A feeder supplies a 65-ampere continuous load plus a 40-ampere noncontinuous load. Before any temperature correction or conductor bundling adjustment, what minimum ampacity must the feeder conductors have?
- A105 amperes
One hundred five amperes is the simple sum of 65 and 40 with no continuous factor applied. It undersizes the conductors for the all-day portion of the load.
- 121.25 amperes
- C131.25 amperes
One hundred thirty-one and a quarter amperes results from multiplying the entire 105-ampere load by 1.25. The noncontinuous portion does not get that factor.
- D150 amperes
One hundred fifty amperes is a standard device rating and a common conductor size, but it is well above the calculated minimum and does not follow from the numbers given.
Why B is correct
Splitting the load and multiplying only the continuous portion gives 81.25 plus 40, or 121.25 amperes. The noncontinuous portion is always taken at 100 percent. That 121.25-ampere figure is the number you carry into the conductor ampacity table and into the overcurrent device selection, before any ambient correction or bundling adjustment is applied.
What this question is testing
Whether you correctly separate continuous from noncontinuous load before applying the multiplier, which is the defining skill for all feeder and service sizing.
On the job
The next step in the field is choosing a conductor whose table ampacity meets 121.25 amperes in the temperature column matching your terminations. A 1 AWG copper conductor at 75 degrees carries 130 amperes and covers it. Then the overcurrent device is sized to the same 121.25-ampere minimum, giving a 125-ampere device. Skipping the split calculation is how undersized shop feeders end up running warm.
Memory technique
Continuous gets the bonus, noncontinuous rides at face value.
Exam tip
Write the two loads on separate lines. Multiply only the continuous line by 1.25, then add.
Where to look it up
Section 215.2(A)(1) for feeder conductor sizing and 215.3 for the feeder overcurrent device. The branch-circuit equivalents are in Article 210.