RUS Bulletin 1724E-104

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UNITED STATES DEPARTMENT OF AGRICULTURE 

Rural Utilities Service 

Bulletin 1724E-104 

RD-GD-1999-33

SUBJECT: Reduced Size Neutral Conductors for Overhead Rural 

Distribution Lines 

TO: RUS Electric Borrowers and RUS Electric Staff 

EFFECTIVE DATE: Date of Approval 

EXPIRATION DATE: Indefinite 

OFFICE OF PRIMARY INTEREST: Distribution Branch, Electric Staff Division 

FILING INSTRUCTIONS: This bulletin supersedes REA Bulletin 61-4, dated 

November 30, 1973. 

AVAILABILITY: This bulletin can be accessed through the internet at 

http://www.usda.gov/rus/electric/. File hard copy along with 7 CFR 1724. 

PURPOSE: This bulletin covers the principal considerations applicable to 

reduced size neutral conductors and outlines a procedure for their selection and 

installation. Tables of the more commonly used conductor combinations for 

existing Rural Utilities Service (RUS) standard construction are also included. 

Assistant Administrator 

Electric Program 

Disclaimer: The contents of this guidance document does not have the force and

effect of law and is not meant to bind the public in any way. This document is 

intended only to provide clarity to the public regarding existing requirements under 

the law or agency policies.
Bulletin 1724E-104

Page 2

Table of Contents

 1. Introduction............................................................................................................ 3

 2. Neutral Load Current Considerations.................................................................... 3

 3. Fault Current Considerations ................................................................................ 5

 4. Harmonic Considerations ...................................................................................... 5

 5. Minimum Conductivity Requirements .................................................................... 6

 6. Economic Considerations...................................................................................... 6

 7. Mechanical Considerations ................................................................................... 6

 8. Large Phase Conductors....................................................................................... 7

 9. Suggested Procedure for the Selection of Neutral Conductor for ThreePhase Lines not Thermally Loaded ....................................................................... 8

10. Suggested Procedure for the Selection of Neutral Conductor Size for

 Three-Phase Lines with Thermally Loaded Phase Conductors ........................... 9

11. Line Staking and Conductor Sagging................................................................. 10

TABLE 1 - Minimum Acceptable Neutral Conductor Size Based on the 20

Percent Conductivity Rule and 3000 Amp Fault Current Maximum ................... 11

TABLE 2 - Commonly Used Phase Conductor/Reduced Neutral Combinations

for ACSR type, Single Layer Conductors - Three-Phase Lines ........................ 11

ABBREVIATIONS

AAAC All Aluminum Alloy Conductor

AAC All Aluminum Conductor

ACAR All Conductor Alloy Reinforced

ACSR Aluminum Conductor Steel Reinforced

AWG American Wire Gauge

kcmil Thousand Circular Mills

NESC National Electrical Safety Code

RUS Rural Utilities Service

INDEX:

DESIGN, SYSTEM:

Neutral Conductors, Reduced Size
Bulletin 1724E-104

Page 3

1. INTRODUCTION:

1.1 Use of Reduced Size Neutrals: The use of reduced size neutral

conductors can result in significant savings in distribution line construction or

conversion cost with very little impairment to the electrical performance of the

system. However, whenever a line is constructed with phase and neutral

conductors of different sizes, materials, or stranding, it becomes necessary to

observe certain precautions to avoid excessive tensions or reduced clearances

which may not meet the minimum requirements of the National Electrical Safety

Code (NESC). Furthermore, with increasing numbers of heavy and harmonically

rich loads, consideration has to be given in some instances to the possibility of

phase and neutral conductors operating at temperatures near their thermal

limits. This differential temperature operation can cause unexpected reductions

in both ground and phase-to-neutral clearances.

1.2 Scope of Bulletin: This bulletin covers the principal considerations

applicable to reduced size neutral conductors and outlines a procedure for their

selection and installation. Tables of the more commonly used conductor

combinations for existing Rural Utilities Service (RUS) standard construction are

also included.

2. NEUTRAL LOAD CURRENT CONSIDERATIONS:

2.1 Stray Voltage: In areas where stray voltage is a concern, a neutral

conductor of the same size and type as the phase conductor should be used.

2.2 Single-phase Lines: On single-phase multigrounded neutral lines, the

entire load current returns to the source through the neutral conductor and the

earth. The earth carries most of the total current. For single-phase lines, the

minimum conductivity (or current capacity) of the neutral conductor has to be

somewhat more than 50 percent of the conductivity of the phase conductor.

2.3 Two-phase Lines: On two-phase multigrounded neutral distribution lines,

the total vector sum of the two phase load currents, which is always less than

their algebraic sum, returns to the source through the neutral conductor and the

earth. The neutral carries less than 50 percent of this vector sum current. The

neutrals on two-phase distribution lines generally need a minimum conductivity

of at least 70 percent of the conductivity of each phase conductor in order to

carry the above load currents and provide an adequate margin of capacity for

fault and worst case conditions.
Bulletin 1724E-104

Page 4

2.4 RUS Recommendation: RUS strongly recommends that a reduced size

neutral not be used on single-phase or two-phase lines for the following

reasons:

• Simplify engineering, materials and construction of the line;

• Improve overall appearance of the line;

• Minimize possible stray voltage problems;

• Comply with the rule of a minimum of 70 percent (of each phase

conductor) neutral conductivity for two-phase lines; and

• Facilitate future conversion to three-phase where the initially installed full

size neutral may possibly be used as a full or reduced size three-phase

neutral.

2.5 Three-phase Lines: For three-phase lines with multigrounded neutrals, the

neutral and earth together normally carry only the unbalanced system load

current. During normal conditions this load unbalance seldom exceeds

20 percent of the phase currents.

2.5.1 The neutral and earth combination may, however, be forced to carry a

greater percentage of the phase current during system outages when one or two

phases are open and the unbalance is significantly increased. Even for these

conditions, it is unlikely that the neutral and earth combination would carry more

than 50 percent of the peak phase current since three-phase loads presumably

drop off the line and reduce system loading.

2.5.2 There may also be locations on a three-phase feeder where an unbalance

condition is more significant because of heavily loaded single-phase taps fed

from the three-phase line. For properly designed systems, worst case

unbalance conditions should not result in a need for more than a 50 percent

neutral conductivity requirement.

2.6 Secondary Underbuild: Where primary lines are underbuilt with

secondary, the primary neutral may also be used as the secondary neutral

provided that its conductivity is suitable to carry both the secondary neutral

loads plus the expected primary neutral loads as explained above.
Bulletin 1724E-104

Page 5

3. FAULT CURRENT CONSIDERATIONS:

3.1 High Fault Currents: Consideration has to be given to the magnitude of

available fault currents in the selection of neutral conductors. Usually this will be

significant only where the maximum line-to-ground fault current exceeds

3000 amperes or where the neutral conductor is #2 ACSR (or its copper

equivalent) or smaller. For such conditions, the annealing or threshold melting

point of the neutral conductor has to be less than the clearing time of the timecurrent characteristics of all of the protective devices between the power source

and the conductor in question. Where conductor damage is possible, a larger

neutral conductor has to be used in the area of high fault current or else the fault

current clearing time of all the relevant source side protection devices has to be

reduced.

3.2 Low Fault Currents: In areas with low fault current, the clearing time of

protective devices may be extended even where reduced size neutrals are used.

Conversely, larger size neutrals provide increased phase-to-neutral fault current

capability thus reducing the total time to clear a fault involving the neutral.

4. HARMONIC CONSIDERATIONS:

4.1 Increased Neutral Loading: With the increasing use of electronic devices

for both home and industrial applications, the harmonic content on power lines is

increasing. The use of electronic ballasts for lighting, switch mode power

supplies in computers and other consumer electronic loads, and variable speed

drives for commercial and industrial loads has greatly increased the level of

harmonic current in neutrals. Harmonic current has to be considered when

sizing the neutral conductors, especially reduced size neutrals, because overall

harmonic current amplitudes may be greater than the normally expected load

currents, and together they may exceed the conductor’s capacity. If harmonic

problems are suspected on a line, a study should be undertaken to determine

the expected magnitude of harmonic currents and the neutral conductors should

be sized accordingly.

4.2 Increased Neutral Sag: Increased neutral currents, due to harmonics, may

cause the neutral to become thermally loaded and sag more than expected at

any given temperature. This phenomenon has to be considered when staking

and tensioning neutral conductors otherwise inadequate ground clearance may

result.
Bulletin 1724E-104

Page 6

5. MINIMUM CONDUCTIVITY REQUIREMENTS: Evaluation of the neutral load

currents, fault currents and harmonics leads to the conclusion that the neutral

conductivity requirements, particularly for three-phase lines, are lower than the

phase conductor requirements. If conductivity were the only consideration, then

relatively small neutral conductors could be used with phase conductors. 

However, RUS advocates that a neutral conductor have at least 20 percent of

the current carrying capacity of the phase conductor with which it is used. 

Table I shows ACSR neutral and phase conductor combinations based upon this

consideration.

6. ECONOMIC CONSIDERATIONS: While relatively small neutral conductors

could be used to satisfy conductivity requirements, their possible increased sag

characteristics may reduce ground clearances which in turn may require shorter

spans or taller poles. Using larger or full-size neutrals may avert this problem. 

A cost analysis comparing the savings of a reduced size neutral conductor cost

versus any additional cost for taller poles or shorter spans is needed to

determine the least costly alternative. The electronic spreadsheet associated

with RUS Bulletin 1724D-104 can aid in this analysis. However, seldom is a

reduced size neutral cost savings offset by the additional cost of taller poles or

shorter spans when required. For this reason, a neutral conductor larger than

that required only for conductivity is frequently the best overall selection.

7. MECHANICAL CONSIDERATIONS: 

7.1 Sag Differences: It is desirable that the neutral conductor selected has a

final unloaded sag equal to the phase conductor sag under the same conditions.

The normal maximum design sag is usually calculated at 120oF (49oC) maximum

design temperature, or 32oF (0oC) with ice, whichever is greater. If the phase

conductors are electrically loaded such that their temperatures are much higher

than the neutral, then the separation between the phase and neutral conductors

has to be considered. Table 235-5 of the 1997 NESC exhibits mid-span

minimum clearance requirements for various primary voltages. Using the values

in this table and the spacing on standard RUS three-phase pole-top assemblies

in most cases results in an allowable sag difference between the neutral and

phase conductors of generally less than approximately 18 inches (460

millimeters).
Bulletin 1724E-104

Page 7

7.2 Ground Clearances: The 1997 NESC basic ground clearances

(Table 232-1, line 9,) for wires that run along roads are 15.5 feet (4.7 meters) for

the neutral and 18.5 feet (5.6 meters) for phase conductors. The vertical

separation between the points of attachment of the neutral and phase conductor

for a standard RUS three-phase pole-top assembly with crossarms is

approximately 3 feet (0.9 meters). For such standard RUS construction, either

the phase or the neutral conductor sag, at the maximum design sag conditions,

may determine ground clearance. Thus, the line design has to consider sag and

clearances for primary conductors, neutral conductors including reduced sized

neutrals, under all of the conditions prescribed by the NESC. Likewise, the

above calculations may be required to determine clearance to underbuilt jointuse facilities. Consequently, taller poles, shorter spans or increased separation

may be required in the design and staking of the line.

8. LARGE PHASE CONDUCTORS:

8.1 Conductor Sag: Various sizes of single layered ACSR conductors (6/1 or

7/1) tend to have similar sag characteristics. It is relatively simple to find two

different size conductors through 4/0 ACSR whose sag will match well over a

wide range of temperature and loading conditions. The same is not true for

larger phase conductors. Conductors larger than 4/0 in size are multilayered

and are available in various ACSR strandings such as 18/1, 26/7, 54/7 etc. All

aluminum conductors (AAC, AAAC, ACAR, etc.,) are frequently selected for

large conductor distribution construction (where the mechanical strength of

ACSR is not needed) because their electrical losses are lower than ACSR. The

differences in strandings and materials of large conductors cause significant

variations in sag characteristics. This phenomenon makes it difficult to match

the sag of a large phase conductor to a single-layered ACSR neutral conductor.

8.2 Thermal Loading: Large phase conductors are likely to be thermally

loaded and subjected to temperatures well above their corresponding neutral

conductor temperature at certain times. Conductor creep will occur at a more

rapid rate at elevated temperatures than at lower temperatures thus further

reducing the separation between the phase and neutral conductors.
Bulletin 1724E-104

Page 8

9. SUGGESTED PROCEDURE FOR THE SELECTION OF NEUTRAL

CONDUCTOR SIZE FOR THREE-PHASE LINES – NOT THERMALLY

LOADED:

9.1 General Comments: The number of variables involved with multilayered

and different conductor materials makes it impractical to provide tables showing

combinations of neutral/phase conductors which will be satisfactory for use with

conductors above 4/0 ACSR. The engineer has to take all of these factors into

consideration and decide upon a suitable neutral to use with large phase

conductors. The procedure that follows is intended as a guide to assist in that

determination.

9.2 Assumptions: The following procedure assumes that RUS standard poletop assemblies are used and that the staking of the lines is to minimum NESC

clearance requirements.

9.3 Suggested Procedure:

(1) Determine the probable maximum neutral load current over the life

of the line under worst case conditions. Worst case is usually

during the time when one or two phases are open following a fault.

If this is estimated to be 20 percent or less than the maximum

current carrying capability of the size and type of phase conductor

to be used, then the conductor combinations provided in Table I

may be used. Where the maximum neutral load current is

estimated to be more than 20 percent of the current carrying

capability of one of the phase conductors, a suitable minimum

conductor size should be determined on the basis of the estimated

maximum current. 

(2) Based on the criteria for current carrying capacity, tabulate all

possible neutral conductor sizes from the minimum as determined

in Step (1) up to and including the phase conductor size.

(3) Prepare a chart using the ruling span of the line showing the initial

and final sags of the phase conductor and all possible neutral

conductors for various temperatures and loadings.

(4) From these data select those possible neutral conductors whose

use would result in a decrease of vertical clearance between phase

and neutral at the midspan of not more than 17 inches (430
Bulletin 1724E-104

Page 9

millimeters) from the vertical clearance at the pole for all

comparable conditions. Note: Usually a reduced neutral conductor

will have a greater final sag than the phase conductor when the

conductor stranding and material is similar. However, when

dissimilar strandings or conductor materials are used, the

separation between phase and neutral conductors may decrease

more than permissible by NESC Rule 235. In such cases, the

pole-top assembly may have to be modified to increase the

clearance between phase and neutral conductors or other

conductor strandings or conductor materials may be necessary for

acceptable sagging.

(5) From the list of possible neutral conductors, discard any conductor

with a sag of 3 feet (0.9 meters) greater than the sag of the phase

conductor under final loaded conditions, the 120o F (49oC) final

unloaded condition, or final unloaded maximum design

temperature condition.

Where icing is a serious problem, a comparison with the phase

loaded and the neutral unloaded should be made to ensure

necessary separation is maintained if the neutral should lose its ice

coating and the phase conductor does not.

Use the smallest neutral conductor that meets the above

conditions.

10. SUGGESTED PROCEDURE FOR THE SELECTION OF NEUTRAL

CONDUCTOR SIZE FOR THREE-PHASE LINES – WITH THERMALLY

LOADED PHASE CONDUCTORS: Where it is possible that the phase

conductors may be loaded at or near their thermal limit during the life of the line,

provisions have to be made in the design of the line to maintain a minimum

separation between the phase and neutral conductors for the anticipated

conditions. The general procedure outlined in Section 9 above should be

followed in selecting a neutral and phase conductor. However, the tabulation of

sags should include the sag of the phase conductor at its maximum assumed

operating temperature and the sag of the neutral at 60oF (15.5oC). In addition,

the final sags of the various possible phase conductors at their maximum

possible operating temperature should be compared with the final unloaded

sags at 60oF (15.5oC) of the possible neutral conductors. Any reduction in

midspan separation greater than 17 inches (430 millimeters) between the

selected neutral and phase conductors use should be compensated for either by

increasing the vertical separation at the point of attachment or by increasing the
Bulletin 1724E-104

Page 10

60oF (15.5oC) sag of the neutral conductor. It is recommended that various

types, sizes and strandings of conductor be evaluated for best sag

characteristics for lines operating at or near the thermal limit of the phase

conductors.

11. LINE STAKING AND CONDUCTOR SAGGING: In general, the line should

be staked to that conductor, either phase or neutral, whose final sag controls the

relationship between pole height, span and minimum NESC ground clearances. 

The vertical separation between phase and neutral conductors on standard RUS

three-phase assemblies is such that either phase or neutral conductors can

control the staking of the line. In any case all NESC ground clearances and

conductor separations have to be met for all conductor sags both initial and final.

 If special sag and tension conditions are used in the selection of a neutral

conductor, then those special sag and tension tables have to be used when

staking the line and tensioning the conductors.
Bulletin 1724E-104

Page 11

TABLE 1

Minimum acceptable neutral conductor size based on the 20 percent

conductivity rule and 3000 amp fault current maximum

Phase Conductor Size

(ACSR)

Minimum Acceptable Neutral

Conductor Size (ACSR)

#4 AWG # 4 AWG

# 2 AWG # 4 AWG

1/0 AWG # 4 AWG

2/0 AWG # 4 AWG

3/0AWG # 4 AWG

4/0 AWG # 2 AWG

336.4 kcmil 1/0 AWG

557.5 kcmil 2/0 AWG

795.0 kcmil 3/0 AWG

TABLE 2

Commonly used phase conductor/reduced neutral combinations

for ACSR type, single layer conductors – Three-Phase Lines

(Same for Each NESC Loading District)

Phase Wire Size Reduced Neutral Size

1/0 (6/1) # 2 (7/1)

2/0 (6/1) # 2 (7/1)

3/0 (6/1) # 1/0 (6/1)

4/0 (6/1) # 1/0 (6/1)