Hello guys,

I am really confused about the answers of some questions given by my instructor. These answers are from the instructor's version of NEC Prep book.

Here is one pair of some questions/answers:

Question:A 3-phase 36KVA load rated 208V is wired to a panel board with 80ft lengths of 1 AWG THHN aluminum. What is the apprx Voltage drop of the feeder circuit conductors?

I = 36000VA / 208V = 173AEvd = (1.732*K*I*D)/cmil = (1.732*21.2*173*80)/83'690 = 6.07V

Answer (from the book):

Ok because this is a 3-phase system, so P-in = (Sqrt of 3 * E-line * I-line) = (3 * E-phase * I-phase)

My answer:

I-line = P/(1.732*E-line) = 36000/(1.732*208) = 100A

Evd = (1.732*21.2*100*80)/83'690 = 3.51V

The confusion is: why did the author use single phase power formula in a 3-phase system calculation? If the author is right and I'm wrong, please explain. I'm not very convinced that my instructor said I was wrong in my calculations just because the book had different answers.

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