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.

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