50 percent. 
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Figure 5.6.1 Engineering Analysis Results, Design Line 1, 2011 
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Figure 5.6.2 Engineering Analysis Results, Design Line 1, 2010 

Figure 5.6.3 and Figure 5.6.4 present plots of the manufacturer sales prices and efficiency 

levels for the full database of designs for the representative unit from DL2, a 25kVA singlephase, liquid-immersed, pole-mounted distribution transformer. The efficiency levels shown in 

this plot represent transformers at 50 percent of nameplate load and are corrected for 

temperature. The following observations can be made about this scatter plot:

• The current standard efficiency level of 98.91 percent is met most cost-effectively by 

designs using M2 core steel (2011 and 2010). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.10 percent, and can reach efficiencies up to 99.5 percent.
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Figure 5.6.3 Engineering Analysis Results, Design Line 2, 2011 
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Figure 5.6.4 Engineering Analysis Results, Design Line 2, 2010 

Figure 5.6.5 and Figure 5.6.6 present plots of the manufacturer sales prices and efficiency 

levels for the full database of designs for the representative unit from DL3, a 500kVA singlephase, liquid-immersed distribution transformer with radiators. The efficiency levels shown in 

this plot represent transformers at 50 percent of nameplate load and are corrected for 

temperature. The following observations can be made about this scatter plot: 

• The current standard efficiency level of 99.42 percent is most cost-effectively met by

designs using M3 core steel (2010) or ZDMH core steel (2011). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.55 percent, and can reach efficiencies above 99.73 percent.
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Figure 5.6.5 Engineering Analysis Results, Design Line 3, 2011 
5-64

Figure 5.6.6 Engineering Analysis Results, Design Line 3, 2010 

Figure 5.6.7 and Figure 5.6.8 present plots of the manufacturer sales prices and efficiency 

levels for the full database of designs for the representative unit from DL4, a 150kVA threephase, liquid-immersed distribution transformer. The efficiency levels shown in this plot 

represent transformers at 50 percent of nameplate load and are corrected for temperature. The 

following observations can be made about this scatter plot: 

• The current standard efficiency level of 99.08 percent is most cost-effectively met by 

designs using M3 core steel (2011) or ZDMH core steel (2010). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.25 percent, but amorphous designs have a minimum efficiency of 99.27 

percent.

• The amorphous designs can reach efficiencies up to 99.60 percent. 
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Figure 5.6.7 Engineering Analysis Results, Design Line 4, 2011 
5-66

Figure 5.6.8 Engineering Analysis Results, Design Line 4, 2010 

Figure 5.6.9 and Figure 5.6.10 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL5, a 

1500kVA three-phase, liquid-immersed distribution transformer. The efficiency levels shown in 

this plot represent transformers at 50 percent of nameplate load and are corrected for 

temperature. The following observations can be made about this scatter plot: 

• The current standard efficiency level of 99.42 percent is most cost-effectively met by 

designs using M3 core steel (2011) or ZDMH (2010). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.50 percent, and can reach efficiencies up to 99.7 percent.
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Figure 5.6.9 Engineering Analysis Results, Design Line 5, 2011 
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Figure 5.6.10 Engineering Analysis Results, Design Line 5, 2010 

Figure 5.6.11 and Figure 5.6.12 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL6, a 25kVA 

single-phase, low-voltage, dry-type distribution transformer. The efficiency levels shown in this 

plot represent transformers at 35 percent of nameplate load and are corrected for temperature. 

The following observations can be made about this scatter plot: 

• The current standard efficiency level of 98.00 percent is most cost-effectively met by 

designs using M5 core steel (2011 and 2010). 

• The NEMA Premium efficiency level of 98.60 percent is met by designs using M4 core 

steel or better.

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 98.80 percent, and can reach efficiencies up to 99.40 percent.
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Figure 5.6.11 Engineering Analysis Results, Design Line 6, 2011 
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Figure 5.6.12 Engineering Analysis Results, Design Line 6, 2010 

Figure 5.6.13 and Figure 5.6.14 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL7, a 75kVA 

three-phase, low-voltage, dry-type distribution transformer. The efficiency levels shown in this 

plot represent transformers at 35 percent of nameplate load and are corrected for temperature. 

The following observations can be made about this scatter plot: 

• The current standard efficiency level of 98.00 percent is most cost-effectively met by 

designs using M12 core steel (2011 and 2010). 

• The NEMA Premium efficiency level of 98.60 percent is met by designs using M4 core 

steel or better.

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 98.93 percent, and can reach efficiencies up to 99.40 percent. 
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Figure 5.6.13 Engineering Analysis Results, Design Line 7, 2011 
5-72

Figure 5.6.14 Engineering Analysis Results, Design Line 7, 2010 

Figure 5.6.15 and Figure 5.6.16 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL8, a 300kVA 

three-phase, low-voltage, dry-type distribution transformer. The efficiency levels shown in this 

plot represent transformers at 35 percent of nameplate load and are corrected for temperature. 

The following observations can be made about this scatter plot: 

• The current standard efficiency level of 98.60 percent is most cost-effectively met by 

designs using M5 core steel (2011) or M6 core steel (2010). 

• The NEMA Premium efficiency level of 99.02 percent is met by designs using M3 core 

steel or better.

• The amorphous metal (SA1) core is the only design that can achieve an efficiency of 

99.40 percent or greater, and can reach efficiencies up to 99.60 percent. 
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Figure 5.6.15 Engineering Analysis Results, Design Line 8, 2011 
5-74

Figure 5.6.16 Engineering Analysis Results, Design Line 8, 2010 

Figure 5.6.17 and Figure 5.6.18 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL9, a 300kVA 

three-phase, medium-voltage, dry-type transformer with a 45kV BIL. The efficiency levels 

shown in this plot represent transformers at 50 percent of nameplate load and are corrected for 

temperature. The following observations can be made about this scatter plot: 

• The current standard efficiency level of 98.82 percent is met by designs using M3 core 

steel (2011 and 2010). 

• The five-legged amorphous metal (SA1) core is the most cost-effective design for any 

efficiency level above 99.20 percent, and can reach efficiencies up to 99.59 percent.
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Figure 5.6.17 Engineering Analysis Results, Design Line 9, 2011 
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Figure 5.6.18 Engineering Analysis Results, Design Line 9, 2010 

Figure 5.6.19 and Figure 5.6.20 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL10, a 

1500kVA three-phase, medium-voltage, dry-type transformer with a 45kV BIL. The efficiency 

levels shown in this plot represent transformers at 50 percent of nameplate load and are corrected 

for temperature. The following observations can be made about this scatter plot: 

• The current standard efficiency level of 99.22 percent is most cost-effectively met by 

designs using M4 core steel (2011 and 2010). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.50 percent, and can reach efficiencies up to 99.67 percent.
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Figure 5.6.19 Engineering Analysis Results, Design Line 10, 2011 
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Figure 5.6.20 Engineering Analysis Results, Design Line 10, 2010 

Figure 5.6.21 and Figure 5.6.22 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL11, a 

300kVA three-phase, medium-voltage, dry-type transformer with a 95kV BIL. The efficiency 

levels shown in this plot represent transformers at 50 percent of nameplate load. The following 

observations can be made about this scatter plot: 

• The current standard efficiency level of 98.67 percent is most cost-effectively met by 

designs using M3 core steel (2011) or H0 core steel (2010). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.00 percent, and can reach efficiencies up to 99.50 percent.
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Figure 5.6.21 Engineering Analysis Results, Design Line 11, 2011 
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Figure 5.6.22 Engineering Analysis Results, Design Line 11, 2010 

Figure 5.6.23 and Figure 5.6.24 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL12, a 

1500kVA three-phase, medium-voltage, dry-type transformer with a 95kV BIL. The efficiency 

levels shown in this plot represent transformers at 50 percent of nameplate load. The following 

observations can be made about this scatter plot: 

• The current standard efficiency level of 99.12 percent is most cost-effectively met by 

designs using M5 core steel (2011 and 2010). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.40 percent, and can reach efficiencies above 99.65 percent. 
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Figure 5.6.23 Engineering Analysis Results, Design Line 12, 2011 
5-82

Figure 5.6.24 Engineering Analysis Results, Design Line 12, 2010 

Figure 5.6.25 and Figure 5.6.26 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL13A, a 

300kVA three-phase, medium-voltage, dry-type transformer with a 125kV BIL. The efficiency 

levels shown in this plot represent transformers at 50 percent of nameplate load and are corrected 

for temperature. The following observations can be made about this scatter plot:

• The current standard efficiency level of 98.63 percent is most cost-effectively met by 

designs using M4 core steel (2011 and 2010). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.10 percent, and can reach efficiencies up to 99.48 percent. 
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Figure 5.6.25 Engineering Analysis Results, Design Line 13A, 2011 
5-84

Figure 5.6.26 Engineering Analysis Results, Design Line 13A, 2010 

Figure 5.6.27 and Figure 5.6.28 present plots of the manufacturer sales prices and 

efficiency levels for the full database of designs for the representative unit from DL13B, a 

2000kVA three-phase, medium-voltage, dry-type transformer with a 125kV BIL. The efficiency 

levels shown in this plot represent transformers at 50 percent of nameplate load and are corrected 

for temperature. The following observations can be made about this scatter plot: 

• The current standard efficiency level of 99.15 percent is most cost-effectively met by 

designs using M3 core steel (2011) or M4 core steel (2010). 

• The amorphous metal (SA1) core is the most cost-effective design for any efficiency 

level above 99.43 percent, and can reach efficiencies up to 99.58 percent. 
5-85

Figure 5.6.27 Engineering Analysis Results, Design Line 13B, 2011
5-86

Figure 5.6.28 Engineering Analysis Results, Design Line 13B, 2010 

5.6.2 Symmetric Core Designs 

In the preliminary analysis, DOE generated cost-efficiency relationships for symmetric 

core design transformers by adjusting comparable traditional core design models. To do this, 

DOE reduced core losses and core weight while increasing labor costs to approximate the 

symmetric core designs. DOE based these approximations on conversations with manufacturers, 

and published literature. In the preliminary analysis, DOE requested information and data 

regarding symmetric core designs. However, DOE was unable to obtain sufficient data to more 

accurately approximate the cost and efficiency of symmetric core designs. Without further 

information, DOE did not consider symmetric core designs for the NOPR analysis, although it 

has not screened these designs out of the rulemaking. DOE welcomes comment and submission 

of engineering data that would be useful in analyzing symmetric core designs for the final rule. 

5.7 THREE EXAMPLE DESIGNS AND COST BREAKDOWNS

This section presents some of the OPS transformer designs from DOE’s engineering 

analysis database. As discussed earlier, to prepare a cost-efficiency relationship on selected 
5-87

representative units, DOE contracted Optimized Program Service (OPS), a software company 

specializing in transformer design since 1969. Using a range of input parameters and material 

prices, more than 39,000 transformer designs were created by OPS for DOE’s analysis. For each 

design, the software generates specific information about the core and coil, including physical 

characteristics, dimensions, material requirements and mechanical clearances, as well as a 

complete electrical analysis of the final design. For information on OPS and their software, visit 

their website: http://www.opsprograms.com/home.html. 

To illustrate the typical output from the OPS software, a design from each of the three 

superclasses (i.e., liquid-immersed, low-voltage dry-type and medium-voltage dry-type) are 

presented in this section. As these designs illustrate, the software output is used to create a bill of 

materials, which is marked-up to arrive at the manufacturer’s selling price. The OPS software 

provides an electrical analysis including efficiency, which, when plotted with the manufacturer’s 

selling price, constitutes the primary output of the engineering analysis.

The three distribution transformers presented are from design lines 1, 7, and 12. Across all 

the design lines, the complete database of designs contains 39,618 distribution transformer 

specification and winding sheets, bills of materials, and performance reports. Any infeasible 

designs or designs below the minimum efficiency standard are removed and then this design 

database is used by the LCC analysis (see chapter 8) as it simulates purchases of distribution 

transformers in the marketplace.

• Design Line 1: 50 kVA single-phase, liquid-immersed. M2 core steel with copper 

primary and aluminum secondary windings (M2CuAl) at a $3.00 A and a $1.20 B 

evaluation formula. 

• Design Line 7: 75 kVA three-phase, low-voltage dry-type. M6 full-mitered core 

steel with aluminum primary and secondary windings (M6AlAl) at a $0.50 A and 

a $0.10 B evaluation formula. 

• Design Line 12: 1500 kVA three-phase, medium-voltage dry-type. M3 step-lap 

core steel with aluminum primary and secondary windings (M3AlAl) at a $1.50 A 

and a $0.30 B evaluation formula. 

For the three designs presented, the design detail report is followed by a bill of materials 

showing the cost calculation, and a pie chart providing a breakdown of the final selling price. 

5.7.1 Design Details Report for Transformer from Design Line 1

A design specification report for a 50kVA single-phase liquid-immersed transformer 

appears below. This design incorporates M2 core steel, with a copper primary and an aluminum 

secondary. The evaluation factors for this design are $3.00 A and $1.20 B. The bill of materials 

and associated breakdown of costs for this design are also reported, after the design and electrical 

analysis reports.
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5-89

Table 5.35 provides the bill of materials which was calculated from the OPS design 

details report. This bill of materials uses the raw material prices given in this chapter for fixed 
5-90

and variable materials used in building the transformer. These materials are then marked-up at 

the bottom of the table to arrive at the manufacturer’s selling price. 

Table 5.35 Bill of Materials for Transformer from Design Line 1

* Indicates those items to which the scrap factor (1.0%) and the handling and slitting 

factor (1.5% are applied.

** Price based on rounded estimations. The non-rounded price may vary slightly. 

$ Each $ Total

$2.00 $430.42 

$4.84 $261.14 

$2.08 $49.64 

$1.52 $6.94 

$3.35 $125.93 

$140.34 $140.34 

$25.00 $25.00 

$0.65 $0.65 

$58.00 $58.00 

$16.00 $16.00 

$7.48 

$1,122 

$ Total

$25.76 

$5.15 

$15.46 

$5.15 

$7.73 

$25.76 

$18.03 

$9.79 

$23.18 

$34.52 

$12.88 

$11.22 

$194.65 

$1,316 

$140 

$210 

$364 

$2,031 

B$ Input $1.20 

Efficiency at 99.00%

Bill of Materials and Labor for liquid-immersed, single-phase, pad-mount, 50kVA

A$ Input $3.00 

Primary Copper wire, formvar, round 53.93

Secondary 

windings* (lb)

Aluminum strip, thickness 

range 0.02-0.045

23.83

Material Item Type Quantity

Core Steel* (lb) M2-.007 215.75

Tank - 1

Core clamp - 1

Winding form & 

insulation* (lb)

Kraft insulating paper with 

diamond adhesive

4.34

Oil (gal) - 37.6

Misc. hardware - 1

Scrap Factor 1.00%

Total Material Cost

Nameplate - 1

Bushings HV & LV 1

Labor item Hours Rate

Lead dressing 0.5 51.52

Total Material Weight (lb) 748

Tanking and impregnating 0.3 51.52

Preliminary test 0.1 51.52

Inspection 0.1 51.52

Marking and miscellaneous 0.35 51.52

Winding the primary 0.19 51.52

Final test 0.15 51.52

Pallet loading 0.5 51.52

Core assembly 0.25 51.52

Handling and slitting factor (on material) 1.50%

Total Labor 3.56 51.52

Winding the secondary 0.45 51.52

Cutting, forming, and annealing 0.67 51.52

Manufacturer Selling Price**

Shipping Cost (Based on Total Weight) $0.22/lb

Non-production Cost Markup 25.00%

Manufacturing Cost (Material + Labor)

Factory Overhead (Materials only) 12.50%
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Figure 5.7.1 provides a summary of the costs contributing to the total selling price of the 

transformer from design line 1. For this design, approximately 57 percent of the final 

manufacturer selling price is direct material and scrap. Labor accounts for 9 percent of the price, 

factory overhead accounts for 7 percent, and together, shipping and non-production costs account 

for 27 percent. 

Figure 5.7.1 Manufacturer Selling Price Breakdown, Transformer from Design Line 1

5.7.2 Design Details Report for Transformer from Design Line 7

The following design report provides information on one of the several designs prepared 

to study the representative unit from design line 7. This is a 75kVA, three-phase, low-voltage, 

dry-type unit. The design shown here is for M6 full-mitered core steel with aluminum primary 

and secondary windings, and a $0.50A and $0.10B. 

Core/Coils

44%

Non-Production

18%

Shipping

9%

Factory Overhead

7%

Labor

9%

Other Material

13%
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5-93

Table 5.36 provides the bill of materials which was calculated from the OPS design 

details report. This bill of materials uses the raw material prices given in this chapter for fixed 

and variable materials used in building the transformer. These materials are then marked-up at 

the bottom of the table to arrive at the manufacturer’s selling price. 
5-94

Table 5.36 Bill of Materials for Transformer from Design Line 7

* Indicates those items to which the scrap factor (1.0%) and the handling and slitting 

factor (1.5% are applied.

** Price based on rounded estimations. The non-rounded price may vary slightly. 

$ Each $ Total

$1.46 $438.44 

$2.97 $162.33 

$2.97 $91.88 

$24.50 $36.83 

$131.82 $131.82 

$19.00 $19.00 

$0.32 $7.54 

$0.65 $0.65 

$1.50 $10.50 

$9.00 $27.00 

$22.55 $26.53 

$7.00 $7.00 

$7.29 

$967 

$ Total

$12.88 

$2.58 

$2.58 

$5.15 

$10.30 

$10.30 

$77.28 

$30.40 

$52.55 

$69.55 

$51.52 

$10.94 

$336 

$1,303 

$121 

$141 

$356 

$1,921 

Bill of Materials and Labor for low-voltage, dry-type, three-phase, 75kVA

A$ Input $0.50 

Material Item Type Quantity

Core Steel* (lb) M6-.014 301.33

B$ Input $0.10 

Efficiency at 98.12%

Winding form & 

insulation* (lb)

Nomex insulation 0.21

Enclosure 14-gauge steel 1

Primary 

winding* (lb)

Aluminum wire, rectangular, 

0.1x0.2, Nomex

54.6

Secondary 

windings* (lb)

Aluminum wire, rectangular, 

0.1x0.2, Nomex

30.9

Nameplate - 1

LV Buss Bar (ft.) - 7

Core clamp - 1

Duct spacers (ft., 

drop 2/3)

- 23.56

Misc. hardware - 1

Scrap Factor 1.00%

Total Material Cost

HV Terminal - 3

Impregnation - 1.18

Labor item Hours Rate

Lead dressing 0.25 51.52

Total Material Weight (lb) 502

Final test 0.1 51.52

Packing 0.2 51.52

Inspection 0.05 51.52

Preliminary test 0.05 51.52

Winding the primary 0.59 51.52

Winding the secondary 1.02 51.52

Marking and miscellaneous 0.2 51.52

Enclosure manufacturing 1.5 51.52

Handling and slitting factor (on material) 1.50%

Total Labor 6.31 51.52

Core stacking 1.35 51.52

Core assembly 1 51.52

Non-production Cost Markup 25.00%

Manufacturer Selling Price**

Manufacturing Cost (Material + Labor)

Factory Overhead (Materials only) 12.50%

Shipping Cost (Based on Total Weight) $0.22/lb
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Figure 5.7.2 provides a summary of the costs contributing to the total selling price of the 

transformer from design line 7. For this design, approximately 50 percent of the final 

manufacturer selling price is direct material and scrap. Labor accounts for 18 percent of the 

price, factory overhead accounts for 6 percent, and together, shipping and non-production costs 

account for 26 percent.

Figure 5.7.2 Manufacturer Selling Price Breakdown, Transformer from Design Line 7

5.7.3 Design Details Report for Transformer from Design Line 12

The following design report provides information on one of several designs prepared to 

study the representative unit from design line 12. This is a 1500kVA, three-phase, mediumvoltage, dry-type unit at 95kV BIL. The design shown here is for M3 step-lap mitered core steel 

with aluminum primary and secondary windings, and a $1.50A and $0.30B. This is a different 

design option combination than the original design selected for the preliminary analysis, which 

was for M4 core steel with copper primary and aluminum secondary windings, and a $1.50A and 

$0.30B. 

Core/Coils

36%

Non-Production

19%

Shipping

7%

Factory 

Overhead

6%

Labor

18%

Other Material

14%
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5-97
5-98

Table 5.37 provides the bill of materials which was calculated from the OPS design 

details report. This bill of materials uses the raw material prices given in this chapter for fixed 

and variable materials used in building the transformer. These materials are then marked-up at 

the bottom of the table to arrive at the manufacturer’s selling price. 
5-99

Table 5.37 Bill of Materials for Transformer from Design Line 12

* Indicates those items to which the scrap factor (1.0%) and the handling and slitting 

factor (1.5% are applied.

** Additional scrap on core due to mitering process. 

*** Price based on rounded estimations. The non-rounded price may vary slightly. 

$ Each $ Total

$1.88 $10,760 

$2.97 $1,851 

$2.08 $755 

$24.50 $4,153 

$820.43 $820 

$125.00 $125 

$0.56 $890 

$0.65 $0.65 

$12.00 $192 

$9.00 $27 

$135.00 $135 

$12.34 $1,186 

$22.55 $742 

$54.00 $54 

$606 

$22,296 

$ Total

$52 

$13 

$26 

$39 

$103 

$113 

$412 

$1,461 

$185 

$311 

$309 

$263 

$3,286 

$25,582 

$2,787 

$2,265 

$7,092 

Manufacturer Selling Price*** $37,727 

Shipping Cost (Based on Total Weight) $0.22/lb

Non-production Cost Markup 25.00%

Manufacturing Cost (Material + Labor)

Factory Overhead (Materials only) 12.50%

Core assembly 6 51.52

Handling and slitting factor (on material) 1.50%

Total Labor 58.7 51.52

Winding the secondary 3.6 51.52

Core stacking 6.04 51.52

Enclosure manufacturing 8 51.52

Winding the primary 28.35 51.52

Packing 2 51.52

Marking and miscellaneous 2.2 51.52

Preliminary test 0.5 51.52

Final test 0.75 51.52

Lead dressing 1 51.52

Inspection 0.25 51.52

Labor item Hours Rate

Total Material Cost

Total Material Weight (lb) 8,055

Misc. hardware - 1

Scrap Factor 1.00%

Winding combs - 96.11

Impregnation - 32.89

HV tap board - 3

HV Terminals - 1

Nameplate - 1

LV Buss Bar (ft.) - 16

Core clamp - 1

Duct spacers - 1,589.03

Winding form & 

insulation* (lb)

Nomex insulation 134

Enclosure 12-gauge steel 1

Primary 

winding* (lb)

Copper wire, rectangular, 

0.1x0.2, Nomex

622.58

Secondary 

windings* (lb)

Aluminum strip, thickness 

range 0.02 - 0.045

362.33

Material Item Type Quantity

Core Steel* (lb) M4-.011 5,738.61

B$ Input $0.30 

Efficiency at 99.14%

Bill of Materials and Labor for medium-voltage, dry-type, three-phase, 1500kVA

A$ Input $1.50 
5-100

Figure 5.7.3 Manufacturer Selling Price Breakdown, Transformer from Design Line 12

provides a summary of the costs contributing to the total selling price of the transformer from 

design line 12. For this design, approximately 59 percent of the final manufacturer selling price 

is direct material and scrap. Labor accounts for 9 percent of the price, factory overhead accounts 

for 7 percent, and together, shipping and non-production costs account for 25 percent.

Figure 5.7.3 Manufacturer Selling Price Breakdown, Transformer from Design Line 12

Core/Coils

35%

Non-Production

19%

Shipping

6%

Factory Overhead

7%

Labor

9%

Other Material

24%