Motors, Controls, and HVAC Equipment
Motors, Controls, and HVAC Equipment
This domain tests NEC Article 430 motor circuits and Article 440 HVAC equipment. You need to size branch-circuit conductors, overload protection, short-circuit protection, disconnects, and controllers from motor nameplate and table values.
Core rules
- Motor branch-circuit conductors are at least 125% of the motor full-load current (FLC) from Tables 430.247–430.250.
- Overload protection is sized from the motor nameplate current, typically 125% for continuous-duty motors; it protects the motor from overheating.
- Short-circuit and ground-fault protection is sized from Table 430.52, often 250% of FLC for an inverse-time breaker, with next-standard-size allowances.
- The disconnecting means must be within sight and readily accessible, and rated at least 115% of the motor FLC.
- HVAC equipment uses the nameplate minimum circuit ampacity (MCA) for conductors and maximum overcurrent protection (MOP) for the branch-circuit protector.
Worked example
A 10 hp, 460 V, three-phase motor has a Table 430.250 FLC of 14 A. Branch conductors: 14 × 1.25 = 17.5 A, so 12 AWG copper at 20 A is acceptable. An inverse-time breaker may be sized up to 250% of FLC = 35 A, subject to Table 430.52. The overload is set from the nameplate current, not the 14 A table value.
Traps
- Using table FLC for overload sizing instead of nameplate current.
- Forgetting the 125% multiplier for conductors.
- Using MCA and MOP interchangeably on HVAC nameplates.
- Omitting the disconnect or locating it out of sight.