UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Utilities Service

BULLETIN 1724E-154 

RD-GD-2003-49

SUBJECT: Distribution Conductor Clearances and Span Limitations

TO: RUS Electric Borrowers and RUS Electric Staff

EFFECTIVE DATE: Date of Approval 

OFFICE OF PRIMARY INTEREST: Distribution Branch, Electric Staff Division

FILING INSTRUCTIONS: This bulletin replaces Chapter II-6 and V-6 of REA Bulletin 160-2, 

“Mechanical Design Manual for Overhead Distribution Lines,” dated April 1982. This is a new 

bulletin.

AVAILABILITY: This bulletin is available on the Rural Utilities Service (RUS) website at 

http://www.usda.gov/rus/electric. 

PURPOSE: The conductor clearance requirements of Rule 235 of the National Electrical Safety

Code (NESC) may limit overhead distribution span lengths. This bulletin presents information 

and the equations needed to determine the maximum span lengths that will meet NESC mid-span 

and supporting structure clearance requirements between conductors. 

July 30, 2003 

BLAINE D. STOCKTON Date

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-154

Page 2

TABLE OF CONTENTS

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

2. Conductor Clearances on Same Support Structures ................................................................... 3

3. Horizontal Clearances Between Conductors on the Same Supporting Structure ....................... 4

4. Vertical Clearances Between Conductors on the Same Supporting Structure............................ 6

5. Maximum Spans of Non-Parallel Conductors Based on Clearances........................................ 10

EXHIBIT A: Contributors ............................................................................................................ 18

INDEX

CONDUCTORS, DISTRIBUTION:

Conductor clearances 

Span limitations

ABBREVIATIONS

ACSR Aluminum conductor, steel reinforced 

ANSI American National Standards Institute 

AWG American Wire Gage

IEEE Institute of Electrical and Electronics Engineers 

NESC National Electrical Safety Code

RUS Rural Utilities Service

UNITS OF MEASURE

inch 1 inch = 2.54 centimeters = 0.0254 meters

foot 1 foot = 0.3048 meters

kV kilovolts [1 kilovolt = 1,000 volts] o

F Degrees Fahrenheit [oF = 9/5 x oC + 32] o

C Degrees Centigrade [oC = 5/9 x (oF - 32)]
Bulletin 1724E-154

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Distribution Conductor Clearances and Span Limitations

1. INTRODUCTION

1.1 Purpose and Scope of Bulletin: The conductor clearance requirements of Rule 235 of the 

National Electrical Safety Code (NESC) may limit overhead distribution span lengths. This 

bulletin presents information and the equations needed to determine the maximum span lengths 

that will meet NESC mid-span and supporting structure clearance requirements between 

conductors. Only bare electric supply conductors supported by the Rural Utilities Service (RUS) 

standard distribution primary, pole-top assemblies are analyzed in this bulletin. However, the 

equations presented in this bulletin can be applied to other types of conductors and support 

assemblies. Diagrams and example solved problems are included in this bulletin to clarify the 

presentation.

1.2 National Electrical Safety Code: Throughout this bulletin are references to rules and 

selected data contained specifically in the 2002 Edition of the National Electrical Safety Code 

(NESC). The NESC is published by the Institute of Electrical and Electronics Engineers, Inc. 

(IEEE).1/ At the time this bulletin was written, the 2002 Edition was the latest edition of the 

NESC. Periodically the NESC is updated and revised. Users of this bulletin should use the rules 

and data, as may be revised and renumbered, from the most recent edition of the NESC.

1.3 RUS Standard Distribution Assemblies: Within this bulletin are references to RUS 

standard distribution pole-top assemblies. RUS standard distribution assemblies can be found in 

RUS Bulletin 1728F-803, “Specifications and Drawings for 24.9/14.4 kV Line Construction” 

(1998), or in RUS Bulletin 50-3 (D-804), “Specifications and Drawings for 12.5/7.2 kV Line 

Construction” (1983).2/

2. CONDUCTOR CLEARANCES ON SAME SUPPORT STRUCTURES

2.1 NESC Conductor Clearance Requirements: NESC Rule 235 specifies horizontal and 

vertical clearances for conductors attached to the same supporting structure. Conductors need to 

be attached far enough apart on their supporting structures (and span lengths need to be short 

enough) such that for all loading conditions all of the NESC horizontal and vertical clearance 

requirements between conductors are maintained.

2.2 Definitions: For the purposes of this bulletin, Separation is defined as the distance between 

the centers of conductors and Clearance is defined as the clear surface-to-surface distance

1/ Copies of the NESC may be purchase from IEEE Customer Service

445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331. Telephone: 1-800-678-4333

2/ Copies available from: RUS-USDA, Publications Office – PDRA 

STOP 1522, Washington DC 20250-1522, Telephone (202) 720-8674
Bulletin 1724E-154

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between conductors. Generally, and specifically in this bulletin, where spacings or distances 

between conductors are mentioned they are intended to be interpreted as separations unless 

otherwise specifically defined as clearances. In this bulletin, RUS assumes that conductors have 

a 1⁄2-inch radius and thus conductor clearances are one inch less than given conductor 

separations.

2.3 Depiction of NESC Rule 235D Clearances: NESC Rule 235D pertains to the diagonal 

clearance between conductors located at different levels on the same supporting structure. The 

rule requires that the surface of any conductor be a minimum horizontal distance (H) and a 

minimum vertical distance (V) away from the surface of any another conductor, (shown in the 

center of the rectangle in Figure 2-1 of this bulletin). The relationship between these 

simultaneous minimum horizontal and vertical clearance requirements, determined in other parts 

of Rule 235, is shown in Figure 2-1.

Figure 2-1: Minimum Clearances Between Conductors

H = Minimum HorizontalClearance 

(NESC Rule 235B)

V = Minimum VerticalClearance 

(NESC Rule 235C)

3. HORIZONTAL CLEARANCES BETWEEN CONDUCTORS ON THESAME 

SUPPORTING STRUCTURE

3.1 Minimum Horizontal Clearances: As per NESC Rule 235B1a and NESC Table 235-1, the 

minimum horizontal clearance between any two electric supply conductors attached to a pole-top 

assembly is: 12 inches plus 0.4 inches per kilovolt (kV) for voltages over 8.7 kV and up to 50 

kV. The following minimum horizontal clearances (rounded up to the nearest whole numbers) 

between electric supply conductors for standard RUS distribution voltages are based on NESC 

Rule 235B1a:

12.47Y/7.2 kV - Phase-to-Neutral: 12 inches 

12.47Y/7.2 kV - Phase-to-Phase: 14 inches 

24.94Y/14.4 kV - Phase-to-Neutral: 15 inches 

24.94Y/14.4 kV - Phase-to-Phase: 19 inches

3.2 Horizontal Clearances Based on Sags: NESC Rule 235B1b specifies minimum horizontal 

clearances between conductors on the same supporting structure based on conductor size and 

sag. The horizontal separations between conductor attachments on RUS standard distribution 

pole-top assemblies usually allow relatively long span lengths. Table 3.1 of this bulletin shows
Bulletin 1724E-154

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the maximum conductor sags allowed by NESC Rule 235B1b for the standard horizontal 

conductor clearances on RUS standard primary pole-top assemblies.

Table 3.1: Conductor Sags Allowed on Standard RUS Assemblies

Maximum Allowed Sags (inches)(2)

Conductor 

Horizontal 

Clearances 

(inches)

Conductor Smaller 

Than No. 2 AWG

No 2 AWG Conductor 

or Larger

Standard RUS 

Assembly

24.94 kV 12.47 kV 24.94 kV 12.47 kV

8-foot Crossarm 43 101 118 236 288

10-foot Crossarm 36 73 87 152 195

Narrow Profile (1) 29 52 63 86 119

(1) Proposed standard design.

(2) RUS recommends these sags be reduced by at least 5 percent to compensate for errors in 

design or construction.

The conductor sags for NESC Rule 235B1b (and Table 3.1 of this bulletin) are to be calculated at 

60 o

F (15 oC) at final unloaded sag with no wind. The maximum allowed sag values in Table 3.1 

of this bulletin were calculated using the following equations that were derived from the 

equations of NESC Rules 235B1b(1) and 235B1b(2):

Sag = 0 .1875 × [C − 0 .3 (in / kV )× V ]

2 inches EQ 3.1

Sag = 24 + [C − 0 .3 (in / kV

16 .3216

)× V ]

2

inches EQ 3.2

Where;

C = the clearance between the primary conductors (inches)

V = Supply voltage (kV)

EQ 3.1 is applicable for No. 2 American Wire Gage (AWG) or larger conductors, and, 

EQ 3.2 is applicable for conductors smaller than No. 2 AWG

For the sag calculations for Table 3.1 of this bulletin (using equations EQ 3.1 and EQ 3.2), 1 inch 

was subtracted from the RUS standard assembly center-to-center separation of the conductors to 

convert to the surface-to-surface clearance of the conductors shown in the table.

When using RUS standard pole-top assemblies, spans may need to be shortened such that 

maximum conductor sags do not exceed the values shown in Table 3.1 of this bulletin.

3.3 Maximum Allowable Spans Based on Horizontal Clearances: The maximum allowable 

span based on horizontal conductor clearances can be determined using the following equation:
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Sm

Where:

= Sr × EQ 3.3

Sm = Maximum allowable span base on horizontal clearances

Sr = Design ruling span

Dm = Defined sag limit (Table 3.1 for RUS standard pole-top assemblies)

Dr = Design ruling span sag (final unloaded sag at 60 oF with no wind)

3.4 Example Problem 1: Maximum Span Based on Horizontal Clearances: Determine the 

maximum span allowed for #4/0 aluminum conductor, steel reinforced (ACSR) primary 

conductors supported at each end by RUS standard 24.9/14.4 kV, VC1.11 (8-foot crossarm), 

pole-top assemblies. (See Figure 5-1 in paragraph 5.1 of this bulletin.) The phase-to-phase 

horizontal conductor separation for these RUS standard assemblies is 44 inches; the conductor 

clearances are 43 inches. The allowed sag for this design is 224 inches (236 inches less 5 

percent, from Table 3.1 of this bulletin). Assume for this problem that the design ruling span is 

350 feet and the ruling span sag, unloaded with no wind at 60 o

F, is 51 inches.

The maximum allowable span for the conditions given in this problem is calculated using 

equation EQ 3.3 as follows:

Sm = Sr × = 350× = 733 feet

4. VERTICAL CLEARANCES BETWEEN CONDUCTORS ON THESAME 

SUPPORTING STRUCTURE

4.1 Vertical Clearance Requirements: NESC Rule 235C requires a basic vertical clearance 

between conductors attached to the same supporting structure and also requires additional 

“voltage-related” and “sag-related” clearances. However, additional “voltage-related” clearances 

need not be considered for voltages less than 50 kV.

4.2 Basic Vertical Conductor Clearances: The basic vertical clearances for open supply 

conductors belonging to the same utility can be calculated using the values in NESC Table 235-5 

which are: 16 inches plus 0.4 inches per kV over 8.7 kV. Table 4.2 of this bulletin compares the 

calculated NESC basic vertical clearance requirements to the standard vertical clearances of RUS 

standard primary pole-top assemblies.

Dm 

Dr

Dm 

Dr

224

51
Bulletin 1724E-154

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Table 4.2: Vertical Clearances Between Supply Conductors on the Same Structure

Standard Clearances Between Supply Conductors 

on RUS Standard Pole-Top Assemblies (inches) (1)

Voltage NESC Basic Vertical

Clearances (inches)

Crossarms

e.g., VC1.11

Vertical

e.g., VC3.1

Narrow Profile

(Proposed Standard)

12.47Y/7.2 kV

Phase-to-Neutral

16 47 47 47

12.47Y/7.2 kV

Phase-to-Phase

18 - - 47 47

24.94Y/14.4 kV

Phase-to-Neutral

19 47 47 47

24.94Y/14.4 kV

Phase-to-Phase

23 - - 47 47

(1) 1 inch, assumed to be the sum of the radii of the adjacent conductors, has been subtracted from 

the standard separation between the conductors to convert to clearances between the conductors.

4.3 Sag-Related Vertical Conductor Clearances: NESC Rule 235C2b(1)(a) requires that the 

conductor attachments at the supporting structure be adjusted such that the vertical clearance 

between conductors at any point in the span be at least 75% of the required vertical clearances 

at the supports. Note that this rule is (only) applicable when the horizontal clearances of NESC 

Rule 235B are not satisfied (see Figure 2-1 in paragraph 2.3 of this bulletin). Thus, for RUS 

standard distribution voltages, the following minimum vertical clearances (rounded up to the 

nearest whole number) between supply conductors throughout any span needs to be maintained:

12.47Y/7.2 kV - Phase-to-Neutral: 12 inches 

12.47Y/7.2 kV - Phase-to-Phase: 14 inches 

24.94Y/14.4 kV - Phase-to-Neutral: 15 inches 

24.94Y/14.4 kV - Phase-to-Phase: 18 inches

4.4 Conductor Sag Calculation Requirements: NESC Rule 235C2b(1)(a) further specifies that 

mid-span vertical conductor clearances are to be considered under the worst conductor loading 

conditions. To that end, NESC Rule 235C2b(1)(c) requires that the conductor sags be calculated 

twice, and that each calculation uses the following sets of conductor loading conditions:

i. “The upper conductor is at the maximum operating temperature for which the line is 

designed to operate and the lower conductor is at final sag at the same ambient 

conditions as the upper conductor without electrical loading, and without ice loading, 

or,

ii. The upper conductor is at final sag at 32 oF with the radial thickness of ice, if any, 

specified in Rule 250B for the loading district concerned and the lower conductor is 

at final sag at the same ambient conditions as the upper conductor without electrical 

loading, and without ice loading.”
Bulletin 1724E-154

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4.5 Exception to NESC Rule 235C2b(1)(c): The above conductor sag calculation requirements 

do not apply if the conductors are: owned by the same utility, the same size and type, and,

installed at the same sag and tension. However NESC Rule 235C2b(1)(c) remains in effect 

where experience shows different icing conditions between the upper and lower conductors.

4.6 Maximum Span Based on Vertical Clearance of Conductors with Different Sags: The 

equation to calculate the maximum span based on the vertical clearance of conductors is:

Sm = Sr × EQ 4.1

Where:

Sm = Maximum span based on vertical clearances

Sr = Design ruling span

V1 = Vertical clearance between conductors at support 1

V2 = Vertical clearance between conductors at support 2

Cv = Required mid-span vertical clearance between conductors

Dum = Final ruling span sag of the upper conductor at its:

(1) maximum design operating temperature; or,

(2) final sag at 32 oF with radial thickness of ice

Db = Corresponding final ruling span sag of the lower conductor at:

(1) same ambient temperature as Dum(1) without electrical or ice loading; or,

(2) same ambient temperature as Dum(2) without electrical or ice loading

Ct = Clearance tolerance for engineering and construction errors (RUS recommends 

adding a minimum of 5 percent to Dum to compensate for engineering and 

construction errors.)

If V1 equals V2, which is usually the case when using RUS standard primary pole-top assemblies 

(V1 = V2 = V = 47 inches), then equation EQ 4.1 reduces to:

Sm = Sr × EQ 4.2

4.7 Example Problem 2: Maximum Span Based on Vertical Clearances: Determine the 

maximum span allowed (Sm) for a single-phase distribution span supported at each end by 

standard RUS 24.9/14.4 kV, VA1.1, primary pole-top assemblies. The primary conductor for 

this span is #1/0 ACSR and the neutral is #2 ACSR. The standard phase-to-neutral vertical 

conductor clearance (V) for these assemblies is 47 inches (48-inch separation). The allowed 

mid-span sag clearance (Cv) between the conductors is 15 inches (See paragraph 4.3 of this 

bulletin).



 V1 − Cv + V2 − Cv 

 2× Dum + Ct − Db













V −Cv 

Dum + Ct − Db 




Bulletin 1724E-154

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The distribution line section, assumed to be situated in the NESC Heavy Loading District, has 

been designed with a design ruling span (Sr) of 325 feet. The following final sags have been 

determined for the primary and neutral conductors for a 325-foot span:

Condition 1: (Primary conductor is at 120 oF; Neutral conductor is at an ambient temperature 

of 60 oF and has no electrical or ice loading);

Dum = 57 inches (plus 3 inches recommended by RUS); Db = 47 inches 

The decrease in clearance equals (57 + 3) – 47 = 13 inches

Condition 2: (Primary conductor is at 32 oF with one-half inch of ice loading; Neutral 

conductor is at an ambient temperature of 32 oF with no electrical or ice loading);

Dum = 73 inches (plus 4 inches recommended by RUS); Db = 37 inches 

The decrease in clearance equals (73 + 4) – 37 = 40 inches

Condition 2 is the worst loading condition because the clearance between the conductors 

decreases the most. The maximum span based on the vertical clearances and loading 

Condition 2 is calculated using equation EQ 4.2 as follows:

Sm = 325 × = 325 x 0.894 = 290 feet

4.8 Example Problem 3: Vertical Clearance on Assemblies Based on Span Length: For the 

data and information given in Example Problem 2 in paragraph 4.7 of this bulletin, what vertical 

clearance (V) between the primary and neutral conductors on the poles is required to achieve a 

350 foot span (Sm) that meets the mid-span conductor clearance (Cv) of 15 inches?

To solve this problem, Equation 4.2 can be rewritten to solve for the required vertical clearance

(V) in terms of the other independent variables of the equation as follows:

 S 

2

V = m 

 Sr 

×(Dum +Ct −Db )+Cv EQ 4.3

By substituting the known data into equation EQ 4.3, V can be determined as:

 350

2

V =  ×(73+ 4−37)+15 = 61.4 inches

 325

Thus, a 350 foot span than complies with the NESC mid-span vertical clearance requirements 

can be achieved by lowering the neutral 15 inches (62 inches – 47 inches) at each adjacent pole.





 73 + 4 −37  47 −15 
Bulletin 1724E-154

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4.9 Alternative Solution for Example Problem 3: For Example Problem 3 in paragraph 4.8 of 

this bulletin, a 350 foot span that meets the NESC clearance requirements can alternatively be 

achieved by re-installing the neutral (at the same pole height) on offset neutral brackets. 

However, the horizontal clearance between the primary conductor and the relocated neutral 

conductor has to be at least 15 inches. If the NESC Rule 235B requirements for both the (15 

inch) basic and (236 inch) sag-related horizontal clearance between conductors attached at the 

same supporting structure are met, then the vertical clearance requirements of NESC Rule 235C 

need not be applied.

5. MAXIMUM SPANS OF NON-PARALLEL CONDUCTORS BASED ON 

CLEARANCES

5.1 Parallel and Non-parallel Conductors: Only supply conductors that are generally 

horizontally and vertically parallel (except for conductor sag), and supported by identical poletop assemblies, are analyzed in Sections 3 and 4 of this bulletin. Frequently in distribution 

construction, adjacent pole-top assemblies are not identical and thus the conductors between the 

assemblies are not parallel. Figure 5-1 of this bulletin shows a common distribution line section 

(vertical roll to turn a large angle) in which the adjacent pole-top assemblies are not identical and 

thus the supply conductors are not parallel.

Figure 5-1: Crossarm to Vertical Construction 

1

2

3

N

2

1

Assembly 1 

VC1.11

Assembly 2 

VC3.1

N

3
Bulletin 1724E-154

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5.2 Application of Clearance Equations: The equations in Sections 3 and 4 of this bulletin are 

used to determine maximum allowable spans independently based on either the horizontal or the 

vertical NESC conductor clearances. When the conductors in a span are not parallel, both the 

horizontal and vertical conductor clearances have to be calculated simultaneously for all points 

in the span for each pair of conductors to determine compliance with NESC conductor 

clearances. Thus, as shown in Figure 5-1 of this bulletin, both the horizontal and the vertical 

conductor clearances between the closest phase-to-phase conductors (phases 1 and 2) and the 

closest phase-to-neutral (phase 3 and N) need to be determined.

Note that for the conductor configuration shown in Figure 5-1 of this bulletin, the horizontal 

clearance requirements are applicable to the conductors extending from the VC1.11 assembly 

and the vertical clearance requirements are applicable to the conductors extending from the 

VC3.1 assembly. It is unknown where in the span the applicable conductor clearance 

requirements shift from horizontal to vertical.

5.3 Maximum Spans Limited by Non-parallel Phase Conductors: The maximum allowable 

span for non-parallel phase conductors (phases 1 and 2 in Figure 5-1 of this bulletin) can be 

calculated with the following equation:

K × Sr   H1 ×(V2 −Cv )− H2 ×(V1 −Cv )

Sm =  ×  D  (V −V )  feet EQ 5.1

 r   2 1 

Where,

Sm = Maximum allowable span (feet)

K = Calculation constant = 1.5

Sr = Design ruling span (feet)

Dr = Phase conductor final ruling span sag at 60 oF (feet)

H1 = Horizontal clearance at support 1 (feet) 

H2 = Horizontal clearance at support 2 (feet) 

V1 = Vertical clearance at support 1 (feet) 

V2 = Vertical clearance at support 2 (feet)

Cv = Required vertical clearance (feet) (See paragraph 4.3 of this bulletin) 

Inherent to the application of equation EQ 5.1 are the following reasonable assumptions:

• The size, types and lengths of the 2 phase conductors are the same, and

• The sags and tensions of the conductors are the same. (See paragraph 4.5 of this 

bulletin.)

5.4 Example Problem 4: Maximum Allowable Span of Non-parallel Phase Conductors: For 

the RUS standard 24.9/14.4 kV distribution pole-top assemblies and conductor configuration 

shown in Figure 5-1 of this bulletin and the following given data, determine the maximumspan
Bulletin 1724E-154

Page 12

4.26

(Sm) based on the clearance of the uppermost primary phase conductors. The final ruling span 

sag (Dr) is based on #4/0 ACSR conductor in the NESC Heavy Loading District.

K = 1.5 (Calculation constant)

Sr = 375 feet (Design ruling span)

Dr = 4.26 feet (Phase conductor final ruling span sag at 60 o

F )

H1 = 3.583 feet (Horizontal clearance at support 1) (43 inches)

H2 = 0 feet (Horizontal clearance at support 2)

V1 = 1.25 feet (Vertical clearance at support 1) (15 inches)

V2 = 3.917 feet (Vertical clearance at support 2) (47 inches)

Cv = 1.5 feet (Required vertical clearance) (18 inches)

The maximum span (Sm) based on the clearance of the uppermost primary phase conductors can 

be determined by substituting the above given values into equation EQ 5.1 as follows:

1.5 ×375  3.583 × (3.917 − 1.5) − 0 × (1.25 − 1.5)

Sm =   ×  (3.917−1.25)  = 885 feet

   

5.5 Maximum Spans Limited by Clearances of Non-Parallel Phase-to-Neutral Conductors: 

The following paragraphs of this bulletin examines non-parallel, phase-to-neutral conductors 

whose separation is controlled by horizontal clearance requirements at one end of the span and 

vertical clearance requirements at the other end of the span. The determination of the maximum 

allowable span based on clearances between these conductors needs to consider:

• The simultaneously horizontal and vertical clearances between the conductors throughout 

the span, and

• The different sags of the conductors as required by NESC Rule 235C2b(1)(a). (See 

paragraph 4.4 of this bulletin.)

The allowable span determination is further complicated because the conductor sags are a 

function of the length of the span. The maximum allowable span based on NESC clearances can 

be determined by performing the following steps:

Step 1: Select the maximum span length desired or required for the distribution line section 

being designed.

Step 2: Determine the maximum allowable span based on phase-to-phase horizontal and vertical 

conductor clearances as presented in paragraphs 5.3 and 5.4 of this bulletin. If the span 

under consideration (Step 1) is longer than maximum allowable span length based on 

phase-to-phase clearances, then stop! The maximum allowable span based on conductor 

clearances has already been determined by the phase-to-phase clearances.
Bulletin 1724E-154

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Step 3: Determine the final unloaded sag (Dm) at 60 oF, for both the phase and the neutral 

conductors, for the span length selected in Step 1 above. The following equation EQ 5.2 

can be used to calculate the sags:

 S 

2

Dm = Dr ×  m  inches EQ 5.2

Where:

 Sr 

Dm = The final unloaded sag (Dm) at 60 oF

Dr = Design ruling span sag (final unloaded sag at 60 oF with no wind) (inches)

Sm = Selected span length (from Step 1) (feet)

Sr = Design ruling span (feet) 

Select the greater of the 2 sags calculated.

Step 4: Calculate the NESC minimum horizontal clearance (Chm) between the conductors based 

on the greater of the 2 sags calculated in Step 3 above. The following equation EQ 5.3 

can be used to calculate (Chm):

Where,

Chm =0.3(inches/kV)×V(kV)+8× inches EQ 5.3

Chm = NESC minimum horizontal clearance (inches)

V = Voltage (kV)

S = Sag (inches) (calculated in Step 3 above)

Step 5: Calculate the distance (Dx) from Assembly 1 as shown in Figure 5-1 in paragraph 5.1 of 

this bulletin to the point in the span where the actual horizontal clearance between the 

conductors equals the required horizontal clearance (Chm) for the given pair of 

conductors. This distance can be calculated using the following equation EQ 5.4:

 H1 − Chm 

Where,

Dx = Sm ×

1 − H2

inches EQ 5.4



Dx = Distance from Assembly 1 (feet)

Sm = Selected span (feet) (from Step 1)

H1 = Horizontal clearance at Assembly 1 (inches)

H2 = Horizontal clearance at Assembly 2 (inches)

Chm = NESC minimum horizontal clearance (inches) (from Step 4)

S(inches)

12

 H
Bulletin 1724E-154

Page 14

 S

Step 6: Calculate the actual vertical clearance between the conductors at the distance (Dx) 

calculated in Step 5 above. The sags used for the conductors have to be the worst case 

sag condition as per NESC Rule 235C2b(1)(c). The methodology used to select the 

worst condition sags is presented in paragraphs 4.6 and 4.7 of this bulletin. The vertical 

clearance between the conductors (Vx) at distance Dx can be calculated using the 

following equation EQ 5.5:

 D   S 2   D   D 

2 

V = V +  x × (V −V )−  m × (D + C − D )× 4 ×  x  −  x   inches EQ 5.5 x 1 S 2 1  2   r  um t b

 Sm   Sm  



Where,

Vx = Vertical clearance between conductors at distance Dx from Assembly 1

V1 = Vertical clearance between conductors at Assembly 1(inches)

V2 = Vertical clearance between conductors at Assembly 2(inches)

Dx = Distance from Assembly 1 where horizontal clearance equals required 

horizontal clearance (feet) (from Step 5)

Sm = Selected span (feet) (from Step 1)

Sr = Design ruling span (feet)

Dum = Final ruling span sag (feet) of the upper conductor at its:

(1) maximum design operating temperature; or,

(2) final sag at 32 oF with radial thickness of ice

Db = Corresponding final ruling span sag (feet) of the lower conductor at:

(1) same ambient temperature as Dum(1) without electrical or ice 

loading; or,

(2) same ambient conditions as Dum(2) without electrical or ice loading

Ct = Clearance tolerance for engineering and construction errors (RUS 

recommends adding a minimum of 5 percent to Dum to compensate 

for engineering and construction errors.)

Step 7: If the vertical clearance (Vx) between the conductors, calculated in Step 6 above, is 

greater than 12 inches (for 12.47/7.2 kV) or 15 inches (for 24.9/14.4 kV) (see paragraph

4.3 of this bulletin), then the NESC clearance requirements are met for the span selected 

in Step 1. Otherwise, select a shorter span and repeat Steps 3 through 6 above.

5.6 Example Problem 5: Maximum Allowable Span of Non-parallel Phase-to-Neutral 

Conductors: Determine if a 275 foot span (Sm) meets the NESC conductor clearance 

requirements for the 24.9/14.4 kV distribution line section shown in Figure 5-1 in paragraph 5.1 

of this bulletin. The following ruling span sag (Dr) data is known for the #4/0 ACSR primary 

and #1/0 ACSR neutral conductors. The line has a 375 foot design ruling span (Rs) and is 

located in the NESC Heavy Loading District.

m 
Bulletin 1724E-154

Page 15

Temperature

(

o

F)

Ice Loading 

(inches)

#1/0 ACSR Neutral

Final Sag (feet)

#4/0 ACSR Primary

Final Sag (feet)

32 0 4.72 3.24

32 1/2 8.31 6.21

60 5.83 4.26

120 6.90 6.21

Step 1:

A 275 foot span has been selected.

Step 2:

In Example Problem 4 in paragraph 5.4 of this bulletin, the maximum allowed span for the #4/0 

ACSR primary conductors based on NESC clearances has been determined to be 885 feet which 

is longer than the span selected in this problem.

Step 3:

The final unloaded sag (Dm) at 60 oF, for both the phase and the neutral conductors for a 375-foot 

span is calculated using equation EQ 5.2 as follows:

 S 

2

 275 

2

Dm = Dr × m  = 70 ×   = 38 inches (#1/0 ACSR neutral conductor)

 Sr   375 

 S 

2

 275 

2

Dm = Dr × m  = 51×   = 28 inches (#4/0 ACSR primary conductor)

 Sr   375 

The #1/0 ACSR neutral conductor has the greater sag and thus is selected.

Step 4:

The NESC minimum horizontal clearance (Chm) between the conductors based on the #1/0 

ACSR neutral conductor sags calculated in Step 3 above is calculated using equation EQ 5.3 as 

follows:

Chm

Chm

= 0.3(inches/ kV)×V (kV)+ 8×

= 0.3(inches/ kV)× 7.2(kV)+8×

inches

= 16.4 inches

Step 5:

The distance (Dx) from Assembly 1 of Figure 5-1 of this bulletin to the point in the span where 

the actual horizontal clearance between the conductors equals the required horizontal clearance 

(Chm) for this problem is calculated using equation EQ 5.4 as follows:

S(inches)

12

38(inches)

12
Bulletin 1724E-154

 S

Page 16

 H1 − Chm   51 −16.4 

Dx = Sm ×

H − H  = 275 × 51− 7  = 216 feet

 1 2   

As depicted in Figure 5-1 in paragraph 5.1 of this bulletin, the horizontal separation (H1) 

between the neutral (N) and the bottom phase conductor (3) on Assembly 1 is approximately 52 

inches (~ 51 inches clearance). If the neutral were installed on the other side of the pole the 

separation would be approximately 36 inches. Also as shown in Figure 5-1 of this bulletin, the 

horizontal separation (H2) between the neutral and the bottom phase conductor on Assembly 2 is 

approximately 8 inches (~ 7 inch clearance). Depending on the use of eyebolts, angle brackets or 

fiberglass primary extension links, H2 can vary from 0 inches to approximately 42 inches.

Step 6:

The worst case sag condition for the primary and neutral conductor pair as per NESC Rule 

235C2b(1)(c) for a 375-foot design ruling span (see paragraph 4.7 of this bulletin) is:

Condition 1; (Primary conductor at 120 oF; Neutral conductor at an ambient temperature of 

60 oF with no electrical or ice loading)

Dum = 75 inches (plus 4 inches recommended by RUS) Db = 70 inches 

The decrease in clearance equals (75 + 4) – 70 = 9 inches

Condition 2; (Primary conductor at 32 oF with one-half inch of ice loading; Neutral conductor 

at an ambient temperature of 32 oF with no electrical or ice loading)

Dum = 75 inches (plus 4 inches recommended by RUS); Db = 57 inches 

The decrease in clearance equals (75 + 4) – 56 = 23 inches

Condition 2 is the worst loading condition.

The actual vertical clearance (Vx) between the conductors at the distance (Dx) calculated in Step 5 

above is calculated using equation EQ 5.5 as follows:

 D   S 2   D   D 

2 

V = V +  x ×(V −V )−  m × (D + C − D )× 4 ×  x  −  x   inches x 1 S 2 1  2   r  um t b

 Sm   Sm  



 216   2752   216   216 

2 

Vx = 31 +   × (47 − 31)−   × (75 + 4 − 56)× 4 ×   −   inches  275   3752   275   275      

Vx = 35 inches

m 
Bulletin 1724E-154

Page 17

Step 7:

The vertical clearance (Vx) between the conductors, calculated in Step 6 above, is greater than the 

required vertical clearance of 15 inches for 24.9/14.4 kV, at the point Dx on the span where the 

horizontal clearances become critical. Thus, the NESC clearance requirements are met for a 

275-foot span for the conditions given in Example Problem 5 of this bulletin.
Bulletin 1724E-154

Exhibit A

Page 18

EXHIBIT A: CONTRIBUTORS

The following members of the Overhead Distribution Lines Subcommittee of the National Rural 

Electric Cooperative Association, Transmission and Distribution Engineering Committee 

provided invaluable assistance in preparing this document:

• James Byrne, Poudre Valley REA, Fort Collins, CO

• Titus (Ty) Diamond, Flint Energy, Warner Robins, GA

• Allan Glidewell, Southwest Tennessee EMC, Brownsville, TN

• Tom Hoffman, Agralite Electric Cooperative, Benson, MN

• Brian Nelson, Intercounty Electric Cooperative Assn., Licking, MO

• Ernest Neubauer, Pioneer Electric Cooperative, Piqua, Ohio

• Terry Rosenthal, Laclede Electric Cooperative, Lebanon, MO

• Gene Smith, SGS Witter Inc., Lubbock, TX

• Thomas Suggs, Jr., Middle Tennessee EMC, Murfreesboro,TN

• James Bohlk, Rural Utilities Service, USDA, Washington,D.C.