Motors, Controls, and HVAC Equipment
Article 430 (motors) and Article 440 (air conditioning and refrigeration) test a specific discipline: circuit conductors and short-circuit protection are sized from Code tables, while overload protection is sized from the nameplate.
Rules to keep straight
- 430.6(A)(1): use Table 430.248, 430.249, or 430.250 for full-load current, not the motor nameplate, when sizing branch-circuit conductors, disconnects, and short-circuit protection.
- 430.22: branch-circuit conductors at 125% of FLC. 430.24: feeder at 125% of the largest motor plus 100% of the rest.
- 430.52: short-circuit and ground-fault protection from the table (250% for an inverse-time breaker, 175% for a dual-element fuse), with permission to go to the next standard size if the table value does not correspond to one.
- 430.32: overload relays at 115% to 125% of nameplate current.
- 430.102: a disconnecting means within sight of the motor; 430.110: rated at least 115% of FLC.
- 440.32 and 440.52 do the same job for HVAC equipment using the nameplate minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP).
Worked example
7.5 hp, 230 V, three-phase motor. Table 430.250 FLC = 22 A.
- Conductors: 22 × 1.25 = 27.5 A, so 10 AWG copper at 75°C (35 A).
- Breaker: 22 × 2.5 = 55 A, so the next standard size, 60 A.
- Overload: nameplate 21 A × 1.15 = 24.2 A, so a 25 A heater or relay.
- Disconnect: 22 × 1.15 = 25.3 A, so a 30 A switch. 10 AWG on a 60 A breaker is legal here because 240.4(G) points to Article 430; the overload, not the breaker, protects the conductor.
Traps
- Sizing motor conductors from nameplate amps.
- Applying 240.4(D) to a motor branch circuit and "correcting" the 60 A breaker.
- Using table FLC to size overloads.
- Ignoring the disconnect requirement when the controller is out of sight.