Section 7: Miniature and Molded Case Circuit Breakers PDF

Section 7: Miniature and Molded Case Circuit Breakers PDF

< Previous
Next >

Motor Protection Selection Tables

Class 601

Motor Circuit Protection Selection

The selection tables below provide detailed information for motor circuit protection components to help you choose the correct conductors, safety switches, and thermal magnetic circuit breakers for motor applications in accordance with the 2017 National Electrical Code (NEC). The selection tables provide guidance to simplify and standardize the selection of conductor sizes (AWG/kcmil), metallic conduit sizes, thermal-magnetic inverse time circuit breakers.

The selections are based on motor horsepower, full-load amperage (FLA), and voltage ratings, ensuring compliance with NEC Articles 430.22, 430.52, and related tables.

Selection Tables for Conductors, Safety Switches and Thermal-Magnetic Circuit Breakers — Based on 2017 NEC® Tables 430.247, 430.248 and 430.250

Horsepower Ratings

Full
Load Amperage [1]

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

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

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

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

1Ø
10 Hz ac

Average Direct Current Motors Operating at Base Speed

For Motor Code
Letter B to E

For Motor Code Letter F to V [5]

AWG

kcmil

Conduit 3 W

3Ø 60 Hz

Ordinary Service[6]

Heavy Service and Energy Efficient [7]

THHN

THWN

XHHW

THW

200 Vac[8]

230 Vac

460 Vac

575 Vac

115 Vac

200 Vac[8]

230 Vac

120 Vdc

240 Vdc
           

3/4

    

6.9 A

15 A

15 A

20 A

30 A

14

1/2 in.

N/A
       

1/3

        

7.2 A
   

5

        

3.4

  

7.6 A

20 A

2

                

7.8 A
         

3/4

      

7.9 A
           

1

    

8.0 A
               

2

8.5 A
     

7-1/2

          

9.0 A

25 A
         

1

      

9.2 A
             

1

  

9.5 A
 

3

              

9.6 A
       

1/2

        

9.8 A
           

1-1/2

    

10.0 A

20 A

3

  

7-1/2

10

          

11.0 A

25 A

30 A
         

1-1/2

      

11.5 A
           

2

    

12.0 A
               

3

12.2 A

25 A

35 A
             

1-1/2

  

13.2 A
       

3/4

2

      

13.8 A

  
   

10

            

14.0 A
 

5

              

15.2 A

30 A

35 A

40 A
       

1

        

16.0 A
     

15

    

3

2

  

17.0 A

45 A

12

1/2 in.

N/A

5

                

17.5 A

35 A
         

3

      

19.6 A

40 A

50 A
       

1-1/2

      

5

20.0 A

40 A
   

15

            

21.0 A

45 A

60 A

10

1/2 in.

N/A
 

7-1/2

              

22.0 A
       

2

        

24.0 A

45 A

50 A

60 A
             

3

  

25.0 A

50 A

70 A

7-1/2

                

25.3 A

60 A
   

20

25

          

27.0 A
 

10

      

5

      

28.0 A
               

7-1/2

29.0 A

60 A

80 A

8

1/2 in. [9]

N/A
     

30

          

32.0 A

70 A

10

                

32.2 A

90 A
   

25

  

3

        

34.0 A
               

10

38.0 A

80 A

80 A

100 A
           

7-1/2

5

  

40.0 A
                 

41.0 A

90 A

110 A

6

3/4 in.

1 in.
 

15

              

42.0 A
         

7--1/2

      

46.0 A

90 A

110 A

125 A

15

                

48.3 A
           

10

    

50.0 A
   

40

50

          

52.0 A

150 A

100 A
 

20

              

54.0 A

4

1 in.

1 in.
               

15

55.0 A
       

5

        

56.0 A

125 A
         

10

      

57.5 A
             

7-1/2

  

58.0 A
     

60

          

62.0 A

100 A

175 A

20

                

62.1 A
   

50

            

65.0 A

150 A
 

25

              

68.0 A
               

20

72.0 A

110 A

200 A

3

1 in.

1-1/4 in.
             

10

  

76.0 A

125 A

175 A
   

60

75

          

77.0 A

110 A

25

                

78.2 A
 

30

    

7-1/2

        

80.0 A
               

25

89.0 A

125 A

225 A

200 A

2

1 in.

1-1/4 in.

30

                

92.0 A

200 A

250 A
   

75

            

96.0 A

1

1-1/4 in.

1-1/2 in.
     

100

          

99.0 A

150 A
       

10

        

100.0 A
 

40

              

104.0 A

225 A

300 A
               

30

106.0 A

175 A

1/0

1-1/4 in.

1-1/2 in.

40

                

120.0 A

250 A
   

100

            

124.0 A

200 A

350 A

2/0

1-1/2 in.

1-1/2 in.
     

125

          

125.0 A

250 A
 

50

              

130.0 A

300 A
               

40

140.0 A
     

150

          

144.0 A

400 A

3/0

1-1/2 in.

2 in.

50

                

150.0 A
 

60

              

154.0 A

225 A

350 A
   

125

            

156.0 A
               

50

173.0 A

250 A

400 A

4/0

2 in.

2 in.

60

                

177.0 A

400 A

500 A
   

150

            

180.0 A
 

75

  

200

          

192.0 A

250

2 in.

2 in.

75

                

221.0 A

300 A

450 A

600 A

300

2 in.

2-1/2 in.
   

200

            

240.0 A

350 A

500 A

350

2-1/2 in.

2-1/2 in.
     

250

          

242.0 A

700 A
 

100

              

248.0 A

100

                

285.0 A

400 A

600 A

800 A

500

3 in.

3 in.
     

300

          

289.0 A
   

250

            

302.0 A

700 A
 

125

              

312.0 A

450 A

(2) 3/0

(2) 2-1/2 in.

(2) 2 in.
     

350

          

336.0 A

500 A

900 A

(2) 4/0

(2) 2 in.

(2) 2 in.

125

                

359.0 A

600 A

800 A

600 A
 

150

              

360.0 A

1000 A
   

300

            

361.0 A
     

400

          

382.0 A

(2)300

(2) 2 in.

(2) 2-1/2 in.

150

  

350

            

414.0 A

900 A

1200 A
       

500

        

472.0 A

800 A

1000 A

(2) 350

(2) 2-1/2 in.

(2) 2-1/2 in.
     

400

          

477.0 A
   

200

            

480.0 A

200

                

552.0 A

1200 A

1600 A

—

(3) 300

(3) 2 in.

(3) 2-1/2 in.
   

500

            

590.0 A

900 A
 

250

              

602.0 A

Contact your local Field Office for circuit breaker selection on constant horsepower multi-speed motors.

Footnotes

  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. 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.
< Previous
Next >
Feedback