CSLB C-10 Electrical ContractorEquipment and InstallationMedium
A 3-phase induction motor has a nameplate full-load current (FLC) of 28 amperes and a marked service factor of 1.15. Per NEC 430.32, what is the maximum permitted overload device setting for continuous-duty protection of this motor?
- A42 amperes
- B28 amperes
- C35 amperes
- D31.5 amperes
Show answer & explanationAnswer & explanation
Correct answer: C. 35 amperes
NEC 430.32(A)(1) permits overload protection sized at up to 125% of motor FLC for motors with a service factor of 1.15 or greater. 28A × 1.25 = 35A.
Why the other options are wrong
- A. 42A corresponds to 150%, which exceeds the overload limit (that percentage applies to short-circuit protection, not overloads).
- B. 28A is 100% of FLC, not the maximum allowed overload setting.
- D. 31.5A corresponds to 112.5%, which is not the applicable multiplier here.
Motor Overload Sizing
NEC 430.32 sets maximum overload relay/device settings based on motor service factor or temperature rise to protect against sustained overcurrent.
- SF ≥1.15 or temp rise ≤40°C: max 125% of FLC
- All other motors: max 115% of FLC
- Overload protects windings from long-term overheating, not short circuits
Memory trick: High service factor motors get a bigger 125% cushion.