After all demand factors, the maximum unbalanced load on a 120/240-volt single-phase dwelling feeder works out to 320 amperes. The Code permits the portion of the unbalanced load that exceeds 200 amperes to be taken at 70 percent for feeders and services, with the first 200 amperes counted in full. What is the calculated neutral load in amperes?
- 284 A
- B320 A
This ignores the permitted reduction entirely and carries the full unbalanced load. It is conservative and never unsafe, but it is not the calculated value the question asks for and it can force an unnecessarily large grounded conductor.
- C224 A
This applies 70 percent to the whole 320 amperes. The first 200 amperes is specifically excluded from the reduction, so discounting it understates the neutral and can leave the grounded conductor undersized.
- D260 A
This reverses the split, discounting the first 200 amperes and carrying the excess at full value. The rule is the other way around: full value up to 200 amperes, reduced value above it.
Why A is correct
The permission is written as a two-step split. Two hundred amperes stays at full value, and only the 120 amperes above that line is multiplied by 0.70, giving 84 amperes. Adding the two portions produces 284 amperes as the calculated neutral load, which is what you carry into the conductor table.
What this question is testing
Tests whether you can apply a two-tier demand factor correctly, keeping the base portion at full value and discounting only the excess, and whether you understand that the neutral of a feeder is sized on maximum unbalance rather than on total load. It also probes awareness that the reduction has exclusions.
On the job
Services above 200 amperes are routine on large homes and small commercial buildings, and the grounded conductor is often the most expensive single conductor in the run because paralleled sets multiply the cost. Knowing that only the excess is discounted keeps you from either overspending on copper or, worse, undersizing a conductor that has to carry real unbalanced current all day. Note also that this reduction is off the table for the nonlinear portion of a load fed from a four-wire wye system.
Memory technique
The first 200 pays full fare; the overflow rides at seven-tenths.
Exam tip
Draw a line at 200 amperes. Everything below it is full price; only what sticks out above gets the 70 percent.
Where to look it up
Article 220, feeder and service neutral load. The same section lists the cases where no reduction is permitted, so read the whole subsection before you commit to an answer.