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Thread: Practice exam question

  1. #1
    Join Date
    Jun 2017
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    Kalispell, Mt
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    Practice exam question

    Howdy gentlemen and all the rest of you.
    Can someone explain to me? Question: "The demand load for a circuit that has a 5 kW heat strip and a 5 HP, 240 V compressor is ___ W." The answer is 6720W. I can't figure how they got it. I'm working out of 2014 NEC and have been looking at 220.82(C)(1-6). Thank you -Don

  2. #2
    Join Date
    Jun 2006
    Location
    Ohio
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    21,385
    How they got their answer is by looking up the motor FLA in Table 430.248 (28A × 240V = 6,720VA).

    The answer you were given is wrong as I see it. To me, the 5kW heat strip means it is a heat pump unit. Under defined heating parameters, both the motor-compressor and heat strip can be active during a heat cycle. Demand load should include 5kW at 65% under 220.82(C)(3) as the question does not state they are exclusive loads.

    In all, this is another botched question. A heat pump will have a nameplate rating and that is what you are supposed to use for the demand calculation... not the HP rating of the motor compressor.
    I will have achieved my life's goal if I die with a smile on my face.

  3. #3
    Join Date
    Dec 2007
    Location
    NE Nebraska
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    I agree with Smart$ that the question has some flaws in it when it comes to reality of what you likely find in the field.

    That said if this is a heat pump and controls are arranged so that the heat strips and compressor will not run simultaneously - then the larger load is all you need to account for in load calculations - which may be what the question writer was attempting to make you use for an answer. As said though this kind of equipment falls under art 440 and will have a MCA for the equipment - and if you do need to consider the compressor individually for some reason you will be going off the RLA of the compressor and not the motor current tables in art 430.

  4. #4
    Join Date
    Oct 2009
    Location
    Austin, TX, USA
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    8,477
    Exam question: What is the smallest conductor size that will service this load?

    Real world question: Can I use some of the #10 I have in the truck?

  5. #5
    Join Date
    Jun 2017
    Location
    Kalispell, Mt
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    That question was straight out of the text book, and there was no preamble to go with it. I thought it was a bit lacking but figured they were smarter than me. I was focusing on that heat strip too much, should have caught the FLC. Thank you -don

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