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Thread: Exam Question

  1. #1
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    Exam Question

    I hope some can help?My question is From my exam I took and got it wrong .Determine the minimum required conductor ampacity for an induction type motor 25 HP 460 volts 3 phase squirrel cage motor name plate full load 32 amp service factor 1.15?My answer was 34x1.25 =42.5A there answer was 43 A .The only thing I can figure is they rounded up.Any help would be greatly appreciated

  2. #2
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    42.5a is not a standard breaker size , though neither is 43 .45 would be the best answer inmo as you can round up for branch ckts but not for feeders

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  3. #3
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    Hmm, doing the math off of that question, I would have gotten 40 amps (nameplate FLA x 1.25). Not sure what the annex of the NEC has to say about it.

    Nickelec, the breaker can be up to 250 percent of the FLA. The motor could very well have a 60 amp breaker with #10 THHN
    Last edited by JFletcher; 04-02-18 at 08:50 PM.
    Electricians do it until it Hertz!

  4. #4
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    The question is about sizing the conductor so forget about the OCPD. You need to also ignore the nameplate current and find the appropriate value in T430.250 (as you did) and use that for the calculation. For rounding up or down (B) applies.

    220.5 Calculations.
    (A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347,480Y/277, 480, 600Y/347, and 600 volts shall be used.
    (B) Fractions of an Ampere. Calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  5. #5
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    Quote Originally Posted by Dsm View Post
    Determine the minimum required conductor ampacity . . . .
    As Rob pointed out, this question is not about the breaker. It is also not about sizing the conductor. It is about determining a specific number. I think your method is right, and the exam author's answer is wrong.

    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  6. #6
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    Quote Originally Posted by charlie b View Post
    As Rob pointed out, this question is not about the breaker. It is also not about sizing the conductor. It is about determining a specific number. I think your method is right, and the exam author's answer is wrong.

    Wouldn't 42.5 be rounded up to 43 which is the answer given?
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  7. #7
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    43 amps is correct you round down fractions smaller than .5.

    That said, I don't know why using the name plate is incorrect.
    Electricians do it until it Hertz!

  8. #8
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    Quote Originally Posted by infinity View Post
    Wouldn't 42.5 be rounded up to 43 which is the answer given?
    I didn't check the calculations, but if he did calculations correctly and came up with 42.5, then IMO 42.5 and 43 are both correct answers.

    It says right in the section you quoted that you are permitted to round this up - not that you shall round it up.
    I live for today, I'm just a day behind.

  9. #9
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    Quote Originally Posted by JFletcher View Post
    43 amps is correct you round down fractions smaller than .5.

    That said, I don't know why using the name plate is incorrect.
    I would assume that the nameplate figure of 32A is correct for that particular motor assuming it is a real motor.
    Making typical assumptions for effy and pf that's about what I come up with.

    However, the tables probably have to be conservative.
    Si hoc legere scis nimium eruditionis habes.

  10. #10
    I thought ampacity of conductors was determined by the table values, not the nameplate.

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