Several recent e-mail questions concern the selection of conductors, fuses or circuit breakers, and overload relay settings for motors. I don't usually get too deep into electrical design issues in this column, thinking that is the realm of the electrical rather than the HVACR designer. However, given the interest level in this topic, I'll try to provide some general understanding of these issues, based on the requirements of the National Electrical Code (NEC).
NEC Article 430 provides appropriate design guidelines for most motor applications, but be cautioned that the NEC (by its own scope definition) provides only minimum standards, and upward departures may be necessary in some cases. I'll also limit this discussion to three-phase AC induction motors, the most commonly used type.