For a 277/480-volt, 3Ø, 4-W service with a maximum unbalanced load of 140 amps, what is the minimum size of the neutral?

To determine the minimum size of the neutral conductor for a 277/480-volt, 3-phase, 4-wire service with a maximum unbalanced load of 140 amps, it is essential to consider both the NEC (National Electrical Code) guidelines and the specifics of how the neutral is sized in relation to the load.

In a three-phase system, the neutral conductor needs to handle the unbalanced load that may occur when the loads on the three phases are not equal. Based on NEC guidelines, for unbalanced loads, the neutral conductor should typically be sized at least 100% of the maximum expected unbalanced load if the balancing is between phases is not guaranteed.

In this case, the maximum unbalanced load is specified at 140 amps. According to NEC standards, when calculating the neutral conductor size, it is often allowed to use aluminum conductors, which usually requires a larger size to carry the same current as copper conductors. When following the NEC sizing tables for conductors, 3/0 AWG aluminum is the appropriate size to ensure that it can accommodate the unbalanced load safely.

Choosing 3/0 AWG aluminum as the correct answer ensures that the neutral provides adequate capacity to handle the unbalanced load without overheating or