SE Digest 178

Motor Circuit Protectors

Motor Protection Selection Tables

Class 601

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Table 7.96: Selection Tables for Conductors, Safety Switches and Thermal-Magnetic Circuit Breakers Based on 2017 NEC ® Tables 430.247, 430.248 & 430.250 Horsepower Ratings

Amperage of Thermal-Magnetic [2] Inverse Time Circuit Breaker

Minimum Size metallic Conduit 75° C, C Wire Field-Installed Sized for 125% FLA [4]

QMB and Heavy Duty Switch with Time Delay Fuses [3]

Squirrel-Cage and Wound Rotor Motors with Norm. Torque Characteristics Operating at Usual Speeds

Average Direct Current Motors Operating at BaseSpeed

For Motor Code Letter B to E

1Ø 10Hzac

Full Load Amperage [1]

For Motor Code LetterF toV [5]

Conduit 3W

3Ø60Hz

AWG kcmil

Heavy Service andEnergy Efficient [7]

THHN THWN XHHW

Ordinary Service [6]

200 Vac [8]

200 Vac [8]

THW

230 Vac

460 Vac

575 Vac

115 Vac

230 Vac

120 Vdc

240 Vdc

3/4

6.9A 7.2A 7.6A 7.8A 7.9A 8.0A 8.5A 9.0A 9.2A 9.5A 9.6A 9.8A

15A

1/3

5

3.4

20A

2

3/4

1

15A

2

7-1/2

20A

1

1

25A

3

14

1/2 in.

N/A

1/2

1-1/2

10.0A 11.0A 11.5A 12.0A 12.2A 13.2A 13.8A 14.0A 15.2A 16.0A 17.0A 17.5A 19.6A 20.0A 21.0A 22.0A 24.0A 25.0A 25.3A 27.0A 28.0A 29.0A 32.0A 32.2A 34.0A 38.0A 40.0A 41.0A 42.0A 46.0A 48.3A 50.0A 52.0A 54.0A 55.0A 56.0A 57.5A 58.0A 62.0A 62.1A 65.0A 68.0A 72.0A 76.0A 77.0A 78.2A 80.0A 89.0A

3

7-1/2 10

20A

30A

1-1/2

30A

2

25A

3

1-1/2

25A

35A

3/4 2

10

5

40A

1

30A

35A

15

3

2

45A

5

35A

12

1/2 in.

N/A

3

40A

50A

1-1/2

5

15

40A

45A

7-1/2

60A

2

45A

50A

3

10

1/2 in.

N/A

7-1/2

50A

70A

20 25

60A

10

5

7-1/2

80A

30

60A

10

70A

1/2 in. [9]

90A

60A

8

N/A

25

3

10

80A

100A

7-1/2

5

80A

90A

110A

15

7–1/2

6

3/4 in.

1 in.

15

125A

10

110A

40 50

20

90A

15

150A

5

10

7-1/2

125A

4

1 in.

1 in.

60

100A

20

100A

175A

50

25

150A

20

110A 125A

10

60 75

200A

3

1 in.

1-1/4 in.

25

110A

175A

30

7-1/2

25

125A

225A 200A 2

1 in.

1-1/4 in.

7 MINIATURE AND MOLDED CASE

CIRCUIT BREAKERS Thermal-magnetic circuit breaker ampere ratings recommended are approximate for average conditions, based on trip characteristics of Square D circuit breakers and NEC Table 430.52. Under some conditions, the next size larger switch or circuit breaker rating may be necessary to accommodate the motor starting current and is permitted by NEC 430.52(C)(1) Exception 2. High starting currents are anticipated with Design E and other energy efficient motors. For explanation of Code letter markings, see NEC 430.7(B). For Busway Plug-in units, see page 9-7. [3] Switch size only is shown in table. Selected fuses should not exceed maximum percent of full-load current as given in NEC Table 430.52. Above 50 hp dc switches are not hp rated by UL as Motor Circuit Switches, but as General Use Switches only and are not necessarily capable of interrupting the max. operating overload current of a motor. See NEC 100 for definition of General Use Switch. When protecting a 3Ø, Design E energy efficient motor, the switch is required by NEC 430.109 to have a hp rating of not less than 1.4 times that of a motor rated 3–100 hp, or not less than 1.3 times that of a motor rated over 100 hp. Switches shown in this table do not necessarily comply with that requirement. [4] NEC 430.22 for Single Motor, Smaller conductors may be permitted for light duty-cycle service per 430.22 (B) Exception No. 1. DC motors operating from rectified 1Ø power supply will require larger conductors per 430.22 (A) Exception No. 1. For motor-generator arc welders, see 630.11 [5] Thermal-magnetic breaker ampere ratings recommended are approximate for average conditions and based on trip characteristics of Square D circuit breakers and NEC Tables 430.7(B) and 430.52. [6] Ordinary service for normal starting duty only, acceleration time of 10 sec. or less. [7] Heavy service is jogging or plugging duty or cycling load with over 25 starts per hour or over 5 starts per minute. Energy efficient motors are polyphase motors defined in NEMA Standard MG1 and exhibit high starting current. [8] 200 V motors are commonly used on 208 V services. [9] 8 XHHW requires 3/4 in. conduit for 3W. [1] Motor full load currents thru 200 hp are taken from NEC Tables 430.247, 248 and 250. Above 200 hp from UL 98. Select wire size, circuit breakers, or fuses on basis of hp rather than nameplate full load current per NEC 430.6. Do not use these values to select overload relay thermal units. See Digest pages 16-129—16152 for selection of thermal units when actual full load current is not known. Voltages listed are rated motor voltages. Corresponding nominal system voltages are 110–120 V, 200–208 V, 220–240 V, 440–480 V and 550–600 V [2]

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