Motor Feeders and Starters

Motor Feeders and Starters

10.1.0 Introduction10.1.1 Sizing Motor-Circuit Feeders and Their Overcurrent Protection10.1.2 NEC Table 430.7(B), Locked-Rotor-Indicating Code Letters10.1.3 Motor-Circuit Data Sheets10.1.4 480-V System (460-V Motors) Three-Phase Motor-Circuit Feeders10.1.5 208-V System (200-V Motors) Three-Phase Motor-Circuit Feeders10.1.6 115-V Single-Phase Motor-Circuit Feeders10.1.7 200-V Single-Phase Motor-Circuit Feeders10.1.8 230-V Single-Phase Motor-Circuit Feeders10.1.9 Motor Starter Characteristics (for Squirrel-Cage Motors)10.1.10 Reduced-Voltage Starter Characteristics10.1.11 Reduced-Voltage Starter Selection Table

10.1.0 Introduction

Motors comprise a significant portion of a building’s electrical system loads. Theyare needed to power fans and pumps for basic mechanical building infrastructuresuch as heating, ventilation, air conditioning, plumbing, fire protection, elevators,and escalators. They are also needed to power equipment endemic to the occupancy,such as commercial kitchen equipment in an institutional facility, CT and MRIscanners in a hospital, and process equipment such as conveyors and machinery inan industrial plant or stone quarry. Consequently, designing motor-circuit feeders isvery much in the mainstream of the electrical design professional’s daily work. Tosave time in this process, the following information is provided.

10.1.1 Sizing Motor-Circuit Feedersand Their Overcurrent Protection

AC Single-Phase Motors, Polyphase Motors Other Than Wound-Rotor (Synchronous*

and Induction Other Than Code E†)

1. Feeder wire size 125 percent of motor full-load current minimum.

2. Feeder breaker (thermal-magnetic fixed-trip type) 250 percent of full-load currentmaximum.

3. Feeder breaker (instantaneous magnetic-only type) 800 percent of full-loadcurrent maximum.

Section

10.1

*Synchronous motors of the low-torque, low-speed type (usually 450 rpm or lower), such as those used to drive reciprocating compressors, pumps,etc., that start unloaded, do not require a fuse rating or circuit breaker setting in excess of 200 percent of full-load current.†For code E induction motors, everything is the same as above except that if an instantaneous magnetic-only type circuit breaker is used, it shallhave a maximum setting of 1100 percent.

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4. Feeder fuse (dual-element time-delay type) 175 percent of full-load currentmaximum.

5. Feeder fuse (NEC non-time-delay type) 300 percent of full-load current maximum.

For Wound-Rotor Motors

1. Feeder wire size 125 percent of motor full-load current minimum.


3. Feeder breaker (instantaneous magnetic-only type) 800 percent of full-loadcurrent maximum.



For Hermetic Motors (Special Case). Hermetic motors are actually a combinationconsisting of a compressor and motor, both of which are enclosed in the same hous-ing, with no external shaft or shaft seals, the motor operating in the refrigerant;thus their characteristics are different from those of standard induction motors.Calculating their feeder size and overcurrent protection is based on their nameplatebranch-circuit selection current (BCSC) or their rated-load current (RLC),whichever is greater. The BCSC is always equal to or greater than the RLC. Hence,the following:

1. Feeder wire size 125 percent of BCSC/RLC maximum.

2. Feeder breaker (thermal-magnetic fixed-trip type) 175 to 225 percent ofBCSC/RLC maximum.

3. Feeder breaker (instantaneous magnetic-only type) 800 percent of BCSC/RLCmaximum.

4. Feeder fuse (dual-element time-delay type) 175 to 225 percent of BCSC/RLC maxi-mum.

5. Feeder fuse (NEC non-time-delay type) not recommended—Do not use.

DC (Constant-Voltage) Motors

1. Feeder wire size 125 percent of motor full-load current maximum.


3. Feeder breaker (instantaneous magnetic-only type) 250 percent of full-load cur-rent maximum.



For Multiple Motors on One Feeder. First, size feeder and overcurrent protection forlargest motor and add the full-load current of the remaining motors to size theoverall feeder and overcurrent protection.

Application Tips

1. Refer to NEC Articles 430 and 440 for further details on sizing motor feedersand overcurrent protection.

10.2 Section Ten

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Motor Feeders and Starters 10.3

2. For elevator motors, always try to get the full-load current because the nameplatehorsepower on many machines is about 10 to 25 percent below the actual rating.

3. For packaged-type evaporative condensers with many small fans nominallyrated 1 hp (for example), be sure to get the full-load current because these arereally equivalent to about 2 hp (for example) each, and feeders sized on nominalhorsepower ratings will be inadequate. Remember to size the feeder and over-current protection as a multiple motor load. Also refer to NEC Article 440.

4. Note that maximum and minimum have precise meanings and that feedersizes shall not be less than the calculated minimum within 3 or 4 percent (e.g.,30-A-rated no. 10 wire is OK for a 31-A load) and breaker sizes shall not bemore than the maximum indicated. In general, for larger motor sizes, the over-current protection needed decreases considerably from the maximum limit.

5. In sizing nonfused disconnects for motors, use the horsepower rating table in themanufacturer’s catalog or realize that, in general, a nonfused disconnect switchshould be rated the same as a switch fused with a dual-element time-delay fuse.

6. When sizing feeders for tape drives in mainframe data centers, it is usuallynecessary to oversize both the overcurrent protection and the feeder to accom-modate the long acceleration time characteristic of this equipment.

7. Today’s highly energy-efficient motors are characterized by low losses and highinrush currents, thus requiring overcurrent protection sized at or near themaximum limit prescribed by the NEC when these motors are used.

8. For NEC locked-rotor-indicating code letters, refer to Table 10.1.2 [NEC Table430.7(B)].

10.1.2 NEC Table 430.7(B),Locked-Rotor-Indicating Code Letters

TABLE 10.1.2

10.1.3 Motor-Circuit Data Sheets

The following motor-circuit data sheets provide recommended design standardsfor branch-circuit protection and wiring of squirrel-cage induction motors of thesizes and voltages most frequently encountered in commercial, institutional, andindustrial facilities. Experience has shown that most facilities of this type use cop-per wire and use no. 12 AWG wire and 3⁄4-in conduit as minimum sizes for powerdistribution. These standards are reflected in the tables that follow. Refer also tothe notes to these tables for assumptions and other criteria used.

(© 2001, NFPA)

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10.1.4 480-V System (460-V Motors) Three-PhaseMotor-Circuit Feeders

10.4 Section Ten

TABLE 10.1.4

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10.1.5 208-V System (200-V Motors) Three-PhaseMotor-Circuit Feeders


TABLE 10.1.5

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10.1.6 115-V Single-Phase Motor-Circuit Feeders

10.6 Section Ten

TABLE 10.1.6

TABLE 10.1.7


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TABLE 10.1.8

10.1.9 Motor Starter Characteristics(for Squirrel-Cage Motors)

There are fundamentally two types of motor starters, full voltage (both reversingand nonreversing) and reduced voltage. In the information that follows, theircharacteristics and selection criteria are briefly summarized.

Full-voltage starters. A squirrel-cage motor draws high starting current(inrush) and produces high starting torque when started at full voltage.While these values differ for different motor designs, for a typical NEMAdesign B motor, the inrush will be approximately 600 percent of the motorfull-load amperage (FLA) rating, and the starting torque will be approximately150 percent of full-load torque at full voltage. High current inrush and startingtorque can cause problems in the electrical and mechanical systems and mayeven cause damage to utilization equipment or materials being processed.

Reduced-voltage starters. When a motor is started at reduced voltage, thecurrent at the motor terminals is reduced in direct proportion to the voltagereduction, whereas the torque is reduced by the square of the voltage reduction.If the “typical” NEMA design B motor is started at 70 percent of line voltage,the starting current would be 70 percent of the full-voltage value (i.e., 0.70 �600% � 420% FLA). The torque would then be 0.702, or 49%, of the normalstarting torque (i.e., 0.49 � 150% � 74% full-load torque). Therefore,reduced-voltage starting provides an effective means of both reducing inrushcurrent and starting torque.

If the motor has a high inertia, or if the motor rating is marginal for the appliedload, reducing the starting torque may prevent the motor from reaching full speedbefore the thermal overloads trip. Applications that require high starting torqueshould be reviewed carefully to determine if reduced-voltage starting is suitable. Asa rule, motors with a horsepower rating in excess of 15 percent of the kVA rating ofthe transformer feeding it should use reduced-voltage start.

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10.1.11 Reduced-Voltage Starter Selection Table

There are several types of electromechanical as well as solid-state reduced-voltagestarters that provide different starting characteristics. The following tables fromSquare D Company are a good representation of industry standard characteristics.Table 10.1.10 shows the starting characteristics for Square D’s class 8600 series ofreduced-voltage starters compared with full-voltage starting along with the advan-tages and disadvantages of each type. Table 10.1.11 provides an aid in the selection ofthe starter best suited for a particular application and desired starting characteristic.

10.1.10 Reduced-Voltage Starter Characteristics

10.8 Section Ten

TABLE 10.1.10

TABLE 10.1.11

(Courtesy of Square D/Schneider Electric)

(Courtesy of Square D/Schneider Electric)

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Motor Feeders and Starters

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