When is feeder conductor size typically adjusted?

Feeder conductor size is typically adjusted when there are more than three current-carrying conductors in a raceway. This adjustment is based on the National Electrical Code (NEC) guidelines, which recognize that when multiple conductors are grouped together, they can degrade each other's heat dissipation capabilities, leading to potential overheating. In such cases, the NEC requires applying derating factors to ensure that the conductors can safely carry the intended load without exceeding their temperature ratings.

For example, if there are four or more conductors in the same raceway, the allowable ampacity of each conductor must be reduced to accommodate the higher ambient temperature caused by the additional heat generated. This derating ensures that the conductors remain safe and operational under expected load conditions.

The other situations listed do not necessarily require adjustments to feeder conductor sizes based specifically on code requirements. The adjustment related to conductor length and damp locations may have safety and performance implications, but they are not the primary reasons for ampacity adjustments in the same way that the number of current-carrying conductors is.