UNITED STATES DEPARTMENT OF AGRICULTURE

RURAL UTILITIES SERVICE 

BULLETIN 1724D-114 

RD-GD-2017-90

SUBJECT: Voltage Regulator Application on Rural Distribution Systems

TO: RUS Electric Borrowers and RUS Electric Staff

EFFECTIVE DATE: Date of Approval. 

OFFICE OF PRIMARY INTEREST: Electric Staff Division. 

INSTRUCTIONS: This is a new bulletin.

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

www.rd.usda.gov/publications/regulations-guidelines/bulletins/electric.

PURPOSE: This bulletin provides fundamental information about voltage regulators, their 

controls, and their application on rural distribution systems for Rural Utilities Service (RUS) 

borrowers and others. 

December 4, 2017 

Date

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-114

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1 INTRODUCTION ...................................................................................................................4

2 VOLTAGE STANDARDS......................................................................................................4

3 VOLTAGE REGULATION ON DISTRIBUTION SYSTEMS.............................................6

4 VOLTAGE REGULATORS ...................................................................................................9

5 SUBSTATION VOLTAGE REGULATORS AND SYSTEM VOLTAGE LEVELS .........26

6 LINE VOLTAGE REGULATOR APPLICATION ..............................................................32

7 VOLTAGE REGULATOR INSTALLATION .....................................................................35

8 VOLTAGE REGULATOR AND CAPACITOR APPLICATIONS.....................................36

9 VOLTAGE REGULATOR PROTECTION..........................................................................42

10 REFERENCES ......................................................................................................................43

Exhibits

Exhibit A – Derivation of Formulas for Calculating Voltage Drop and Voltage Regulation

Exhibit B – Effect of Shunt Capacitor on Source Current and Line Voltage

INDEX:

Regulators (Voltage)

Voltage Regulators

Voltages (Distribution System)

ABBREVIATIONS

AC Alternating Current

ANSI American National Standards 

Institute

CFR Code of Federal Regulations

CT Current Transformer

EEPROM Electrically Erasable and 

Programmable Read-Only 

Memory

IEEE Institute of Electrical and 

Electronics Engineers

kV Kilovolt

kVA Kilovolt-Ampere

kvar Kilovolt-Ampere Reactive

LDC Line Drop Compensator

LTC Load Tap Changer

PT Potential Transformer

R Resistance

RLC Resistor-Inductor-Capacitor

rms Root Mean Square

RUS Rural Utilities Service

SCADA Supervisory Control and Data 

Acquisition

VD Voltage Drop

VR Voltage Regulation

VSC Voltage Sensing Circuit

X Reactance

DEFINITIONS

Bandwidth — The difference between the maximum and minimum allowable voltages within 

which a voltage regulator will hold the output voltage.
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Base Voltage — Base voltage is a reference voltage 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; the base voltage of a distribution 

line having a nominal voltage rating of 7,200 volts is 120 volts. In general, distribution lines and 

associated equipment having a nominal rating from 2,400 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.

Maximum Voltage — Maximum voltage is the greatest sustained five-minute mean or average 

voltage.

Minimum Voltage — Minimum voltage is the smallest sustained five-minute mean or average 

voltage.

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. For example, 

the nominal rural residential voltage is 120 volts, although the voltage may actually range from 

114 volts to 126 volts.

Rated Voltage — Rated voltage is the voltage at which performance and operating 

characteristics of utilization equipment are referred.

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

supplier and the user are connected. In rural electric systems, this is usually considered the 

voltage at the meter socket or service entrance switch.

Utilization Equipment — Utilization equipment refers to consumer electrical loads, such as 

electrical appliances, lighting fixtures, heaters, and motors.

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

equipment. 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 circuit, main, or service.

Voltage Regulation — Voltage regulation of a line is the percent voltage drop with respect to the 

receiving-end voltage.

Voltage Spread — Voltage spread is the difference between maximum and minimum voltage at a 

particular location in the distribution system.
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1 INTRODUCTION

Providing acceptable voltage levels to consumers is vital to the proper operation of 

utilization equipment (i.e., electrical appliances, equipment, and other electrical loads). If 

utilization equipment is operated outside its prescribed voltage limits, equipment damage 

and erratic performance could result. An overvoltage condition, for example, can 

significantly shorten equipment life, decrease power factors of motors, and burn out 

lights and heating elements. On the other hand, an undervoltage condition can result in 

overheating of motors, dim lights, and cause other potential equipment damage. Hence, 

it is important for electric utilities to regulate their distribution system voltage for proper 

operation of electrical equipment.

It is impractical for electric utilities to provide each consumer with constant voltage that 

exactly matches the rated voltage of utilization equipment. Thus, the standard practice 

for utilities is to provide consumers with voltage levels that are within voltage ranges 

established by the American National Standards Institute (ANSI) and the Rural Utilities 

Service (RUS). To ensure that utilization voltage is kept within prescribed limits, electric 

utilities use voltage regulators on their distribution systems as a standard practice.

This bulletin provides fundamental information about voltage regulators, their controls, 

and their application on rural distribution systems for RUS borrowers and others.

2 VOLTAGE STANDARDS

In many states, the limits of voltage spread permitted at consumers’ meters are 

established by the state regulatory commissions. In states where borrowers are not 

subject to such controls, RUS recommends the limits established by the American 

National Standards Institute in ANSI C84.1-2016, “Electric Power Systems and 

Equipment—Voltage Ratings (60 Hertz).”

a Nominal System Voltage Standards

Table 1 presents the nominal voltages for secondary circuits established in ANSI 

C84.1-2016 and is recommended by RUS for its borrowers to serve their 

residential and commercial consumers. The voltage limits established and 

recommended in ANSI C84.1-2016 are used by the majority of the electric 

utilities in the United States.

TABLE 1: Nominal Low-Voltage Standards.

1

Nominal System Voltages Connection and Phase

 120-V Two-wire, single-phase

 120/240-V Three-wire, single-phase

 240/120-V Four-wire, three-phase delta

 208Y/120-V Four-wire, three-phase wye

 480Y/277-V Four-wire, three-phase wye

 480-V Three-wire, three-phase delta
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b ANSI Recommended Voltage Limits

ANSI Standard C84.1-2016 establishes voltage limits. The maximum and 

minimum utilization voltage limits for 120- to 480-volt circuits are shown in 

Table 2.

TABLE 2: ANSI Recommended Service and Utilization Voltage Ranges.

1

Range

Nominal

System

Voltage

(Volts)

Minimum Maximum

Utilization

Voltage

(Volts)

Service

Voltage

(Volts)

Utilization and

Service Voltage

(Volts)

A 120 108 114 126

A 120/240 108/216 114/228 126/252

A 240/120 216/108 228/114 252/126

A 208Y/120 187Y/108 197Y/114 218Y/126

A 480Y/277 432Y/249 456Y/263 504Y/291

A 480 432 456 504

B 120 104 110 127

B 120/240 104/208 110/220 127/254

B 240/120 208/104 220/110 254/127

B 208Y/120 180Y/104 191Y/110 220Y/127

B 480Y/277 416Y/240 440Y/254 508Y/293

B 480 416 440 508

As shown in Table 2, for any given nominal system voltage, the actual operating 

values can vary over various ranges of voltage limits. ANSI C84.1-2016

categorizes voltage ranges into Range A and Range B. Range A is bounded by 

voltage limits within which service voltages have to be held. Range A includes 

the majority of the existing utilization equipment operating voltages that produce 

satisfactory operation of consumer equipment. Range B is bounded by voltage 

limits that are slightly above and below the Range A voltage limits. In this range, 

consumer equipment may still operate satisfactorily, although its performance 

may be less than warranted by the manufacturer. Range B voltage limits can be 

tolerated for limited frequency and duration only.

c RUS Recommendations on Voltage Levels

RUS recommends that rural electric distribution systems be designed and 

operated to meet the voltage level requirements of Range A, per ANSI C84.1-

2016, as presented in Table 2 of this Bulletin. The optimum utilization of voltage 

regulators as an integral part of the design and operation of the distribution system 

is the basic approach to meeting these requirements.

The effects of adopting Range A voltage limits on a rural distribution system 

result in voltage spreads shown in Tables 3 and 4 of this Bulletin. Table 3 shows 
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the voltage spreads and levels (on a 120-volt base) for a rural distribution line 

using a suitable voltage regulator with line drop compensation. Table 4 shows the 

voltage drops and percent voltage drops (on a 120-volt base) permitted in each 

segment of a rural distribution line.

TABLE 3: Allowable Voltage Levels for Rural Distribution System Design (120-

Volt Base).

2

Location on System Voltage Levels

Minimum Maximum

Substation Regulated Bus 118 126

Distribution Transformer Primary Terminals 

 Closest to substation bus 118 126

 At end of line 118 126

Service Connection (Meter Socket)

 First consumer off transformer closest to substation bus 114 126

 Last consumer off transformer at end of line 114 126

Point of Utilization

 First consumer off transformer closest to substation bus 110 126

 Last consumer off transformer at end of line 110 126

TABLE 4: Voltage Drops for Rural Distribution System Design (120-Volt Base).

2

Section of 

Rural Distribution System

Voltage Drop

(Volts)

Voltage 

Drop (%)

Substation Bus to End of Primary Distribution Line 8 6.67

Distribution Transformer and Service Conductors to 

Metering Point 4 3.33

Consumer’s Service Point to Utilization Point

 Loads Include Lighting

 Non-lighting Loads

4

6

3.33

5.00

3 VOLTAGE REGULATION ON DISTRIBUTION SYSTEMS

a Primary Distribution System

Table 5 shows the preferred nominal primary voltages for wye-connected electric 

distribution systems. These are the typical RUS distribution voltages.

TABLE 5: Preferred Nominal Distribution System Voltages.

Wye-Connected Systems (Volts)

12,470Y/7,200

13,200Y/7,620

24,940Y/14,400

34,500Y/19,920
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b Voltage Regulation on a Distribution System

The maximum load of a primary distribution circuit is limited by the currentcarrying capacity and voltage regulation of the circuit. Of these two factors, 

voltage regulation is more often the controlling factor. If not limited by the 

current-carrying capacity, the maximum load of the primary distribution circuit is 

determined by the maximum service voltage allowed at the first customer on the 

circuit and the minimum service voltage allowed at the last customer on the 

circuit during maximum load demand. The latter is known as a voltage 

constrained distribution circuit.

On rural distribution systems, voltage regulation of primary distribution circuits is 

normally accomplished using voltage regulators at distribution substations and on 

distribution lines. Capacitors are applied both at substations and on distribution 

lines, in conjunction with voltage regulators, to obtain the maximum economic 

benefits in improving voltage regulation of the lines. However, capacitors cannot 

replace circuit voltage regulators in maintaining constant voltage at the service 

point. For voltage control, shunt capacitors are useful in boosting the line voltage 

only, whereas voltage regulators automatically adjust line voltage to within 

prescribed voltage limits as load conditions vary.

(1) Voltage Spread. The range of voltage variation allowable at a strategic 

point of a distribution system is referred to as the voltage spread. In 

general, the strategic points in a distribution system where voltages are 

allowed to vary within designated voltage spreads are: (1) substation bus, 

(2) primary terminals of distribution transformers, (3) service points 

(service entrance) of consumers, and (4) utilization points (e.g., outlets) at 

consumers’ premises. The voltage spread at the consumer service point is 

important to the overall design and operation of a distribution system.

(2) Voltage Drop and Regulation

Voltage drop is an important element in distribution circuit design. 

Maintenance of adequate voltage levels throughout a distribution circuit 

requires holding voltage drops on sections of the circuit to within 

acceptable limits as described in Table 4 of this Bulletin. By definition, 

voltage drop is the difference between the voltages at the sending and 

receiving ends of a feeder, main, or service.

3

Voltage regulation is a measure of performance of a distribution circuit 

with respect to voltage drop. The voltage regulation of a distribution 

circuit is the ratio of the voltage drop of the line to the load voltage which 

can be presented in percent regulation. The formulation for calculating 

voltage drop and voltage regulation of a simple circuit is presented in 

Appendix A.
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(3) Voltage-Limited Distribution Circuit Design

The primary considerations in designing a voltage-constrained distribution 

circuit are the allocated voltage drops and voltage spreads at strategic 

points of the circuit for the first and last consumers. The aggregate 

voltage drop between the substation bus and the first or last consumer 

includes voltage drops in the primary circuit, distribution transformer, 

secondary circuit, and service point. As shown in Table 4 of this Bulletin, 

the allocated voltage drops allow a 12-volt or 10% voltage drop maximum 

between the regulated bus and service point of the last consumer on the 

circuit. Since the voltage drop from the substation bus to the service point 

of the last consumer is at maximum, the primary design consideration is to 

maintain a service voltage no less than the minimum limit of 114 volts. 

On the other hand, due to the minimal voltage drop between the substation 

bus and the service point of the first consumer, the primary design 

consideration is to maintain a service voltage no more than the allowed 

maximum limit of 126 volts.

Due to variation in loads, load swings between peak and light load 

conditions are important considerations in a voltage-constrained design of 

a distribution circuit. The last consumer on the circuit is most affected by 

voltage swing. It is important that the voltage change or spread is limited 

to the allowed value. As shown in Table 3 of this Bulletin, the maximum 

voltage spread recommended by RUS for consumers to experience is 8 

volts at the consumer’s service point. 

In addition to voltage drop and voltage spread considerations, the 

following points have to be considered in designing a voltage-constrained 

distribution circuit.

(a) There are several methods for improving voltage levels on a 

distribution circuit other than the use of voltage regulators. The 

following voltage level improvement methods should also be 

examined for feasibility and economics.

(1) Construction or extension of express feeders (that serve no 

transformers or tapped transformers along their route) to 

the circuit’s load center or first service transformer, 

(2) Converting single-phase lateral branch circuits to threephase circuits,

(3) Reconductoring of circuits using larger conductors, 

(4) Rearrangement of circuits,

(5) Balancing loads (taps and/or transformer connections) on 

three-phase lines,

(6) Installation of capacitors,

(7) Construction of new substation or distribution feeders, and 
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(8) Converting to a higher distribution voltage.

(b) The voltage range of substation voltage regulators is limited by the 

allowable primary voltage levels of the first distribution 

transformer.

(c) Distribution circuits have to be designed to maintain acceptable 

voltage levels per ANSI C84.1-2016. Future load growth also has 

to be taken into consideration.

(d) The allocation of voltage drops between the primary circuits, 

distribution transformers, secondaries, and services have to be 

optimized with the over-all cost.

4 VOLTAGE REGULATORS

Voltage regulators are the most commonly used regulating equipment on distribution 

systems. The function of a voltage regulator is to measure the line voltage and 

automatically adjust the voltage to a preset level. Voltage regulators are effective in 

automatically correcting high-voltage and low-voltage conditions. They are simple to 

apply and they provide voltage correction, within close tolerances, needed to maintain 

desired voltage levels.

a Types of Voltage Regulators. There are two general types of voltage regulators: 

(1) induction-voltage regulators and (2) step-voltage regulators. For all practical 

purposes, the induction-voltage regulator has been replaced by the modern stepvoltage regulator. Hence, the following discussion will focus only on stepvoltage regulators.

(1) Step-Voltage Regulators

A step-voltage regulator is “a voltage regulator in which the voltage and 

(or) phase angle of the regulated circuit are controlled in steps by means of 

taps without interrupting the load.”4 There are two types of step-voltage 

regulators as defined by the ANSI: (1) Type A, also known as “straight” 

design, and (2) Type B, also known as “inverted” design. Schematic 

diagrams of Type A and Type B step-voltage regulators are shown in 

Figures 1 and 2, respectively, of this Bulletin.

(a) In a Type A step-voltage regulator, the primary circuit (source 

side) is directly connected to the shunt winding of the regulator 

(Figure 1). The symbol “S” represents the source-side terminal of 

the voltage regulator. The shunt winding is connected to the series 

winding via a reversing switch. The other end of the shunt 

winding is connected to ground or common conductor at terminal 

“SL.” With the reversing switch, the polarity of the induced
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voltage across the series winding (secondary voltage) can be 

reversed with respect to the polarity of the shunt winding voltage, 

allowing the secondary voltage to be subtracted from or added to 

the primary voltage.

FIGURE 1: Schematic Diagram of a Single-Phase,

Type A Step-Voltage Regulator.

In essence, the reversing switch allows the regulator to switch 

between buck and boost positions. The series winding, in turn, is 

connected to the regulated circuit, at terminal “L,” via an automatic 

tap-changing mechanism, composed of multiple taps, “two-finger” 

contacts, and bridging reactor, also known as preventive autotransformer. The multiple taps allow the raising and lowering of 

line voltage by increments or steps without interrupting 

consumers’ loads. Load interruption is prevented by using “twofinger” contacts which are separated in a manner that both 

“fingers” cannot break contact at the same time when changing 

taps. The function of the bridging reactor is to prevent a shortcircuit condition when both “fingers” are in contact with different 

taps or in bridging position. In a Type A step-voltage regulator, 

the core excitation varies because the shunt winding is connected 

across the primary circuit.

(b) In a Type B step-voltage regulator, shown in Figure 2, the primary 

circuit from source side is connected, via a bridging reactor and 

taps, to the series winding of the regulator. The series winding in 

turn is connected to the shunt winding through a reversing switch. 

The functions of the components of a Type B step-voltage 

regulator are similar to those of Type A. However, in a Type B 
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step-voltage regulator, the core excitation is constant because the 

shunt winding is connected across the regulated circuit.

FIGURE 2: Schematic Diagram of a Single-Phase,

Type B Step-Voltage Regulator.

(2) 32-Step Voltage Regulator

(a) Step voltage regulators are available as 32-step voltage regulators 

and 4-step voltage regulators. The 32-step voltage regulator is the 

most commonly used regulator on distribution systems in the 

United States. The application of 4-step voltage regulators on rural 

distribution systems is very rare. As a general rule, the 4-step 

voltage regulator is not used in substations. A 4-step regulator is 

designed to be used on a tap line when no special coordination is 

required with the upstream 32-step voltage regulator that is used to 

regulate the main feeder. For brevity purposes, only the 32-step 

voltage regulator is discussed in this bulletin.

(b) Most substation and downline step-voltage regulators are designed 

to correct bus or line voltage from 10% boost to 10% buck (i.e., 

±10%) in 32 steps. These regulators are generally known as 32-

step voltage regulators. The 32-step voltage regulator can be an 

ANSI Type A or Type B step-voltage regulator. As shown in 

Figures 1 and 2 of this Bulletin, the series winding of a 32-step 

voltage regulator has 8 taps. However, because of the reversing 

switch that allows the regulator to switch between boost and buck 

positions, the effective number of taps is 16. As the “fingers” of
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the contact mechanism of the bridging reactor move up and down 

the series winding, they alternately rest on the same contact 

(Figure 1) and in bridging position (Figure 2). These dual contact 

positions result in a total of 32 steps. Since the full voltage 

regulation range is 20%, each step, therefore, represents a 5/8%

voltage change.

(c) To perform automatic voltage control, the 32-step voltage 

regulator is equipped with voltage control circuit. The voltage 

controller allows the regulator to sense voltage changes on the 

substation bus or distribution line and adjust the voltage output 

accordingly by automatically changing the regulator’s taps. 

Modern step-voltage regulators are equipped with communications 

terminals for remote control options, either through a supervisory 

control and data acquisition (SCADA) system or via direct dialing.

b General Description

(1) KVA Rating

In selecting the proper step-voltage regulator for a specific application, the 

required kVA rating has to be determined and, to determine the kVA 

rating of a regulator, the following information has to be known:

Type of bus or circuit connection ( or connection) and whether 

single- or three-phase voltage regulators will be used,

Voltage rating of bus or distribution feeder,

Continuous load current or kVA rating of bus or distribution feeder, 

and

Amount of voltage correction required.

The kilovolt-amperes (kVA) rating of a single-phase, step-voltage 

regulator is the product of the continuous load current and the rated “raise” 

or “lower” voltage regulation range in kilovolts (kV), as expressed by the 

following formula:

Rated kVA = Continuous Load Current

(in Amps) × Regulation Range (in kV)

For example, the kVA rating of a 10%, single-phase voltage regulator 

required to supply a load rated at 100 amperes on a 7,620-volt line-toground single-phase circuit would be:

Rated kVA = 100 A × (0.10 × 7.620 kV) = 76.2 kVA
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Note that, if the rated current is not directly known and the kVA rating of 

the circuit is known, then the rated current can be calculated as follows:

Rated Current = Rated kVA Rated kV (line-ground)

for a single-phase circuit, or

Rated Current = Rated kVA {1.732 × Rated kV (line-line)}

for a three-phase circuit.

For a 10% single-phase, delta-rated, step-voltage regulator supplying a 

load rated at 100 amperes through a 13,800-volt, phase-to-phase 

distribution system, the kVA rating of the voltage regulator would be:

Rated kVA = 100 A × (0.10 × 13.8 kV) = 138 kVA

For three-phase step-voltage regulators, the kVA ratings are determined in 

similar manner as the kVA ratings of single-phase step-voltage regulators. 

The kVA rating of a three-phase step-voltage regulator is the product of 

the rated load current and the rated range of regulation in kilovolts (kV) 

multiplied by 1.732. Three-phase voltage regulators can be applied either 

to three-phase/three-wire or three-phase/four-wire circuits.

In cases where the ranges of regulation are different for the “raise” range 

and the “lower” range, the larger percent regulation is used in determining 

the regulator kVA rating. The preferred kVA ratings for step-voltage 

regulators are based on operation at a frequency of 60 Hz and a voltage 

range of 10% raise and 10% lower. In addition to the preferred kVA 

ratings, step-voltage regulators have to deliver rated kVA output without 

exceeding the temperature rise at the given operating voltage per ANSI 

C57.15-1986.

(2) Extended Range of Ratings

One of the inherent features of a step-voltage regulator is that regulator 

losses decrease as the regulator moves from the extreme tap positions 

(boost or buck) closer to the neutral point. As a result, the range of 

voltage regulation required need not always be a full ±10%. This allows 

for an extended range of regulator operation.

In instances where less than the full 10% regulation is needed, the loadcarrying capabilities of the step-voltage regulator can be extended. 

Single-phase, step-voltage regulators with rated voltage up to 19.92 kV 

and rated current at 668 A and below have extended continuous current
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ratings on intermediate ranges of steps as shown in Table 6 in this 

Bulletin, not to exceed 668A.

TABLE 6: Recommended Continuous-Current Ratings on Intermediate 

Steps of Voltage Regulation for 19.92 kV and Below Single-Phase StepVoltage Regulators.

4

Range of Voltage Regulation 

(Percent)

Continuous-Current Rating 

(Percent)

± 10.00 % 100 %

± 8.75 % 110 %

± 7.50 % 120 %

± 6.25 % 135 %

± 5.00 % 160 %

Three-phase, step-voltage regulators with rated voltage up to 13.8 kV and 

rated current at 668 A and below have extended continuous current ratings 

on intermediate ranges of steps as shown in Table 7 in this Bulletin, not to 

exceed 668 A.

TABLE 7: Recommended Continuous-Current Ratings on Intermediate 

Steps of Voltage Regulation for 13.8 kV and Below Three-Phase StepVoltage Regulators4

Range of Voltage Regulation 

(Percent)

Continuous-Current Rating 

(Percent)

±10.00 % 100 %

± 8.75 % 108 %

± 7.50 % 115 %

± 6.25 % 120 %

± 5.00 % 130 %

It can be seen from Tables 6 and 7 that step-voltage regulators can extend 

their continuous-current rating up to a maximum rating of 668 A when set 

below 10% voltage regulation. For example, the continuous-current rating 

of a 7.62-kV single-phase voltage regulator rated at 438 A and set at 

±5.00% voltage regulation will raise its loading capability to 668 A.

Due to the increased loading capability or “load bonus” of voltage 

regulators, investment cost can be reduced by using lower-rated 

regulators. However, before taking this approach, care has to be taken to 

ensure that opportunity for future load growth, flexibility under sudden 

load changes, and proper voltage regulation in cases of emergency are 

considered.

(3) Terminal Designations
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The terminal designations of single-phase and three-phase step-voltage 

regulators are shown in Figure 3 in this Bulletin. For single-phase stepvoltage regulators, the terminal connected to the load is designated L, the 

terminal connected to the source is designated S, and the common terminal 

is designated SL. For three-phase step-voltage regulators, these 

identifications are S1, S2, S3, L1, L2, L3, and S0L0.

Single-Phase Step- Three-Phase StepVoltage Regulator Voltage Regulator

FIGURE 3: Step-Voltage Regulator Terminal Designations.

(4) Regulator Standards

Regulators for use on RUS borrowers’ systems have to comply with the 

requirements of ANSI C57.15-2009, “IEEE Standard Requirements, 

Terminology, and Test Code for Step-Voltage and Induction-Voltage 

Regulators.”

(5) Short-Circuit Requirements

Step-voltage regulators used on RUS borrowers’ systems must be 

designed and constructed to withstand root-mean-square (rms) 

symmetrical short-circuit currents of 25 times the regulator full-load 

currents for 2 seconds and 40 times the regulator full-load current for 0.8 

seconds without damage.

4

Voltage regulators, especially those installed at substations, are subjected 

to high short-circuit requirements. Care must be exercised in regulator 

applications to ensure that the short-circuit capabilities of the regulator are 
Bulletin 1724D-114

Page 16

not exceeded. The following procedure must be observed in applying 

regulators on a rural distribution system.

(a) Determine the short-circuit duty at the proposed location in terms 

of available fault current and clearing time of the over-current 

protective device, and

(b) If the short-circuit duty on the regulator exceeds its capabilities, 

then one of the following steps must be taken:

(1) Install a larger voltage regulator to obtain greater shortcircuit withstand capabilities,

(2) Install the voltage regulator at a distance far enough from 

the substation to limit the duty at the regulator to safe 

values, or

(3) Install current limiting reactors at the substation to limit the 

available fault current.

c Step-Voltage Regulator Control

(1) Basic Control Operations. In order to automatically and continuously 

correct bus and circuit voltages according to pre-determined voltage 

levels, step-voltage regulators are equipped with automatic controls. In 

general, the automatic control operation of a step-voltage regulator 

involves: (1) voltage-sensing, (2) switching and time delay, and (3) 

driving tap-changing motor. These control functions are accomplished 

with the control components shown in Figure 4.

FIGURE 4: Step-Voltage Regulator Control Components.

(2) Potential and Current Transformers. A potential transformer (PT) is 

installed at the output of the step-voltage regulator (L terminal). The PT 

steps down the actual line voltage to a 120-volt base voltage. The reduced 

voltage at the output of the PT is fed to the line drop compensator (LDC) 

and to the voltage-sensing circuit (VSC). The current transformer (CT) is 

typically internal to the step voltage regulator. It is a toroid through which 

the load current passes. It provides a source of current proportional to 

load current for line-drop compensation calculation and metering 

functions.
Bulletin 1724D-114

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(3) Line Drop Compensator

(a) When a step-voltage regulator is used to regulate the voltage of a load 

center located at some distance, the load cannot be served at the 

desired voltage level due to the line losses between the regulator and 

the load. As the load increases, line losses also increase, causing 

lowest voltage condition during heaviest loading. This means 

consumers at the end or near the end of the line could experience 

undervoltage at times when the distribution system is heavily loaded.

(b) A line drop compensator (LDC) is incorporated in the regulator 

controls in order for the voltage regulator to maintain a constant 

voltage at a “projected” point on a distribution line. The LDC circuit 

is composed of a CT and adjustable resistive and reactive elements. 

The resulting CT current is passed through the resistive and reactive 

elements and produces voltage drops that simulate the voltage drops 

on the primary line between the regulator and the regulated point down 

line. Hence, LDC allows the voltage regulator to extend its voltage 

regulation capability to a distant part of the distribution line. When 

properly applied, the LDC can distribute the voltage variations equally 

between the first and last customer on the distribution feeder.

(c) The LDC circuit is connected with the PT such that the simulated LDC 

voltage drop is vectorially subtracted from the PT voltage. The 

resultant voltage is then fed to the VSC. A simplified diagram of the LDC 

and VSC connection is shown in Figure 5.

Sub. Bus

FIGURE 5: Simple Schematic Diagram of LDC and VSC.

(4) Setting of Line Drop Compensator. The voltage boost produced by a line 

drop compensator (LDC) is a function of the settings of the resistance, R, 

and reactance, X, controls on the control panel and of the amount of the 

line current flowing through the regulator. The LDC settings are 
Bulletin 1724D-114

Page 18

accomplished at the control panel by means of the R and X dials. In 

modern regulator controls, the R and X voltage settings are adjusted by 

pressing the corresponding LDC function codes on a keypad on the 

control panel. The R and X voltage settings have to correspond to the 

amount of resistive and reactive voltage drops expected on the distribution 

circuit at peak load condition. There are several methods for determining 

the proper setting of load drop compensators. Some of these methods are 

discussed below.

(a) Traditional LDC Setting Method

Traditionally, the LDC settings are set to produce a 4-volt rise 

above the 122-volt set voltage during the annual peak load. To 

attain the 4-volt rise, the R and X control settings have to be such 

that R and X voltage values vectorially add up to 4 volts when the 

load current is equal to the current rating of the voltage regulator. 

In most cases, it is desirable to maintain a ratio between the R and 

X control settings equal to the R/X ratio of the actual distribution

line. Although this ratio varies with the size and type of 

conductors used, for most distribution circuits a 5/3 ratio is 

satisfactory for copper conductors and most small non-copper 

conductors. Breaking down the 4 volts vectorially on this basis 

results to approximately 3.5 volts for R and 2 volts for X. These 

are the recommended R and X control settings if the annual peak 

load current is equal to the current rating of the regulator. Since 

the annual peak load current is not often equal to the regulator’s 

current rating, it is necessary to calculate the R and X set values for 

the estimated peak current. This is done in the following steps:

(1) Determine the current rating of the regulator. This is 

shown on the voltage regulator nameplate as the current 

transformer primary rating.

(2) Estimate the peak load current expected during a time 

period between adjustments of voltage regulator controls. 

Regulator control adjustments are typically completed 

annually, but could be shortened to semi-annually or 

quarterly when there is a significant increase in seasonal 

peak demand or the load is increasing rapidly.

(3) Divide the current rating by the peak load current. 

Multiply the result by 3.5 to obtain the R setting and by 2 to 

obtain the X setting. For example, if the voltage regulator 

rating is 150 A and the estimated peak current of the line is 

80 A, the R and X settings can be estimated as follows:
Bulletin 1724D-114

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K = 150 A ÷ 80 A = 1.87

RSET = 3.5 × K = 6.6 volts

XSET = 2 × K = 3.75 volts 4 volts

(Note: X setting is adjustable in 1-volt step only.)

(b) Load Center LDC Setting Method

When it is desired to hold a constant voltage at some specific spot 

on a distribution line (such as at a small town or industrial load), 

the R and X settings are based on the actual resistance and 

inductance of the distribution line between the voltage regulator 

and projected load center. In this case, the determination of the R 

and X settings is achieved by observing the following steps.

(1) Calculate the R and X values of the distribution line by 

multiplying the per mile resistance and inductive reactance 

values by the actual line distance between the voltage 

regulator and load center.

(2) From the nameplate of the voltage regulator, determine the 

current transformer primary rating and potential 

transformer ratio.

(3) Calculate the R and X voltage values using the following 

equations:

RSET = (CTR/NPT) × RL (volts)

XSET = (CTR/NPT) × XL (volts)

Where:

RSET = R voltage setting (dial or function code), in volts

XSET = X voltage setting (dial or function code), in volts

CTR = Current transformer’s primary current rating, in 

amperes

NPT = Power transformer turns ratio, VPRI /VSEC

RL = Effective resistance of a feeder conductor from 

regulator to regulation point or load center

XL = Effective reactance of a feeder conductor from 

regulator to regulation point

For example, if the distance between the regulator and the load is 

6.2 miles on a No. 2 copper equivalent line, the current transformer 
Bulletin 1724D-114

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primary rating is 150 A, and the potential transformer ration is 

60:1, the R and X settings can be calculated as follows:

R per mile for No. 2 copper = 0.885 ohm/mile

X per mile for No. 2 copper = 0.756 ohm/mile

RL = 0.885 ohm/mile × 6.2 miles = 5.5 ohms

XL = 0.756 ohm/mile × 6.2 miles = 4.7 ohms

RSET = (CTR/NPT) × RL = (150 A/60) × 5.5 ohms = 13.7 volts

XSET = (CTR/NPT) × XL = (150 A/60) × 4.7ohms = 11.8 volts

(Note: This method is only applicable when the regulator solely 

serves the line for which the calculations are made. It cannot be 

used for setting substation regulators serving more than one 

circuit.)

(c) Voltmeter LDC Setting Method

The voltmeter method is a simple method for determining R and X 

settings of the LDC when the load on the regulator is at or very 

near the anticipated peak. The determination of the R and X 

settings is achieved by observing the following steps.

(1) With the R and X values set at zero, connect an accurate 

voltmeter to the voltage test terminals of the control panel 

of the voltage regulator. Verify that the output voltage is 

122 volts.

(2) Slowly raise the R and X control settings (approximately in 

5:3 ratio) until the voltmeter reads 126 volts.

With proper care, this method may be used with less than peak 

load on the regulator if the ratio of present load to peak load is 

known. For example, if the present load is one-half of the 

anticipated peak load, the R and X values have to be set to display 

a voltmeter reading of 124 volts, which is a boost of 2 volts or onehalf the desired peak load boost of 4 volts.

(5) Voltage-Sensing Circuit. The voltage-sensing circuit (VSC) provides a 

reference standard for the regulator voltage control. The VSC receives the 

resultant voltage from LDC and power transformer (PT) to determine if 

the voltage is beyond the VSC’s preset limits. If the resultant voltage is 

higher or lower than the preset voltage limits, the VSC then sends a signal 

to a time-delay circuit which activates the operation of a timer. The timer 
Bulletin 1724D-114

Page 21

provides a time delay between the VSC action and the actual tap changing 

action. In essence, the VSC controls the motor-operated switching 

mechanism that selects the proper tap to “raise” or “lower” regulator 

output voltage through the action of the timer. There are two settings that 

are critical to the proper operation of VSC: (1) Set Voltage and (2) 

Bandwidth.

(a) Set Voltage

(1) The set voltage is the voltage level to which the regulator 

control will regulate on the 120-volt base. It is this voltage 

with which all other voltages on the system are compared. The 

set voltage has to be as close to the ideal system voltage as 

possible. VSC can generally be set from 105 volts to 135 volts 

in one-volt increments. Generally, the voltage setting is based 

on the voltage limits delivered to the consumer’s meter, which 

is 114 volts to 126 volts as set forth in the ANSI Standards.

(2) Selecting the mid-point of this range, 122 volts, would 

essentially provide every consumer downline the appropriate 

voltage service at light loads. Since line current is minimum at 

this operating condition, the voltage drop is also minimal on 

the line. The ideal set voltage value, therefore, is 122 volts. In 

some instances where LDC is not used, the set voltage may be 

held at 125 volts with a ±1-volt bandwidth.

(3) The voltage sensing circuit (VSC) receives the resultant 

voltage of the LDC and PT outputs. If the resultant voltage 

received by the VSC is 4 volts less than the set value of 122 

volts, for example, the VSC reacts accordingly and initiates tap 

changing actions of the regulator until the 122-volt set voltage 

is sensed. The tap changing action will, therefore, result in an 

actual regulator output of 126 volts.

(b) Bandwidth

(1) The sensitivity or voltage tolerance of the VSC is known as the 

bandwidth. The bandwidth is the area above and below the set 

voltage level (e.g., 122 volts) within which the regulated 

voltage may fluctuate. It is the difference between the 

maximum and minimum allowable voltages in the regulator 

control. As an example, a 2-volt bandwidth on a 122-volt 

setting means the time-delay circuit will not activate the 

operational timer (time delay) until the voltage is below 121 

volts or above 123 volts.
Bulletin 1724D-114

Page 22

(2) The selection of a narrow bandwidth will cause more tap 

changes to occur, but will provide a more tightly regulated line. 

Conversely, a wider bandwidth results in fewer tap changes, 

but at the expense of better regulation. Selection of the 

bandwidth settings has to be made recognizing the trade-off 

between better regulation and longer equipment life. The 

recommended bandwidth for RUS borrowers is 1.5 volts (±3/4 

volt) or 2 volts (±1 volt).

(6) Time Delay Circuit. The primary function of the time-delay circuit in the 

regulator control is to interject a preset “waiting period” or time delay into 

the tap-changing operation of the regulator. The time-delay circuit has a 

relay that activates the tap-changing mechanism after the time delay 

period expires. The preset time delay prevents the voltage regulator from 

responding to momentary voltage excursions that are outside the 

bandwidth limits. Responding to every momentary voltage flicker would 

result in significant life reduction of the tap-changing mechanism.

(a) Time Delay Setting

(1) The time delay relay, set in the control panel, provides a 

predetermined response time for the regulator. When two 

voltage regulators are used on a circuit, their time delays have 

to be coordinated such that the substation voltage regulator

operates first before the downstream regulator. This time delay 

coordination strategy allows the substation regulator to respond 

to changes before the line regulator and prevent unnecessary 

corrective actions by the line regulator. As a general rule, the 

time delay setting for substation regulators is 30 to 40 seconds. 

If excessive regulator operations are encountered, the setting 

may be readjusted up to 60 seconds, maximum.

(2) The time delay for line regulators has to be longer than that for 

substation regulators. Generally, the time delay of a line 

voltage regulator has to be longer by at least 15 seconds than 

that of the substation voltage regulator. By coordinating the 

time delay in this manner, the substation regulators are allowed 

to correct source voltage variations before line regulators 

operate. When more than one line regulator is used (see the 

last paragraph of Section 6a of this Bulletin), the time delay 

setting of the voltage regulator farthest from the substation has 

to be the highest.

(b) Bandwidth vs. Time Delay Settings
Bulletin 1724D-114

Page 23

(1) The number of tap changing operations which a regulator 

performs has a bearing on both the life of the regulator and on 

the interval between scheduled inspection and maintenance. 

The number of operations which a regulator will make during a 

given period is determined by the characteristics of the supply 

voltage and the load, and by the bandwidth and time delay 

settings of the regulator. A regulator operated under typical 

conditions will probably make, on the average, 100 to 200 tap 

changes per day. If the daily average number of operations 

falls much below 100, the regulator is possibly not used 

effectively. If the average tap changes per day are more than

200, additional maintenance service on the regulator would be 

required.

(2) An increase in time delay or bandwidth will tend to reduce the 

number of regulator operations. Of the two approaches, 

increasing time delay reduces the number of tap-changing 

operations with the least sacrifice in voltage control. Examples 

of these approaches are shown in Figure 6.

FIGURE 6: Effects of Time Delay and Bandwidth Settings on 

Voltage Regulator Output

(3) As shown in Figure 6, the graph of the regulated output voltage 

for Case I (fixed bandwidth with zero time delay) depicts seven 
Bulletin 1724D-114

Page 24

tap-changing operations. Note that each tap-changing

operation is characterized by an abrupt drop in voltage level.

(4) As can be seen with Cases II and III, the number of tapchanging operations decreases as the time delay settings 

increase. Case IV shows that the number of regulator 

operations can be decreased by increasing the bandwidth. 

However, by increasing the bandwidth as clearly shown in the 

Case IV graph, voltage limits can be violated without being 

detected and corrected. Hence, the voltage quality is sacrificed 

in favor of reducing regulator operations.

(7) Range of Regulation Setting

(a) The voltage regulation range is set at the regulator position indicator,

which is generally mounted on the regulator tank. Setting is 

accomplished by adjusting the limit switches which stop the travel of 

the tap changer in either direction. These limit switches are located in 

the position indicator of the regulator, and their adjustments to provide 

the desired range of regulation is a simple matter (differing somewhat 

among the different brands of regulators).

(b) At nameplate rating, a regulator will deliver rated current while either 

boosting or bucking the incoming voltage 10%. Reducing this 

regulation range, say 5%, will allow the regulator to carry more than 

the rated current. This extended range of load carrying capability at 

reduced regulation can play an important role in selecting the size of a 

voltage regulator to be used in a particular application.

(8) Tap-Changing Mechanism

(a) When the load voltage goes out-of-band, the time delay circuit is 

activated. At the end of the time delay (e.g., 30 seconds), the output 

signal of the time delay circuit is relayed to the tap-changing 

mechanism, which, in turn, raises or lowers the tap by one step. After 

a tap change, a time period or pause is provided before the operational 

timer is reset. The pause, which is typically a duration of 2 seconds, 

allows the regulator control to sample the load voltage and determine 

if it is already in-band or still out-of-band. If the load voltage is inband, as a result of the tap change, the operational timer is reset. If the 

load voltage is still out-of-band, a control signal is relayed to the tapchanging mechanism to perform another tap change. The sequence is 

continued until the load voltage is in-band.

(b) There are two types of tap changers: spring-drive and direct-drive. 

For low-current applications, stored-energy spring-drive tap changers 
Bulletin 1724D-114

Page 25

are commonly used. Spring-drive tap changers are equipped with 

capacitor-run, reversing gear-motor suitable for operation at 120 V ac, 

single-phase, 60 Hz. For medium- and high-current applications, 

direct motor-driven tap changers are used. Direct-drive tap changers 

are equipped with capacitor-start, capacitor-run, high-torque, and 

reversing gear motors rated at 120 V, single-phase, 60 Hz.

d Modern Regulator Controls. Modern regulator controls incorporate digital logic 

and microprocessor technology. The microprocessors used in a modern regulator 

control typically include an electrically erasable and programmable read-only 

memory (EEPROM). The EEPROM has a special memory storage where critical 

information, such as control settings and metering values, is stored indefinitely 

even during a power outage. Instead of dials and analog meter displays, the 

modern regulator control is equipped with liquid crystal displays (LCD) for 

displaying of control settings, accessory settings, metering values, and 

annunciator information. The regulator control is also equipped with an analogto-digital converter that converts the analog voltage and current waveforms into 

digital signals. Digital signal processing techniques are applied to enhance the 

resolution of voltage and current signals. Modern voltage regulator controls are 

equipped with remote metering and control capabilities. These controls can 

communicate with a supervisory control and data acquisition (SCADA) system. 

Because of these added control capabilities, voltage regulators are used in other 

distribution operation functions, such as voltage reduction to support load 

management, voltage limiting, remote monitoring, and telemetering.

(1) Voltage Reduction. Distribution voltage regulators are ideal devices for 

supporting distribution system load management. In this application, 

voltage regulators are manipulated to force the distribution system to 

temporarily operate at lower voltages. The reduction of system voltage 

reduces line losses and, thus, helps alleviate distribution system overloads 

and minimizes peak load demand. The voltage reduction function of the 

control is generally automatic. However, the automatic voltage reduction 

mode can be overridden by an operator taking local control, by a remote 

operator, by a SCADA system, or by the regulator’s voltage limiter. 

When the control receives a voltage reduction signal, it triggers the 

voltage regulator to reduce the set voltage based on a preset percent 

voltage reduction. The voltage reduction control works by calculating an 

effective set voltage as follows:

Eff. VSET = VSET × (1 - %R)

Where:

Eff. VSET = Effective set voltage (after voltage reduction is applied)

VSET = Regulator set voltage

%R = Percent voltage reduction
Bulletin 1724D-114

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As an example, if the set voltage is 122 V and the percent voltage 

reduction is 4.9%, the effective set voltage would be 116 V, i.e., a tap 

down of 6 V.

(2) Voltage Limiting. The voltage limiting capability of a modern voltage 

regulator control is used to place an upper and lower limit on the output 

voltage of the regulator. The purpose of the voltage limiter is to protect 

consumers from abnormally high or low voltages resulting from various 

causes, such as rapid changes in transmission voltage, abnormal loading of 

the feeder, and inaccurate voltage regulator settings. The voltage limiting 

function is automatic; however, it can be overridden by local and remote 

control or by an interconnected SCADA system.

(3) Remote Control and Telemetering. Modern voltage regulator controls 

have remote access capabilities that allow users to monitor, control, and 

set voltage regulators from a remote location. The control is equipped 

with analog and digital communications ports that allow interconnection 

with the utility’s SCADA system.

Due to the strategic placement of voltage regulators within a distribution 

system and the availability of PTs and CTs, voltage regulators are ideal 

points for remote monitoring and telemetering. Advanced monitoring and 

telemetering functions could include tap position indication and metering 

of load voltage, source voltage, compensated voltage, load current, line 

frequency, load kVA, load kW, and load kvar.

5 SUBSTATION VOLTAGE REGULATORS AND SYSTEM VOLTAGE LEVELS

a Substation Voltage Regulators. Voltage regulators are extensively used in rural 

electric systems to provide voltage control on distribution feeders or circuits. For 

substation applications, the standard practice on rural electric systems is to use 

substation voltage regulators in bus regulation rather than in individual feeder 

regulation. Bus regulation provides practical benefits in terms of lower cost, ease 

of operation, and less space requirements than feeder regulation. Bus voltage 

regulators are used to regulate the voltage level of the distribution substation 

secondary bus which supports multiple distribution feeders. In this application, 

voltage regulators are generally set to provide bus voltage at the maximum 

allowable voltage setting, e.g., 125 volts (on a 120-volt base) with ±1-volt 

bandwidth. In cases where the projected load center is located far from the 

substation, the load drop compensator (LDC) is normally used. Individual feeder 

regulation at the substation is only used on a few rural electric systems. Although 

this practice is not as popular as bus regulation, regulating the circuits 

individually provides greater flexibility in the overall voltage regulation of each 

circuit. With individual feeder regulation, the LDCs of substation voltage 

regulators can be employed to maximize the effect of the regulators. The 

principal drawback of feeder or circuit regulation is the cost of using multiple 
Bulletin 1724D-114

Page 27

banks of voltage regulators instead of one bank as in bus regulation. In bus 

regulation, single-phase or three-phase regulators can be used. Single-phase, 

step-voltage regulators are the most commonly used substation regulators on rural 

electric systems. However, in large distribution substations, typically 20 MVA 

and above, bus regulation in distribution substations is accomplished by using 

transformers with load-tap-changers. In selecting substation bus regulation 

equipment, there are three choices: (1) single-phase regulators, (2) three-phase 

regulators, and (3) load tap-changers built into the power transformer. The 

following are several factors that could influence the selection.

(1) For the typical rural distribution substation size, single-phase regulators 

are more popular than three-phase regulators and LTCs. Using singlephase regulators allows flexibility in maintaining balanced phase voltages 

under conditions of unbalanced loading.

(2) In large distribution substations, the choice of three-phase regulators may 

be based on costs or on the non-availability of single-phase regulators of 

the required size. The primary advantage of using three-phase regulators 

is the relatively smaller space requirement.

(3) Power transformers with under load tap changers (LTC) are being used 

more in large distribution substations. Basically, they consist of a threephase regulator built into a three-phase power transformer. The relative 

cost of this combination—compared to a separate transformer and voltage 

regulators—varies depending on the size of the substation. Aside from the 

base cost of the equipment, the LTC method of voltage regulation could 

result in savings from lesser requirements in space, buswork, and 

supporting structures. Failure of the LTC will likely render the transformer 

bank inoperative, requiring an outage to repair the LTC.

b System Voltage Profiles. When a distribution line is initially energized, the 

maximum drop along the line is generally less than the full 8-volt drop permitted on 

primary lines. This allows for future load growth to develop in a normal manner. When 

such a line is supplied with a constant (regulated) bus voltage at its source, its voltage 

profile resembles that of curve A of Figure 7. With load growth, the voltage drop along

this line will increase, causing line voltages to eventually fall below the minimum limit as 

shown by curve B. Continued load growth beyond this point will result in excessive line 

voltage drop, greater than 8 volts on the primary, for some sections of the line. This is 

shown by curve C. Curve B demonstrates why voltage regulators alone, that regulate 

system voltage without consideration of system loading, are not sufficient for maintaining 

proper voltage levels.
Bulletin 1724D-114

Page 28

 FIGURE 7: Line Voltage Profile.

(1) Although the voltage drop represented by curve B is not greater than the full 

8-volt limit permitted on primary lines, the voltage level for some segments of 

the line may still be below the minimum recommended line voltage. 

Distribution transformers served from these portions of line will have 

insufficient input voltage to their primary terminals and the consumers 

probably will have low voltage at their point of utilization. This results from 

the fact that, at peak load conditions, the full 8-volt voltage drop is based from 

a fixed substation bus voltage, regulated independently of loading.

(2) To be effective, system voltage regulators have to do more than provide a 

constant source voltage. They regulate the source voltage such that the 

normal distribution line voltage drops are maintained during peak load and 

light load conditions.

c Effects of Line Drop Compensator. The line drop compensator (LDC) of a 

voltage regulator gives the latter the capability of regulating line voltage in 

response to changing load conditions. This capability allows the regulator to be 

more effective in maintaining a constant voltage output throughout all types of 

load conditions. An example of a voltage profile of a bus-regulated line at peak 

load condition without the application of LDC is shown in Figure 8.
Bulletin 1724D-114

Page 29

 FIGURE 8: Line Voltage Drop at Peak Load

Using Substation Regulator Without LDC

(1) In this example, the regulated substation bus voltage is held to a constant 122 

volts with a bandwith of ±1 volt. Notice that restricting the primary voltage 

drop to 8 volts maximum at peak load, while holding the substation output 

voltage constant, does not prevent segments of the line from receiving less 

than the minimum recommended line voltage (118 volts).

(2) Figure 9 shows the same line except, in this instance, the load-sensing 

capabilities of the LDC are being utilized to monitor the line loading and to 

provide the necessary voltage adjustment to the regulator output. This 

adjustment is in proportion to the amount of line voltage drop caused by the 

loading.
Bulletin 1724D-114

Page 30

 FIGURE 9: Line Voltage Drop at Peak Load

Using Substation Regulator with LDC

(3) During peak load conditions, the LDC adjustment is in the form of an increase 

in regulator output voltage at the substation bus. In effect, the regulator has 

increased its preset output voltage level from 122 volts to 126 volts, i.e., a 4-

volt line drop compensation is added to the regulator output. This regulator 

output voltage compensation allows the primary line to experience a full 8-

volt drop from 126 volts, without going below the minimum recommended 

line voltage level.

(d) Mitigating Voltage Swings. One of the inherent advantages gained by using the 

LDC is the reduction of voltage swings throughout the different parts of the 

distribution system. This can be illustrated by the example shown in Figure 9, 

above. Assume that the substation voltage regulator is set to provide a constant 

output voltage of 126 volts without line drop compensation. Under peak load 

conditions, the 126-volt setting allows an 8-volt drop on the primary distribution 

line without violating the minimum allowable voltage limit of 118 volts. During 

light load conditions, the voltage on the primary side of the last transformer would 

be close to the 126-volt setting at the substation bus. Hence, from peak load 
Bulletin 1724D-114

Page 31

conditions to light load conditions, consumers toward the end of the circuit could 

experience a voltage swing of up to 8 volts at utilization points.

(1) Without the LDC, consumers served from segments of the line closest to the 

substation will have nearly a constant voltage level, close to the 126-volt 

substation setting, while the consumers near the end of the distribution line 

experience maximum voltage swings at their utilization points. However, 

with LDC, these voltage swings can be minimized as shown by curve B in 

Figure 10.

 FIGURE 10: Voltage Spreads from First Consumer to Last Consumer.

(2) In the example, the voltage is set at 122 volts. During light load conditions, it 

is not necessary to increase the output voltage of the regulator because the line 

drop compensation is at minimum. At peak loading periods, the regulator 

control senses a voltage below 122 volts at the system load center and 

introduces an additive voltage correction to the 122-volt set voltage 

proportional to the voltage drop on the line from the voltage regulator output 

to the system load center.
Bulletin 1724D-114

Page 32

(3) Without the LDC (curve A), the maximum voltage deviation from the set 

voltage of 122 volts would be ±4 volts, or an 8-volt swing (or spread) from 

peak to light load conditions. The voltage swing is mostly experienced by 

consumers at the far end of the line. When LDC is properly employed, this 

deviation still remains ±4 volts except the +4 volts is experienced at the 

substation end and the 4 volts is experienced at the far end of the line. Thus, 

with the use of LDC, the maximum voltage swing of 8 volts at one end of the 

line is redistributed to a more balanced 4-volt voltage swing at each end of the 

line.

6 LINE VOLTAGE REGULATOR APPLICATION

a Line Voltage Regulators

(1) As the load grows and as the distribution line is extended, the voltage drop 

at the end of the line may exceed the 8-volt drop limit. The most common 

approach to maintaining the line voltage and getting it back to an 

acceptable level is by installing voltage regulators on the line. An 

example of a line voltage profile with excessive voltage drop is shown in 

Figure 11 of this Bulletin. In the example, the bus is regulated at 126 

volts. During peak load condition, the minimum voltage limit of 118 volts 

is exceeded at the primaries of distribution transformers from the middle 

to the end of the line. Consumers served by these transformers will 

experience undervoltage conditions. This voltage level violation is shown 

by the dashed line below the 118-volt line in Figure 11.

(2) If a voltage regulator is installed out on the line at the point where the 8-

volt drop occurs, the abnormal voltage levels will be corrected, as shown 

in Figure 11. The addition of the line voltage regulator at this point 

provides the same voltage control as the substation voltage regulator does 

upstream. The line voltage at this point is raised up to 126 volts. If LDC 

is used, the output of the voltage regulator will be 122 volts at light load 

condition and 126 volts at peak load condition.

(3) The use of line voltage regulators allows an extension of a distribution line 

without violating allowable voltage limits. It defers costly replacement or 

uprating of conductors. In some applications, two or more line-type 

voltage regulators are used in a series. Line-type step-voltage regulators 

are only available in single-phase units. These regulators are polemounted on overhead primary distribution circuits and pad-mounted on 

underground distribution lines.
Bulletin 1724D-114

Page 33

 FIGURE 11: Line Voltage Profile at Peak Load

 Using Substation and Line Voltage Regulators.

(4) Line-type voltage regulators are generally placed on the line where the 

voltage drop approaches 8 volts. More than one line-type voltage 

regulator may be used in a series on a distribution circuit. The decision to 

use more than one voltage regulator is based on economic considerations. 

RUS recommends that not more than two voltage regulators be installed in 

series on the same circuit. However, in emergency cases, and for 

temporary voltage correction downline, more than two cascaded voltage 

regulators may be used.

b When to Install a Line Voltage Regulator

(1) A voltage regulator should be installed on a distribution feeder when the 

actual or projected primary voltage of the feeder is approaching or has 

dropped below the allowable limit. Line voltage regulator location has to

take into consideration both present and anticipated future load conditions.

(2) Voltage profile studies on all distribution feeders have to be performed 

regularly as part of the Construction Work Plan. Unscheduled voltage 

studies should be promptly performed on distribution feeders that display
Bulletin 1724D-114

Page 34

voltage problems. Voltage profile analyses are normally conducted using 

commercially available computer software applications. Many of these 

computer programs can be run on a personal computer and are becoming 

more user-friendly.

(3) Voltage recording devices installed at critical points of distribution feeders 

and on consumer service points can be used to effectively supplement 

voltage profile studies. These devices provide recorded minimum and 

maximum voltage levels and the time periods they occur. Modern voltage 

regulator controls are equipped with profile recording devices that record 

the regulator’s voltage output into a microcomputer memory in the 

regulator’s controller, where this information can be downloaded locally 

or remotely. Such control is available on both substation and line-type 

voltage regulators.

c Where to Install a Line Voltage Regulator

In determining the actual distance from a regulated substation to install a line 

voltage regulator, the following criteria need to be addressed.

(1) Critical Loads. The location of the line regulator should benefit, if 

possible, voltage-sensitive or critical loads on the distribution feeder.

(2) Long Taps. A thorough evaluation should be made to determine whether 

to place the line voltage regulator before or after the tap point of a long 

branch line or lateral. Projected load growth of the branch line must be 

considered.

(3) Sectionalizing. The effects of sectionalizing and reclosing devices on 

voltage regulated circuits have to be taken into account. If the location is 

subject to a high degree of sectionalizing and recloser operations, seeking 

an alternative location could be an option.

(4) Accessibility. The location has to be readily accessible for ease of 

maintenance and adjustment of the voltage regulator.

d Line Voltage Regulator Settings

Once the need for a regulator is established and the site is selected (in terms of 

physical and electrical location), the next step is to determine the following 

regulator control settings:

Voltage level,

Bandwidth,

Time delay,

Regulation range, and
Bulletin 1724D-114

Page 35

Line drop compensation.

These setting requirements are the same as those of substation voltage regulators. 

The voltage level and bandwidth are normally set the same for all voltage 

regulators in the distribution system, while the settings for time delay, range of 

regulation, and line drop compensation vary with each application.

7 VOLTAGE REGULATOR INSTALLATION

a Regulator Connections for Single-Phase Circuit. 

A voltage regulator can be installed to regulate a single-phase circuit or on one of 

the phases of a three-phase circuit. Such applications are normally found in 

feeder regulation. A voltage regulator can be installed by mounting it on a pole or 

on a crossarm platform. Voltage regulator connections for single-phase circuit 

regulation, such as laterals, are shown in Figure 12 of this Bulletin. Connections 

for regulating one phase of a three-phase circuit, such as a main distribution 

feeder, are shown in Figure 13 of this Bulletin.

FIGURE 12: Regulator Connections for Regulating a Single-Phase Circuit.

5
Bulletin 1724D-114

Page 36

FIGURE 13: Regulator Connection for Regulating

One Phase of a Three-Phase/Four-Wire Circuit.

5

b Regulator Connection for Three-Phase Circuit. Three single-phase regulators are 

typically used for three-phase bus and feeder regulation applications on rural 

distributions systems. The typical connections of three single-phase voltage 

regulators on four-wire/wye-connected circuits are shown in Figure 14.

FIGURE 14: Wye Connections for Regulating a

Three-Phase/Four-Wire Circuit.

5

8 VOLTAGE REGULATOR AND CAPACITOR APPLICATIONS

a Application Considerations. Voltage regulators are sometimes used in 

conjunction with power correction capacitors to significantly improve voltage 

profiles of distribution systems. However, careful planning for their proper 

coordination is needed when using both voltage regulators and capacitors. 

Voltage regulators are used to increase voltage levels at specific locations on a 

distribution system. Capacitors are used to reduce line impedances by improving 
Bulletin 1724D-114

Page 37

power factor and, thereby, reduce the voltage drop on the line. Voltage regulators 

have integral automatic controls that actively and continuously monitor and 

correct voltage problems on distribution circuits. Capacitors, on the other hand, 

are passive devices that can be installed on a fixed or switched basis. Fixed 

capacitors are connected to distribution systems for a long duration, while 

switched capacitors are installed where they can be switched on- and off-line 

more than once a day. Because of their differences in function and operation, 

capacitors can easily interfere with voltage regulators.

(1) Capacitors banks are relatively inexpensive, easy to install, and require 

little maintenance. For this reason, they are popular for improving power 

factor, reducing current flow, and reducing voltage drops on distribution 

lines. Capacitor banks can be installed at the substation, at a midpoint, or 

at the end of a distribution line. The location of the capacitor bank is 

critical to its effectiveness in improving the voltage profile of the line. 

When on the same circuit as voltage regulators, capacitor locations have to 

be carefully considered in order to properly coordinate the functions of 

these two types of devices.

(2) Another important issue that has to be considered when installing 

capacitors on the same circuit as voltage regulators is the possible creation 

of a deleterious harmonic resonant condition created by distributed 

resistor-inductor-capacitor (RLC) line components. Such resonant 

conditions can also cause unwanted interference by inductively coupling 

into nearby paralleling telecommunications circuits, thereby creating 

public relations problems with consumers who own the 

telecommunications system. If the natural resonant frequency of the RLC 

line components coincide with any of the frequencies of the harmonic 

voltages normally occurring on the power line, overvoltages and 

overcurrents at this frequency can occur, resulting in overheating of 

conductors, transformers, and motors.

b Effects of Shunt Capacitors. Shunt capacitors are used extensively in distribution 

systems rather than series-connected capacitors. Although series capacitors 

directly compensate inductive reactance on a distribution line, they present 

problems that complicate the operation of a distribution line. Examples of such 

problems include ferroresonance in transformers, subsynchronous resonance 

during motor starting, shunting of motors during normal operation, and difficulty 

in protection of capacitors, all resulting in high system fault current condition.6 

For these reasons, series capacitors are not popularly employed in distribution 

systems. Therefore, the capacitors or capacitor banks that are described and 

discussed in this section are shunt capacitors.

(1) Shunt capacitors supply reactive power or current that counteracts the 

inductive power or current required by conductor inductances and inductive 

type loads. A shunt capacitor bank installed on a distribution circuit reduces
Bulletin 1724D-114

Page 38

the magnitude of the source current which consequently results in the 

improvement of the power factor and reduction of voltage drop. The 

reduction of resultant source current and, consequently, the voltage drop of 

the line are illustrated in Appendix B.

c Voltage Regulator and Shunt Capacitor Coordination

(1) When a capacitor bank is installed on the source side of a line or circuit 

voltage regulator, as shown in Figure 15 of this Bulletin, coordination will 

not be a problem between the capacitor bank and the regulator’s load drop 

compensator (LDC). The leading current drawn by the capacitor bank (IC)

counteracts the lagging line current, which results in reduced source 

current (IS) flowing from the source to the capacitor bank’s point of 

installation. The magnitude of current flowing from the capacitor bank to 

the load (IL) is the same as the magnitude of the load current when the 

capacitor bank was not present. Since the same current flows through the 

voltage regulator, no adjustment in voltage regulator settings will be 

necessary.

FIGURE 15: Capacitor Bank Located on the Source Side of Voltage Regulator.

(2) When the capacitor bank is located between the voltage regulator and 

load, such as shown in Figure 16 of this Bulletin, the current through the 

voltage regulator is different from the load current. This condition 

presents a problem in the proper operation of the voltage regulator. 

During peak load condition, the regulator may see the line voltage to be 

sufficient because of the reduced voltage drop between the source and the 

capacitor bank. However, the low voltage condition across the load is not 

reflected at the voltage regulator since the load current is different from 

the regulator current. Therefore, the voltage regulator does not properly 

react to the low-voltage condition at the load.
Bulletin 1724D-114

Page 39

FIGURE 16: Capacitor Bank Located on the Load Side of Voltage Regulator.

(3) To properly coordinate the voltage regulator when the capacitor bank is 

located between the regulator and the load, the regulator’s voltage setting 

has to be adjusted accordingly. If no other load is served between the 

regulator and the load or regulating point, the new set voltage of the 

regulator can be calculated using the following equation.

3

 VSET(NEW) = VSET(OLD) + CKVA . (X)(1 - L1 )

 KVA L

Where:

VSET(NEW) = New set voltage

VSET(OLD) = Old set voltage

CKVA = Capacitor bank rating

KVA = KVA rating based on regulator CT primary current

X = Old regulator LDC X-setting

L1 = Distance between voltage regulator and capacitor bank

L = Distance between voltage regulator and load or regulating 

point

(4) When the capacitor bank is located on the regulating point or projected 

load center, coordination between the capacitor bank and voltage regulator 

can be accomplished by adjusting the LDC’s R and X settings, taking into 

account the effect of leading current of the capacitor bank. The new R and 

X settings can be calculated by the following equations:

RSET(NEW) = (CTR/NPT) × REFF (volts)

XSET(NEW) = (CTR/NPT) × XEFF (volts)

Where:

(IL1 + IC)r1d1 + (IL2 + IC)r2d2 + ...

REFF = ________________________________________________________

(IL + IC)
Bulletin 1724D-114

Page 40

(IL1 + IC)x1d1 + (IL2 + IC)x2d2 + ...

XEFF = ________________________________________________________

(IL + IC)

RSET(NEW) = R voltage setting (dial or function code), in volts

XSET(NEW) = X voltage setting (dial or function code), in volts

CTR = Current transformer’s primary current rating, in amperes

NPT = Power transformer turns ratio, VPRI / VSEC

REFF = Effective resistance of a feeder conductor from regulator to 

regulation point or load center

XEFF = Effective reactance of a feeder conductor from regulator to 

regulation point

IL = Line current at regulator location

IC = Current drawn by capacitor bank current

IL1 = Current in first section of the line before the first load is tapped 

off

IL2 = Current in second section of the line before the second load is 

tapped off

r1 = Conductor resistance in ohms/mile of first line section

r2 = Conductor resistance in ohms/mile of second line section

x1 = Conductor reactance in ohms/mile of first line section

x2 = Conductor reactance in ohms/mile of second line section

d1 = Length of first line section in miles

d2 = Length of second line section in miles

d Coordination with Switched Capacitor Banks. Switched capacitor banks are 

more difficult to coordinate with voltage regulators. When the capacitor bank is 

switched on or off, the voltage regulator settings may have to be adjusted to 

perform properly. Since most of the settings are done locally, their adjustments 

and readjustments will not be practical if the capacitor bank is switched several 

times a day. Hence, this coordination problem has to be considered when 

selecting the type of capacitor to be installed.

Coordination with switched capacitor banks can be practical when the capacitor 

bank and voltage regulator are located close to each other. By interconnecting the 

two control circuits, correct voltage regulator operation can be attained 

throughout all load conditions. There are several possible methods for the voltage 

regulator control circuit to obtain the correct line drop compensation. Two of 

these methods are discussed below.

(1) A current transformer can be installed in the capacitor bank circuit and 

interconnected with the current transformer of the voltage regulator 

control circuit, such as shown in Figure 17 of this Bulletin. With the 

current transformer interconnection, the secondary capacitor current flows
Bulletin 1724D-114

Page 41

only between the two transformers and through the LDC R and X 

elements of the regulator control. In effect, the actual line drop 

compensation will result regardless of whether the capacitor bank is online or off-line.

FIGURE 17: Interconnection of Current Transformers.

7

(2) Another possible method is to insert an impedance in the regulator control 

circuit with a magnitude such that voltage drop across it is equal to the 

voltage rise in the LDC elements due to capacitor current, such as shown 

in Figure 18. An auxiliary contact is connected across the impedance Z so 

that when the capacitor bank is switched off line, the impedance is shorted 

out. The shorting contact is interlocked with the capacitor switch.

FIGURE 18: Alternative Interconnection of Current Transformers.

3
Bulletin 1724D-114

Page 42

9 VOLTAGE REGULATOR PROTECTION

a Surge Protection. Voltage regulators, like other elements of the distribution 

system, require protection from lightning and other high-voltage surges. Because 

voltage regulators are constructed like auto-transformers—having one of the 

windings in series with the primary line—additional protection is required for the 

series winding. Regulators are normally factory-equipped with by-pass arresters 

across their series windings. The arresters limit the voltage developed across the 

series winding during surges to keep them within safe values. However, bypass 

arresters protect only the series winding of the regulator and do not eliminate the 

need for distribution class arresters to protect the regulator itself. For protection 

of shunt windings, an optional shunt arrester may be used and mounted between 

the L bushing and ground. For additional protection, another shunt arrester may 

also be installed between the S bushing and ground.

b Reverse Power Flow Protection. Typically, voltage regulators are used on 

distribution circuits with well-defined power flow from source to load. Voltage 

regulators designed to operate only in one power flow direction will not operate 

as desired when the power flow direction is reversed. During a reverse power 

flow condition, the regulator will respond to a low-voltage condition by further 

lowering the output voltage and will respond to a high-voltage condition by 

further raising the output voltage. To prevent misoperation of a voltage regulator 

in the event of reverse power flow, the regulator control has to have a safeguard 

that senses the reverse power flow and automatically switches the regulator into 

idle or neutral mode.

Some regulator applications, however, may involve interconnected or looped 

circuits in which power is meant to flow through the regulator from the opposite

direction. For optimum distribution system performance, a regulator installed in 

such circuits has to have the capability of detecting reverse power flow, and of 

sensing and controlling the voltage regardless of the power flow direction. In 

order to accomplish automatic reverse operation, the regulator control has to be 

able to sense the source-to-load differential voltage in addition to load voltage. A 

source-to-load differential potential transformer (PT) or an external source-side 

PT will be required. A second transformer called the Ratio Correcting 

Transformer will also be required to properly correct the source voltage.

c Voltage Regulator Bypassing. It is sometimes necessary to take a regulator out 

of service for maintenance or replacement. It is very important that proper 

bypassing procedures are followed. Of primary concern is making sure that the 

bypass switch of a regulator is never closed without the regulator itself being in 

the “Neutral” position.

(1) When a regulator is in any position other than “Neutral,” there is a potential 

difference between the source and load terminals. Closing the bypass switch 

of a regulator that is off “Neutral” will cause a circulating current to flow 
Bulletin 1724D-114

Page 43

within the regulator that will likely cause failure of the unit. This current can 

easily reach magnitudes of several hundred or even thousands of amps. In 

many cases, the conductor connecting the regulator to the phase wires will 

physically move back and forth or actually melt in two. This can create both a 

safety and a fire hazard.

(2) Never rely solely on the position indicator to determine whether or not a 

regulator is in the “Neutral” position. Verify proper operation of the indicator 

by making use of the neutral light within the regulator control and/or an 

appropriate voltage-measuring device to ensure that there is no potential 

difference between the source and load terminals of the regulator. Do not use 

an ordinary voltmeter for this purpose.

(3) If a regulator needs to be bypassed, and the unit cannot be placed in the 

“Neutral” position due to equipment problems, the safest course of action is to 

de-energize the primary line that serves the regulator installation. Once the 

line is de-energized, then the source and load switches can be opened and the 

bypass switch closed. The line can then be re-energized.

10 REFERENCES

1 American National Standards Institute, “American National Standard for Electric Power 

Systems and Equipment—Voltage Ratings (60 Hz),” ANSI C84.1-2016.

2 RUS Bulletin 1724D-113, “Voltage Levels on Rural Distribution Systems.”

3 Westinghouse Electric Corp., “Distribution Systems: Electric Utility Engineering Reference 

Book,” Westinghouse, 1965.

4 American National Standards Instate Publication, “IEEE Standards Requirements, Terminology 

and Test Code for Step-Voltage and Induction-Voltage Regulators,” ANSI/IEEE C57.15-2009.

5 Cooper Power Systems, “Voltage Regulators,” Service Information S225-20-1, 1992.

6 Gönen, Turan, “Electric Power Distribution System Engineering,” McGraw-Hill, Inc., 1986.

7 Cooper Power Systems, “Voltage Regulator and Capacitor Application Considerations in 

Distribution Systems for Voltage Improvement,” System Engineering Reference Bulletin 

SE9701.
Bulletin 1724D-114

Appendix A

Page 1

APPENDIX A

DERIVATION OF FORMULAS FOR CALCULATING

VOLTAGE DROP AND VOLTAGE REGULATION

As can be observed from Figure A-1, the voltage at the source (VS) is equal to the vector sum of 

the voltage at the load (VR) plus the voltage impressed on the line IZ, or 

VS = VR + IZ.

R - Line resistance

X - Line inductive reactance

Z - Line impedance

I - Line current

VS - Source voltage

VR - Receiving-end voltage

FIGURE A-1: A Simple Distribution Circuit Diagram.

The voltage impressed on the line (IZ) is the actual voltage drop, which can be expressed as a 

vector. In practice, however, voltage drop is determined by taking the difference between the 

known voltage magnitudes of the sending and receiving ends as expressed by the following 

equation:

VD = |VS| - |VR|

The relationship of the voltage and current phasors of the simple circuit of Figure A-1, with a 

lagging power factor, is shown in Figure A-2. As shown in Figure A-2, VD is not equal to IZ 

drop. However, for the purposes of distribution circuit calculations, the difference between VD 

and IZ drop is insignificant. For known sending-end voltage and power factor, VD can be 

approximated by the following equation:

VD = IRCos + IXSin 

Where:

I = Line current
Bulletin 1724D-114

Appendix A

Page 2

= Phase angle between voltage and current and can be derived from known 

power factor

R = Resistance of the line

X = Inductive reactance of the line

It can be observed from the phasor diagram of Figure A-2 that the approximation of VD using 

the above equation is sufficiently accurate.

FIGURE A-2: Voltage-Current Phase Relations.

Specifically, for three-phase laterals and feeder mains, the approximate voltage drop is expressed 

as:

6

VD = I3 (RCos + XSin )

and for the single-phase lateral:

VD = I1 (KRRCos + KX XSin )

where constants KR and KX are each typically assumed to be 2.0.

Voltage drop values on a distribution system are based on the nominal system voltage. To 

convert these values to 120-volt base, the following equation is used.

VD = Actual Voltage Drop . × 120 volts

 Nominal System Voltage
Bulletin 1724D-114

Appendix A

Page 3

Voltage regulation is a measure of performance of a distribution circuit with respect to voltage 

drop. The voltage regulation of a distribution circuit is the ratio of the voltage drop of the line to 

the load voltage which can be presented in percent regulation as expressed in the following 

equation:

Where:

%VR = Percent Voltage Regulation

|VS | = Sending-end voltage of a distribution line

|VR | = Receiving-end voltage of a distribution line

% VR = |VS 

| - |V

R 

|

 × 100

 |VR | 
Bulletin 1724D-114

Appendix B

Page 1

APPENDIX B

EFFECT OF SHUNT CAPACITOR ON SOURCE CURRENT AND LINE VOLTAGE

The effect of shunt capacitors is shown in Figures B-1 and B-2. A schematic diagram of a 

simple distribution circuit is shown in Figure B-1 (a) and the corresponding voltage-phasor 

diagram is shown in Figure B-1 (b). As shown in Figure B-1 (b), load current (I) lags the load 

voltage (VL) by phase angle . Notice that the load current is the same as the source current. 

When a capacitor bank is installed on the circuit, as shown in Figure B-2 (a), the capacitor bank 

draws a leading reactive current (IC) that counteracts the lagging reactive load current, resulting 

in the reduction of the source current up to the point of capacitor bank installation. As shown in 

the resultant voltage and current phasor diagram of Figure B-2 (b), the magnitude of resultant 

source current (I′) is reduced. However, the magnitude of load current (I) remains the same. 

Also, the improvement of the power factor is clearly shown.

(a) (b)

Figure B-1:

(a) Schematic Diagram of a Simple Feeder (b) Voltage and Current Phasor Diagram

(a) (b)

Figure B-2:

(a) Schematic Diagram of a Simple Feeder (b) Voltage and Current Phasor Diagram

with Shunt Capacitor