Master Electrician: Motors, Motor Controls, Generators, and HVAC Equipment

Motors and controls

Motor calculations use the tables for full-load current, not the nameplate FLA, for conductor and short-circuit protection sizing; nameplate values are used for overload protection. Branch-circuit conductors are generally 125% of the table FLC, the short-circuit and ground-fault protective device may be sized up to specified percentages, and the overload device protects the motor from overheating. Motor disconnects, controllers, and overloads have separate requirements. HVAC equipment often uses nameplate MCA and MOP values.

Generators and HVAC

Generators require transfer equipment that prevents backfeed, appropriate grounding and bonding, and conductor and overcurrent sizing based on the generator output. Hermetic refrigerant motor-compressors use nameplate rated-load current and specific multiplier rules. For HVAC, the minimum circuit ampacity and maximum overcurrent protective device are commonly taken directly from the nameplate.

Traps

Using nameplate FLA instead of the motor table is a classic error, as is applying 125% to the overload rather than the branch-circuit conductor. Do not size the short-circuit protective device from the overload setting. For HVAC, MCA is not the same as MOP, and the disconnect must be within sight and correctly rated.

Worked example

A 25 hp, 460 V, three-phase motor has a table FLC of 34 A. Branch conductors: 34 × 1.25 = 42.5 A, so select a conductor with at least that ampacity. A 90 A inverse-time breaker is within the 250% maximum for short-circuit protection, while the overload is set from the nameplate current. Rule of thumb: conductors and breakers from the table; overload from the nameplate.