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Thread: Check my motor sizing calcs pls

  1. #1
    Join Date
    Jun 2016
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    Tampa, FL, USA
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    Check my motor sizing calcs pls

    I'm bidding a small industrial job and I want to check my calculations.

    A motorized machine needs to be hooked up. It's 52 FLA, 3 phase, 460v. I don't have any other data on it such as motor type. It uses no motor starter according to the owner. It's being connected to a miniature bolt on breaker. The panel is 77 feet of conduit from the machine.

    52 FLA so 6AWG THHN seems right.

    Table C.1 says I can put 4 6AWG THHN in 3/4" EMT. I would run 1" for ease of pulling.

    I looked at table 430.52 for breaker sizing and it says I can go a max of 250% of FLA so:

    52 x 2.5 = 130. Next standard size up is 150. Next standard size down is 125 amp.

    I haven't checked yet, but I'm not sure if a miniature 150 amp bolt-on breaker is available. I'm used to working with plug-on and the largest I see for those is 125 amp. Would a 125 amp breaker be sufficient?
    Last edited by Coppersmith; 08-06-17 at 09:09 AM.

  2. #2
    Join Date
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    Durango, CO, 10 h 20 min without traffic from winged horses.
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    If a machine has a rating of 52A then that is your circuit, all of the sizing has already been done by the manufacturer.
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

  3. #3
    Join Date
    Jun 2010
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    Bowmanville, Canada
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    If I was bidding, I would allow (3) 6 awg and a 60 amp breaker

  4. #4
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    Jun 2016
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    Quote Originally Posted by ActionDave View Post
    If a machine has a rating of 52A then that is your circuit, all of the sizing has already been done by the manufacturer.
    I think you are correct Dave, Your comment prompted me to take a much more in depth look at the specs I have for the machine. Even though it says 52 FLA, I think it really should say 52 amps. I don't see anywhere near 40 HP of motors in the thing.

    Quote Originally Posted by controlled View Post
    If I was bidding, I would allow (3) 6 awg and a 60 amp breaker
    I agree. Thanks.

  5. #5
    Join Date
    Aug 2013
    Location
    new york
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    11

    Motor Calulations

    I would think you would take the Full Load and have to add in 125 percent.
    Look on the mane plate of running load if it has it. I not sure they would do that math for you Than your looking at a 70 amp breaker

  6. #6
    Join Date
    Dec 2008
    Posts
    466
    O.p. said: "It uses no motor starter according to the owner"
    If it is about DOL starting, the starting current may be 350 A.
    However, according to NEC Art.430.52(C)(1) Exception 2(c) the maximum setting current for an inverse-time circuit breaker has to be not more than 400% [4*52=208 A].In these case see pos.(3) Instantaneous Trip Circuit Breaker.
    The motor has to be provided with an overload device for 1.30*52=67.5 A-in my opinion.

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