Motor Circuits: Overloads, Conductor Sizing, and Troubleshooting
Motors get their own rules (Article 430) because of huge starting inrush current. The key is separating the two protective jobs: overload (protects the motor) vs. short-circuit/ground-fault (protects the circuit).
1. Two very different protections
- Overload protection guards against sustained overcurrent / overheating while running. It's sized at 115–125% of the motor NAMEPLATE full-load amps (125% if service factor ≥1.15 or temp rise ≤40°C; otherwise 115%). (CEC 430.32)
- Short-circuit/ground-fault protection (the breaker/fuse) is sized much higher to let the motor start without nuisance tripping. For an inverse-time breaker, up to 250% of FLC. (CEC 430.52, Table 430.52(C)(1))
2. Conductor sizing — 430.22
Branch-circuit conductors for a continuous-duty motor must be at least 125% of the motor FLC. (CEC 430.22)