A 120/240-volt, single-phase feeder has a calculated maximum unbalanced load on the grounded conductor of 260 A, with no nonlinear loads present. What minimum ampacity is required for that feeder neutral?
- A182 amperes
One hundred eighty-two amperes results from applying the 70 percent factor to the entire 260-ampere unbalanced load. The reduction applies only to the excess above 200 amperes.
- B200 amperes
Two hundred amperes treats 200 as a hard cap on the neutral. The rule reduces the excess, it does not eliminate it.
- 242 amperes
- D260 amperes
Two hundred sixty amperes ignores the demand factor entirely. It is a safe but oversized answer and not the calculated minimum.
Why C is correct
Only the portion above 200 amperes gets the reduction. Two hundred at full value plus 42 from the reduced excess gives 242 amperes. The demand factor applies only to a three-wire single-phase or comparable system with no nonlinear load content, and the resulting neutral still may not be smaller than the minimum size required for the feeder.
What this question is testing
Whether you apply a stepped demand factor to only the qualifying portion of a load, a pattern that repeats throughout the load calculation article.
On the job
The reduction exists because on a large mixed load, the odds of the two ungrounded legs being severely unbalanced at the same moment drop as the load grows. Two important limits: no reduction is permitted for the portion of the load that is nonlinear on a four-wire wye system, because harmonic currents add in the neutral rather than cancel, and range and dryer neutral loads are handled by their own provisions.
Memory technique
First two hundred at full price, the rest at seventy percent off the top.
Exam tip
Everything through 200 amperes stays whole. Only what is above 200 gets multiplied by 0.70.
Where to look it up
Section 220.61 covers feeder and service neutral load, with the demand factor and the nonlinear-load restriction in separate subdivisions. Section 215.2(A) sets the floor size for a feeder grounded conductor.