1. Junior Member
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## [Request] An explanation for this simple problem/answer

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

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

2. In their calculation it appears they are using single phase amperage for the 36kw.
I get the same number you do.

3. Junior Member
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Originally Posted by augie47
In their calculation it appears they are using single phase amperage for the 36kw.
I get the same number you do.
Thank you for your reply, but I really need some one who can help me to clear this mess up, because I no matter what I tried to explain, my instructor would stick to the answers in his book.
10 out of 10 questions, the author used P = I * E for all the 3-phase calculations. Just what the heck?
I don't want to make my instructor mad, but I want it to be right, and ultimately I don't want to get a big Zero on my paper just because some else made mistakes in his damm answer keys. This is really frustrating.

4. Originally Posted by mhuytran
10 out of 10 questions, the author used P = I * E for all the 3-phase calculations. Just what the heck?
I don't want to make my instructor mad, but I want it to be right, and ultimately I don't want to get a big Zero on my paper just because some else made mistakes in his damm answer keys. This is really frustrating.
this is a key part of your training. dealing with AHJ over a disagreement.
consider it useful to learn how to move past a situation blocked by
someone who won't change either the subject, or their mind.

it's just hidden in the middle of classwork.

5. ask him to do the calculation using I calculated from kva and the Ohm/1k ft value from the NEC tables
he must acknowledge S kva = sqrt3 x V x I ???

6. Moderator
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Originally Posted by Ingenieur
ask him to do the calculation using I calculated from kva and the Ohm/1k ft value from the NEC tables
he must acknowledge S kva = sqrt3 x V x I ???
Where V = line-to-line voltage and I = line current, not "phase current" which is slightly ambiguous because some call A a phase and while others call AB a phase and some are just inconsistent in their language.

7. as mentioned, IMO, he is using the wrong number for "I", the 3 phase current.

8. Junior Member
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I asked my instructor where the answer keys were from. He said they were from "the author" (hrmm... i.e Mr. Mike Holt)
Course code is <ELPT-1429>, Chapter 4, HandOut 1 with 5 originals + 5 extras.

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