Energy Efficient 

Transformers

EPA/CEE Conference on Energy Efficiency

The Background of NEMA TP-1

Phil Hopkinson, P.E.

1

Manager, Transformer Technology

Square D Company
Energy Efficient 

Transformers

Background of NEMA TP-1

• Objective

• Approach

2

• Studies

• Standard
Energy Efficient 

Transformers

Background of NEMA TP-1

• Doable with known technology

Objectives

3

• Economical -- Pays for itself

• Saves energy

NEMA anticipates high market acceptance
Energy Efficient 

Transformers

Background of NEMA TP-1

• Apply total owning cost methods

Approach

4

• Use real loading assumptions and real costs 

as close as possible

• Determine best fit efficiencies
Energy Efficient 

Transformers

Background of NEMA TP-1

• Average energy cost 6.5¢/kwh

• Most people would support a 3-5 year payback

Studies

Square D -- Building Owners

5

• Most people would support a 3-5 year payback

• Loading (RMS equivalent)

35% for LV Dry

50% for all MV transformers

A 3-5 year payback is now the average 

requirement for most US utilities as well as 

building owners
Energy Efficient 

Transformers

DOE’s Main Forrestal Building

% Instantaneous Load

kVA Size Quantity Min. Max. Avg.

15 6 5 24 17

30 11 2 108 36

6

45 39 8 76 30

75 25 1 58 29

112.5 6 19 69 44

225 2 26 26 26

58.1 Avg. 89 2 108 30

Weighted Avg. % (kVA * Qty Avg. load)/Total kVA = 31

Weighted Avg. % load for 30-112.5 kVA ratings only = 32
Energy Efficient 

Transformers

Background of NEMA TP-1

TOC = A (NL) + B (LL) + Sell price

A  $0.065/kwh x 8.76 khr/yr x 3 yr. = $1.71/watt

Total Owning Cost Factors

7

A  $0.065/kwh x 8.76 khr/yr x 3 yr. = $1.71/watt

Note: Using present worth of inflation series, A = $1.50/watt

B = $A/kwh x x Temperature Correction Factor

For LV transformer B ~ $0.15/watt

For MV transformer B ~ $0.35/watt

100

%Load 2
Energy Efficient 

Transformers

Reference Condition Temperature % of Nameplate Load

Low Voltage 75C 35%

Medium Voltage 75C 50%

Single Phase Three Phase

Efficiency Efficiency

Table 4-2

NEMA Class 1 Efficiency Levels for Dry-Type Distribution Transformers

Background of NEMA TP-1

8

8

kVA Low Voltage Medium Voltage kVA Low Voltage Medium Voltage

15 97.7 97. 15 97.0 96.8

25 98.0 97.9 30 97.5 97.3

37.5 98.2 98.1 45 97.7 97.6

50 98.3 98.2 75 98.0 97.9

75 98.5 98.4 112.5 98.2 98.1

100 98.6 98.5 150 98.3 98.2

167 98.7 98.7 225 98.5 98.4

250 98.8 98.8 300 98.6 98.5

333 98.9 98.9 500 98.7 98.7

500 --- 99.0 750 98.8 98.8

667 --- 99.0 1000 98.9 98.9

833 --- 99.1 1500 --- 99.0

2000 --- 99.0

2500 --- 99.1 8
Energy Efficient 

Transformers

Reference Condition Temperature % of Nameplate Load

Load Loss 85C 50%

No Load Loss 20C 50%

Single Phase Three Phase

kVA Efficiency kVA Efficiency

10 98.3 15 98.0

Table 4-1

NEMA Class 1 Efficiency Levels for Liquid-Filled Distribution Transformers

Background of NEMA TP-1

9

10 98.3 15 98.0

15 98.5 30 98.3

25 98.7 45 98.5

37.5 98.8 75 98.7

50 98.9 112.5 98.8

75 99.0 150 98.9

100 99.0 225 99.0

167 99.1 300 99.0

250 99.1 500 99.1

333 99.2 750 99.2

500 99.3 1000 99.2

667 99.4 1500 99.3

833 99.4 2000 99.4

2500 99.4
Energy Efficient 

Transformers

75kVA Transformer Efficiency

Typical Energy Losses for 75 kVA Low Voltage Dry Type Transformer

Energy Efficiency vs. 150oC Rise or 80oC Rise

No Load Load Total

Transformer

Type

Core Loss

(Watts)

100% Coil

Loss (Watts)

35% Coil

Loss (Watts)

100% Total

Loss (Watts)

35% Total

Loss (Watts)

EXAMPLE

10

Type (Watts) Loss (Watts) Loss (Watts) Loss (Watts) Loss (Watts)

Std 150oC Rise 460 2500 250 2960 710

Std 80oC Rise 800 960 96 1760 896

TP1 Compliant

Energy Efficient 288 2370 237 2658 522
Energy Efficient 

Transformers

Total Transformer Losses vs Load

2000

2500

3000

EXAMPLE

75kVA Transformer Efficiency

11

0

500

1000

1500

0 10 20 30 40 50 60 70 80 90 100

Percent Load

Total Loss

in Watts

NEMA TP-1

Standard 150 Rise

Standard 80 Rise
Energy Efficient 

Transformers

96

98

100

E

fficie

n

c

y

(

%

)

EXAMPLE

75kVA Transformer Efficiency

12

90

92

94

96

10 20 30 40 50 60 70 80 90 100

Percent Load

E

fficie

n

c

y

(

%

)

H-150

H-80

TP1
Energy Efficient 

Transformers

2

3

4

5

6

7

B

a

c

k

P

e

rio

d (Y

e

a

r

s)

75kVA Transformer Efficiency

13

0

1

2

Utility Rate ($)/KWH

P

a

y-B

a

c

k

P

e

rio

d (Y

e

a

r

s)

0.0

3

0.0

3

5

0.0

4

0.0

4

5

0.0

5

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5

5

0.0

6

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6

5

0.0

7

0.0

7

5

0.0

8

0.0

8

5

0.0

9

0.0

9

5

0.10
Energy Efficient 

Transformers

Conclusion on NEMA TP-1

• TP-1 Transformers are

Good value

14

Good for the environment

They save energy
Energy Efficient 

Transformers

Square D NEMA TP-1 Transformers are Available Now

• How Identified?

Catalog Suffix “EE”

15

• How to Order

Through Square D Distribution and/or Field Sales

• Visit Square D Website

www.squared.com