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Thread: Calculating FLA, MCA & MOCP

  1. #1
    Join Date
    Mar 2007
    Posts
    5

    Calculating FLA, MCA & MOCP

    I'm trying to make sure I'm calculating FLA, MCA & MOCP correctly. I've been studying the NEC book and here is my understanding.

    For example, a particular warm air furnace has the following:
    (2) 10 hp motors, 460/3/60, 10 amps each,
    (1) 3 hp motor, 460/3/60, 4.1 amps,
    (1) 1.5 hp motor, 460/3/60, 2.5 amps,
    control circuit, 115/1/60, 7 amps.

    Should FLA @ 460/3/60 be calculated this way?
    10 amps + 10 amps + 4.1 amps + 2.5 amps + 1 amp ?

    Should MCA be calculated this way?
    (10*1.25) + 10 + 4.1 + 2.5 + 1?

    Should MOCP be calculated this way?
    FLA * 1.75?

    Help is appreciated!
    Thanks.

  2. #2
    Join Date
    Jul 2005
    Location
    Tennessee
    Posts
    12,422

    amps

    I assume the 1 amp vs 7 amp numbers are typos....From there I'm a little confused to where you are heading. From your calculations, I assume you are looking at numbers for the entire unit, so I;'m a bit confused about the MOCP. Each motor would need individual overload protection. The 175% figure that you quote would be correct for dual element fuses as Short-Circuit, Ground fault Protection.
    For the entire unit this number would be derived by selecting the short circuit protection for the largest motor and then adding the FLA of the others.
    Hope that helps

  3. #3
    Join Date
    Mar 2007
    Posts
    5
    augie47, the 1 amp vs 7 amp was not a typo. This 7 amp 115/1/60 load would be coming from a transformer on the 460/3/60. 7 amps at 115/1/60 = 805 watts. At 460 volts, would 805 watts would be 1.01 amps? (W=V*A*1.73, so would A=W/(V*1.73)???

    I am looking at numbers for the entire unit. Specifically, I'm trying to get the MOCP and MCA for the entire unit so I can size the fuses in the disconnect switch and calculate the proper wire size. Since the motors are general purpose induction motors, I figure I better use RK5 fuses. Based on the above system info, how would I calculate MCA and MOCP?

    Thanks for your help, I'm trying to learn.

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