In a 277/480-volt panelboard, what is the largest permissible 3-pole circuit breaker for a motor feeder?

In a 277/480-volt panelboard, when determining the largest permissible 3-pole circuit breaker for a motor feeder, adherence to the National Electrical Code (NEC) and the characteristics of the motor circuit is essential. The NEC stipulates that the maximum rating for a branch-circuit protection device varies based on the application and the motor's service factor.

For motors, the allowable circuit breaker size is typically calculated based on the full-load current (FLC) of the motor and the duration of the starting current. The NEC generally allows circuit breakers to be sized at 125% of the full-load current for continuous loads. In the context of a 277/480-volt system, a 3-pole circuit breaker rated at 175 A balances the needs for providing sufficient protection while not exceeding the rating permissible for typical motor applications of that voltage range.

The ratings for the other choices are below this optimal size. A breaker rating of 250 A or 200 A may not adhere to the proper protection levels for standard motors, as it is important to avoid oversized breakers that could allow excessive current in the event of a fault without tripping. A 150 A breaker might be under-sized for certain motors, leading to nuisance tr