How to select Fuse or Circuit Breaker for group of motor in same branch circuit as per NEC

To select the proper fuse or circuit breaker for Group Motor installations, you must apply the specific rules of the National Electrical Code (NEC) for Group Motor installations. This applies to a single circuit breaker or fuse for a group of motors in the branch circuit that is single upstream short circuit protection in a Group Motor installation.

                                                    Fig: Motor in the same branch circuit

Refer to NEC (National Electric Code) section 430-53C and section 430-53D for proper conductor ampacity selection. This applies to a single circuit breaker or fuse for a group of motors in the branch circuit that is single upstream short circuit protection in a Group Motor installation.

Calculating the circuit breaker size for group of motors

Example -1

Eight motors with the sizes shown in Table are installed on a conveying system. Time-delay fuses are used.

Motor Quantity	Rating (hp)	Voltage	Full-load ampere (FLA)
1	5	460	7.6
2	3	460	4.8
5	2	460	3.4

In accordance with NEC section 430-52, section 430-53, and table 430-152, the time-delay fuse must be sized as follows:

175% FLA for largest motor + sum of FLAs for all other motors

⇒ (1.75 x 7.6) + (2 x 4.8) + (5 x 3.4) = 39.9 A

NEC 430-52 allows use of the next largest standard-size fuse—which in this case is 40 A. If nuisance tripping is a problem with this fuse selection, NEC does allow 225% of the largest motor FLA to be used in lieu of 175% when calculating the size. In this case, the calculation is as follows (the next largest standard fuse size in this case is 45 A):

(2.25 x 7.6) + (2 x 4.8) + (5 x 3.4) = 43.7 A

Example -2

Motor Quantity	Rating (hp)	Voltage	FLA- Full load ampere
2	10	460	14
1	5	460	7.6
2	3	460	4.8
5	2	460	3.4

In accordance with NEC section 430-52, section 430-53, and NEC table 430-152, the inverse-time circuit breaker must be sized as follows (the next largest standard-size inverse-time circuit breaker is 90 A):

250% FLA for largest motor + sum of FLAs for all other motors

⇒ (2.5 x 14) + 14 +7.6 + (2 x 4.8) + (5 x 3.4) = 83.2 A

If nuisance tripping is a problem, the NEC allows for inverse-time circuit breaker sizes that “shall in no case exceed 400% for full-load currents of 100 amps or less, or 300% for full-load current greater than 100 amps.” In this case, the calculation is as follows (the next largest standard inverse-time circuit breaker size in this case is 110 A):

(4.0 x 14) + 14 +7.6 + (2 x 4.8) + (5 x 3.4) = 104.2 A

                                                                Fig: Motor in branch circuit