Motor Circuit Breaker Sizing

Motor circuits are another exception to the general rule, where the overcurrent protection device shall be permitted to be sized above the conductor’s ampacity. Motor circuits are sized according to the requirements of Article 430. A study of these requirements will reveal that for motor circuits, a fuse size or circuit breaker setting in excess of the ampacity of the conductor shall be permitted by the National Electrical Code. This exception is intended to provide fuse or circuit breaker protection large enough to hold the high momentary inrush current required for starting and running the driven load. Overload protection is provided in Part III of Article 430.

NEC Motor Circuit Breaker Sizing (Minimum and Next Size)

Example Problem: What size inverse-time circuit breaker is required to start and run a 40 HP, three- phase, 460 volt, Design B motor?

Step 1: Finding FLC

240.4(G), 430.6(A)(1), and Table 430.250

40 HP = 52 A

Step 2: Finding percentage

430.52(C)(1) and Table 430.52(C)(1)

Minimum size = 250%

Step 3: Calculating minimum size

430.52(C)(1)

Minimum size = 52 A x 250%

Minimum size = 130 A

130 A = 125 A

Step 4: Calculating maximum size

430.52(C)(1)(a)

130 A requires 150 A OCPD

Step 5: Selecting inverse-time circuit breaker

Table 240.6(A)

Minimum size = 125 A

Next size = 150 A

Solution: The minimum size inverse-time circuit breaker is 125 amps and the next size is 150 amps.

See Illustration 1 for an exercise problem when sizing and selecting the minimum size (rounded down) and next size (rounded up) inverse-time circuit breaker to start and run a motor.

Illustration 1

Answer Key to Illustration

[A] 65 [B] 250 [C] 65 [D] 250 [E] 162.5 [F] 162.5 [G] 175 [H] 150 [I] 175 [J] 150 [K] 175

NEC Motor Circuit Breaker Sizing (Maximum Size)

Example Problem: What size inverse-time circuit breaker is required to start and run a 40 HP, three- phase, 460 volt, Design B motor? (Determine maximum size.)

Step 1: Finding FLC

240.4(G), 430.6(A)(1), and Table 430.250

40 HP = 52 A

Step 2: Finding percentage

430.52(C)(1)(b)(3) and Table 430.52

Maximum size = 400%

Step 3: Calculating maximum size (round down)

430.52(C)(1)(b)(3)

Maximum size = 52 A x 400%

Maximum size = 208 A

Step 4: Selecting inverse-time circuit breaker

Table 240.6(A)

Maximum size = 200 A

Solution: The maximum size inverse-time circuit breaker is 200 amps.

See Illustration 2 for an exercise problem when sizing and selecting the maximum size inverse-time circuit breaker to start and run a motor.

Illustration 2

Answer Key to Illustration

[A] 65 [B] 400 [C] 65 [D] 400 [E] 260 [F] 250 [G] 250

NEC Motor Circuit Breaker Sizing (Two or More Motors)

Example Problem: What size overcurrent protection device is required for a feeder supplying a 10 HP, 15 HP, 20 HP, and 25 HP, three-phase, 460 volt, Design B group of motors?

Step 1: Finding FLC of motors

430.6(A)(1) and Table 430.250

10 HP = 14 A

15 HP = 21 A

20 HP = 27 A

25 HP = 34 A

Step 2: Calculating feeder OCPD

430.52(A), Table 430.52, and 430.62(A)

34 A x 250% = 85A

= 90A (round up) = 14 A

= 21 A

= 27 A

Total load = 152 A

Step 3: Selecting OCPD

430.62(A), Table 240.4(G), and Table 240.6(A)

152 A requires 150 A OCPD (round down)

Solution: The size overcurrent protection device is 150 amps.

See Illustration 3 for an exercise problem when sizing and selecting the overcurrent protection device for two or more motors supplied by a feeder.

Illustration 3

Answer Key to Illustration

[A] 21 [B] 27 [C] 34 [D] 52 [E] 52 [F] 250 [G] 130 [H] 150 [I] 21 [J] 27 [K] 34 [L] 232 [M] 232 [N] 225 [O] 225

Note: There are no exceptions to 430.62(A), which permits a feeder overcurrent protection device to be rounded up. You shall always round down to the next standard available size per 240.6(A) and Table 240.6(A).