[Request] An explanation for this simple problem/answer

[mhuytran]

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?

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

My answer: 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.

 

[augie47]

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

 

[mhuytran]

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. :rant:

 

[Fulthrotl]

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. :rant:

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.

 

[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 ???

 

[GoldDigger]

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.

 

[augie47]

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

 

[mhuytran]

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