The maximum permitted rating of the breaker in a 120/240-volt, 3Ø, 4-wire panel with the smallest permitted feeder conductors is?

In a 120/240-volt, three-phase, four-wire panel, determining the maximum permitted rating of the breaker involves understanding the relationship between the feeder conductors and the connected load. The smallest permitted feeder conductors are typically determined based on the ampacity of the conductors used, which is defined by the National Electrical Code (NEC) and takes factors like conductor material, insulation type, and ambient temperature into account.

For a three-phase system, the sizing of the overcurrent protection device, such as a breaker, must ensure that it does not exceed the allowable rating based on the conductor sizes. In general, feeder conductors must be sized to safely carry the load they supply, while the breaker must provide appropriate protection without exceeding the thermal limit of the conductors.

The answer of 450 amps corresponds to a breaker rating that would be feasible for a well-sized three-phase system where the smallest permitted conductors have an ampacity that at least matches or exceeds this rating while ensuring compliance with NEC requirements. This size allows for flexibility and accommodates the possible load scenarios typically encountered in commercial or industrial setups.

Choosing this rating allows for ensuring that if any overcurrent event occurs, the breaker will operate appropriately to protect the conductors and connected equipment without