Practice Questions: Motor Circuit
Real questions from our bank, all referenced to NEC 2026. Tap “Show explanation” after you answer.
Per NEC Table 430.250, a 10 HP, 230-volt, 3-phase motor has a full-load current (FLC) of 28 amperes. What is the minimum branch-circuit conductor ampacity per NEC 430.22(A)?
- A28 amperes
- B35 amperes
- C42 amperes
- D56 amperes
Show explanation
NEC 430.22(A) requires conductor ampacity of at least 125% of FLC: 28A × 1.25 = 35A.
A continuous-duty 10 HP, 230V, three-phase squirrel-cage induction motor has a table full-load current of 28 amps per NEC Table 430.250. What minimum branch-circuit conductor ampacity is required per NEC 430.22(A)?
- A28 amps
- B32 amps
- C35 amps
- D40 amps
Show explanation
NEC 430.22(A) requires branch-circuit conductors supplying a single continuous-duty motor to have an ampacity of at least 125% of the motor full-load current from Table 430.250. 28 A × 1.25 = 35 A. Use the table FLC, not the nameplate value (per 430.6(A)(1)).
When sizing motor branch-circuit conductors and short-circuit protection, which current value does NEC 430.6(A)(1) require you to use?
- AThe motor nameplate full-load current
- BThe motor service-factor current
- CThe values given in NEC Tables 430.247 through 430.250
- DThe locked-rotor current
Show explanation
Per NEC 430.6(A)(1), for general motor applications, the conductor ampacity and branch-circuit short-circuit and ground-fault protection must be based on the values from Tables 430.247 through 430.250 — not the nameplate. Nameplate current is used for overload protection sizing (430.32), not branch-circuit sizing.
A three-phase squirrel-cage induction motor (non Design B energy-efficient) has a table full-load current of 40 amps. What is the maximum standard inverse-time circuit breaker rating permitted for branch-circuit short-circuit and ground-fault protection per NEC Table 430.52(C)(1)?
- A50 amps
- B80 amps
- C100 amps
- D125 amps
Show explanation
Per NEC Table 430.52(C)(1), the maximum inverse-time circuit breaker rating for a three-phase AC squirrel-cage motor is 250% of the full-load current. 40 A × 2.50 = 100 A.
Two continuous-duty motors with table full-load currents of 20 A and 12 A are supplied by a single branch circuit. What minimum conductor ampacity is required per NEC 430.24?
- A32 amps
- B37 amps
- C40 amps
- D45 amps
Show explanation
Per NEC 430.24, conductors supplying several motors must have an ampacity of at least 125% of the largest motor full-load current plus the sum of the full-load currents of the other motors. (20 A × 1.25) + 12 A = 25 A + 12 A = 37 A.
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