CSLB C-10 Electrical ContractorEquipment and InstallationHard
A 3-phase, 480V motor has a nameplate FLC of 34 amperes. Using copper THWN-2 conductors terminated on 75°C-rated terminals, what is the minimum standard conductor size required per NEC 430.22 for this motor's branch-circuit conductors? (Use NEC Table 310.16.)
- A4 AWG (85A at 75°C)
- B6 AWG (65A at 75°C)
- C10 AWG (35A at 75°C)
- D8 AWG (50A at 75°C)
Show answer & explanationAnswer & explanation
Correct answer: D. 8 AWG (50A at 75°C)
NEC 430.22 requires branch-circuit conductors for a single motor to have an ampacity of at least 125% of the motor's FLC. 34A × 1.25 = 42.5A. The smallest Table 310.16 conductor at 75°C meeting or exceeding 42.5A is 8 AWG copper, rated 50A.
Why the other options are wrong
- A. 4 AWG at 85A is significantly oversized beyond the calculated minimum.
- B. 6 AWG at 65A is oversized; it exceeds the requirement but is not the minimum standard size.
- C. 10 AWG is rated 35A at 75°C, which is below the required 42.5A minimum.
Motor Conductor Sizing 430.22
Branch-circuit conductors for a single continuous-duty motor must have an ampacity of at least 125% of the motor's full-load current rating.
- Formula: conductor ampacity ≥ 1.25 × FLC
- Use motor FLC from Table 430.250, not nameplate current, when applicable
- Select the smallest Table 310.16 conductor meeting the calculated ampacity
Memory trick: Motors need conductors sized a quarter bigger than FLC.