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Thread: [Request] An explanation for this simple problem/answer

  1. #1
    Join Date
    Feb 2017
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    Texarkana, TX, USA
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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?

    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.

  2. #2
    Join Date
    Jul 2005
    Location
    Tennessee
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    In their calculation it appears they are using single phase amperage for the 36kw.
    I get the same number you do.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  3. #3
    Join Date
    Feb 2017
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    Quote Originally Posted by augie47 View Post
    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. #4
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    Quote Originally Posted by mhuytran View Post
    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.
    ~New signature under construction.~
    ~~~~Please excuse the mess.~~~~

  5. #5
    Join Date
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    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. #6
    Join Date
    Dec 2012
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    Placerville, CA, USA
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    Quote Originally Posted by Ingenieur View Post
    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. #7
    Join Date
    Jul 2005
    Location
    Tennessee
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    as mentioned, IMO, he is using the wrong number for "I", the 3 phase current.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  8. #8
    Join Date
    Feb 2017
    Location
    Texarkana, TX, USA
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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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