UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Utilities Service (RUS)

BULLETIN 1724D-113 

RD-GD-2012-67

SUBJECT: Voltage Levels on Rural Electric Distribution Systems

TO: All RUS Electric Borrowers 

EFFECTIVE DATE: Date of Approval

OFFICE OF PRIMARY INTEREST: Distribution Branch, Electrical Staff Division. 

INSTRUCTIONS: This bulletin replaces REA (Rural Electrification Administration) 

Bulletin 169-4 dated November 1970. 

AVAILABILITY: This bulletin is available on the RUS Electric Program website at http://

www.usda.gov/rus/electric/bulletins.htm. 

PURPOSE: This bulletin provides recommended voltage levels and limits to serve as 

guidelines in the design and operation of electric distribution systems. Because utilities are 

responsible only for power supply up to the point of connection with the consumer’s facilities, 

this bulletin limits its scope to the supply portion of the electric system. The bulletin refers, 

however, to the expected performance of consumer-owned wiring and its effect on utilization 

voltage. 

May 16, 2012

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 1724D-113

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TABLE OF CONTENTS

1 PURPOSE 4

2 VOLTAGE STANDARDS 4

3 AMERICAN NATIONAL STANDARDS INSTITUTE 4

VOLTAGE RANGES

4 RUS RECOMMENDED VOLTAGE LIMIT VALUES 5

ABBREVIATIONS

ANSI – American National Standards Institute

NEMA – National Electrical Manufacturers Association

RUS – Rural Utilities Service

USDA – United States Department of Agriculture

DEFINITIONS

Base Voltage – Base voltage is a reference value which is a common denominator to the nominal 

voltage ratings of transmission and distribution lines, transmission and distribution equipment, 

and utilization equipment. For example, the base voltage of a transmission line having a nominal 

voltage rating of 34,500 volts is 115 volts; and the base voltage of a distribution line having a 

nominal voltage rating of 7200 volts is 120 volts. In general, distribution lines and associated 

equipment having a nominal rating of from 2400 volts through 19,920 volts will have a 120-volt 

base, and the nominal voltage rating of utilization equipment served by these voltages will also 

have a 120-volt base.

Equipment - The term "equipment" is a general term that includes material, fittings, devices, 

appliances, fixtures, apparatus, and the like, used as part of, or in connection with, an electrical 

installation (Reference 1).

Maximum Voltage – Maximum voltage is the greatest sustained root-mean-square (RMS) 

voltage averaged over 10 minutes.

Minimum Voltage - Minimum voltage is the least sustained RMS voltage averaged over 10 

minutes.

Nominal Voltage - The nominal voltage of a circuit or system is a nominal value assigned to the 

circuit or system for the purpose of conveniently designating its voltage class (See References). 
Bulletin 1724D-113

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For example, the nominal residential voltage is 120 volts, although the voltage may actually 

range from 114 volts to 126 volts.

Phase Unbalance (Reference 1)

Percent phase unbalance = 100 × maximum deviation from average phase-to-phase voltage

 average phase-to-phase voltage 

Service Voltage – Service voltage is the voltage at the point where the electric systems of the 

supplier and the user are connected (Reference l). In distribution systems this is usually 

considered the voltage at the meter socket or entrance switch.

Voltage Regulator - A voltage regulator is a device consisting of a regulating transformer and 

means for adjusting the voltage of the circuit without interrupting the load.

Utilization Voltage – Utilization voltage is the voltage at the line terminals of utilization 

equipment (Reference 1). This is generally considered to be the voltage at the terminals of the 

device or appliance or the voltage at the convenience outlet to which these terminals are 

connected.

Voltage Drop – Voltage drop (in a supply system) is the difference between the voltages at the 

transmitting and receiving ends of a feeder, main, or service.

Voltage Level - Voltage level is a generalized term that is synonymous with the RMS voltage 

averaged over 10 minutes (Reference 1). 

REFERENCES

ANSI C84.1 – 2011, American National Standard for Electric Power Systems and Equipment –

Voltage Ratings (60 Hertz) published by National Electrical Manufacturers Association 

(NEMA).
Bulletin 1724D-113

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UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Utilities Service (RUS)

RUS BULLETIN 1723D-113

SUBJECT: Voltage Levels on Rural Electric Distribution Systems

1 Purpose: The purpose of this bulletin is to recommend voltage levels and voltage limits to 

serve as guidelines in the general design and operation of electric distribution systems. 

Because utilities are responsible only for power supply up to the point of connection with 

the consumer’s facilities, this bulletin limits its scope to the supply portion of the electric 

system. The bulletin refers, however, to the expected performance of consumer-owned 

wiring and its effect on utilization voltage.

2 Voltage Standards: The American National Standards Institute (ANSI) Publication ANSI 

C84.1-2011, "American National Standard For Electric Power Systems and Equipment—

Voltage Ratings (60 Hertz)",

* establishes standards on voltage limits for the electric 

distribution supply system. It includes voltage limits within which utilization equipment 

should operate satisfactorily. It thus serves as a guideline for the electrical manufacturer in 

equipment design and for the user in designing an electrical wiring system. The 

recommendations in this bulletin are based on this ANSI voltage standard.

 3 American National Standards Institute Voltage Ranges

Application of voltage ranges (quoted directly from the standard, ANSI C84.1-2011)

a Range A

(1) Service Voltage

Electric supply systems shall be so designed and operated that most service 

voltages are within the limits specified for Range A. The occurrence of 

service voltages outside of these limits is to be infrequent.

(2) Utilization Voltage

User systems shall be so designed and operated that with service voltages 

within Range A limits, most utilization voltages are within the limits 

specified for this range. 

Utilization equipment shall be designed and rated to give fully satisfactory 

performance throughout this range.

b Range B

(1) Service and Utilization Voltages

Range B includes voltages above and below Range A limits that necessarily 

result from practical design and operating conditions on supply or user 

 *

References are listed on page 3.
Bulletin 1724D-113

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systems, or both. Although such conditions are a part of practical operations, 

they shall be limited in extent, frequency, and duration. When they occur, 

corrective measures shall be undertaken within a reasonable time to improve 

voltages to meet Range A requirements.

Insofar as practicable, utilization equipment shall be designed to give 

acceptable performance in the extremes of this range of utilization voltage, 

although not necessarily as good performance as in Range A.

4 RUS Recommended Voltage Limit Values

The voltage limits of Range A and Range B in ANSI-C84.1-2011 refer to utilization and 

service voltages. The effects of these limits on primary distribution voltages are discussed 

later in this bulletin.

Table 1. Voltage Ranges (120-volt base)

Range

Service Voltage

Minimum Maximum

A 114 126

B 110 127

a Basic Design Criteria

(1) Electric distribution systems should be designed and operated to meet the 

voltage level requirements of Range A in Table 1. Users' electrical 

equipment of all types will generally be designed to give satisfactory 

performance in this range.

(2) Maintaining voltage levels within Range A on all parts of the system at all 

times cannot be assured. As a result of the economics of operation, there 

may be some system voltages that fall in the extremes of Range B and 

even beyond. This may occasionally occur as the feeder reaches its design

loading limit at extreme loads.

(3) When voltages extend into Range B during normal conditions or for an 

extended period of time they should be corrected immediately to conform 

to Range A requirements by using interim measures, and plans for a 

permanent solution should be developed.
Bulletin 1724D-113

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(4) If voltages on any part of the system fall outside the limits of Range B, 

corrective action shall be taken upon discovery of this condition to bring 

voltages within Range B. Interim measures should be taken if a permanent 

solution cannot be completed immediately.

(5) As lines become loaded, the voltage can vary independently within the 

three phases that all distribution systems provide. This is termed “phase 

unbalance,” because the voltage levels depend on the method and settings 

of regulation at the substation and downstream along the circuit as well as 

the loading conditions. The utility should endeavor to maintain the 

voltages among the three phases to within a maximum of 3 percent phase 

unbalance under no-load conditions (Reference 1; also see Definitions).

(6) The consumer is responsible for the design of the electric system and its 

subsequent voltage drop beyond the point of connection to the supplier. 

The consumer facility design is governed by applicable local codes as well 

as the National Electrical Code (Reference 2).

Some types of utilization equipment will not perform satisfactorily or efficiently at the 

extremes of Range B voltages. Outside Range B voltage limits, many types of 

utilization equipment may fail to operate and may be seriously damaged or suffer 

shortened operating life. Voltages above the limits of Range B may be especially 

damaging to the users' utilization equipment.

b System Voltage Design Recommendations

Table 1 indicates the allowable range of voltages delivered to consumers at the point of 

delivery. Table 2 provides recommendations on ranges of voltage drops allowable at 

various points in the distribution system. Table 3 provides recommendations on ranges 

of voltage levels allowable at various points in the distribution system. Regardless of 

the voltage drops recommended in Table 2, the voltage levels in Tables 1 and 3 must be 

maintained at all load levels.

(1) Distribution Substation

The output voltage of the distribution substation should be regulated to stay 

within the limits shown in Table 3. This can be accomplished by applying 

regulation equipment, by coordination with the power supplier, or both.

(2) Primary Distribution Lines

This zone extends from the load switches of the substation feeder breaker to the 

high-voltage terminal of the distribution transformer. Voltage drop occurs along 

the distribution feeders, and regulation can be placed as needed, but the 

resulting voltage drop should be within the limits shown in Table 2.
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(3) Distribution Transformer and Services

(a) This zone extends from the high-voltage terminal of the distribution 

transformer to the point of delivery. The total voltage drop in this zone should 

be limited to the range shown in Table 2.

(b) While the voltage levels on ANSI C84.1-2011 are generally accepted as 

sustained voltages, the designer should not ignore the impact of flicker and 

motor starting on the level or quality of service voltage. These issues are 

beyond the scope of this document.

Table 2. Voltage Drops for Electric Distribution System Design (120-volt base)

Maximum Volts Drop Percent Volts Drop

Substation regulated bus 

(output) to last distribution 

transformer (primary)

8 6.67

Distribution transformer 

(primary) to service delivery 

connection to consumers’ 

wiring (meter or entrance 

switch)

4 3.33

Table 3. Design Voltage Level Limits for Electric Distribution Systems. (120-volt base)

Voltage Level (volts)

Minimum Maximum

Substation regulated bus with 

regulator line drop compensator 

in use

122 126

Distribution transformer primary 

terminals

118 126

Service connection to consumer 

wiring

114 126