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Thread: VD to Motors

  1. #1
    Join Date
    Apr 2007
    Mpls, MN

    VD to Motors

    I'm running 3 fans at a distance of 365ft from an MCC.

    Fan are:
    480, 10HP, 3PH

    14amps from table 430-152
    14 * 1.25 = 17.5a for wire size; could run 14THHN, but voltage drop.

    Here's my calculation:
    total distance I could run = (CM * vd)/(K * I * 1.73)
    CM = 10380 from table 8
    vd = 480 * .03 =14.4v
    K = 12.9 for copper
    I= 14a
    and 3ph 1.73

    Multiply all that out and you get about 465ft.
    I'm using #10THHN wire rated at 90C

    Here's my derating:
    3 fans total for 9 wires = .7
    Ambient Temp = .89
    #10 THHN wire 90C = 40 x .7 x .89 = 24.92a; more than enough to handle 17.5a at that distance.
    I'm running all this in 1" conduit. 3 conductors per motor and a GEC.

    So why do I feel uneasy about this, everything checks out I think,,,, I've run the numbers many different ways and still it all adds up to being OK.
    What do other people think?, have experienced?
    Master Electrician
    Control Design

  2. #2
    Join Date
    Feb 2003
    Looks ok to me. Using an online calculator I get a voltage drop of 2.1% for #10s and 3.4% for #12s with 14 amp load at 365 feet and an ambient temperature of 40C.
    Where did the .89 correction factor come from? I don't see that in the code book.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

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