CSLB C-10 Electrical ContractorEquipment and InstallationMedium
A 3-phase feeder supplies three continuous-duty induction motors with full-load currents of 12A, 18A, and 24A per Table 430.250. Per NEC 430.24, what is the minimum ampacity required for the feeder conductors supplying all three motors?
- A72 A
- B66 A
- C60 A
- D54 A
Show answer & explanationAnswer & explanation
Correct answer: C. 60 A
Per 430.24, feeder conductors for multiple motors must have an ampacity equal to the sum of all motor FLCs plus 25% of the highest-rated motor's FLC: 12+18+24=54A, plus 25% of 24A (6A) = 60A minimum conductor ampacity.
Why the other options are wrong
- A. 72A is too high and not supported by the calculation.
- B. 66A overstates the required adder.
- D. 54A is only the sum of FLCs without the required 25% adder.
Multiple Motor Feeder Sizing (430.24)
Feeder conductors supplying multiple motors must be sized for the sum of all motor full-load currents plus 25% of the largest motor's FLC.
- Formula: Sum(FLC) + 0.25 x largest FLC
- Uses Table 430.250 for FLC values, not nameplate current
- Applies to feeders, not individual motor branch circuits
Memory trick: 'Add them all, then boost the biggest by a quarter.'