A feeder supplies two continuous-duty motors whose table full-load currents are 28 amperes and 14 amperes, plus a continuous lighting load of 24 amperes. Motor feeder conductors are sized at 125 percent of the largest motor full-load current plus the full-load current of the remaining motors, and a continuous nonmotor load is added at 125 percent. What minimum feeder ampacity is required?
- A66 amperes
This adds the three loads at face value with no multiplier anywhere. Both the largest-motor rule and the continuous-load rule are ignored, and the feeder ends up undersized by about 20 percent.
- 79 amperes
- C73 amperes
This applies 125 percent to the largest motor but forgets that the lighting is continuous. The lighting multiplier comes from a different rule than the motor multiplier, and both apply on the same feeder.
- D82.5 amperes
This multiplies the entire 66 amperes by 125 percent. Only the largest motor and the continuous nonmotor load earn the multiplier; the second motor is counted at its table value.
Why B is correct
The rule adds 25 percent for exactly one motor, the largest, and takes every other motor at its table value. That gives 35 plus 14. The lighting is not a motor load, so it is handled by the ordinary continuous-load rule and contributes 30 amperes. The three pieces sum to 79 amperes of required feeder ampacity.
What this question is testing
Tests whether you know that only one motor on a feeder receives the 25-percent addition, whether you use table full-load currents rather than nameplate values for conductor sizing, and whether you can apply a second, independent continuous-load multiplier to the nonmotor portion of the same feeder.
On the job
Shop and industrial feeders almost always mix motors with lighting and receptacles, and the two load types follow different rules that happen to share the same 125-percent number. Mixing them up in either direction costs you: undersizing brings nuisance tripping on a hot afternoon, oversizing forces bigger lugs and bigger conduit. The rule also assumes you use the table full-load current for the conductor calculation, not the motor nameplate, which is the opposite of what overload protection uses.
Memory technique
Biggest motor plus a quarter, everybody else plain, and the lights bring their own quarter.
Exam tip
One motor gets the bonus, the rest ride at table value, and any continuous nonmotor load carries its own separate 125 percent.
Where to look it up
Article 430 for motor feeder conductors and Article 215 for the continuous nonmotor portion. The motor full-load current tables are in Article 430, not in Article 310.