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Thread: Voltage drop for new feeder in an existing building

  1. #11
    Join Date
    Jan 2016
    Location
    PA
    Posts
    5,071
    Quote Originally Posted by Grouch1980 View Post
    Got it. So there's no rule of thumb / formula, other than to actually take a field measurement of the voltage at the point where the new connection is being made, or find as-builts / design drawings and calculate it.
    you need 2 v readings
    supply and load at the same time (2 guys, do each ph and avg)
    take i readings at same time, should be steady, avg 3 phases
    feeder Z = (v supply - v load) / i Ohms

    max v drop = Z x cond ampacity (x 80% depending on application)

    eg
    v supply 485
    v load 480
    i = 100
    Z = 5/100 = 0.05 Ohm

    if 250 kcmil rated ampacity = 255
    v drop = 0.05 x 255 = 13 or 2.7%
    if loading is 80% (cond sized 1.25 x load)
    v drop = 0.05 x 0.8 x 255 = 10 v or 2.1%

    the closer the i is to the rated ampacity the better
    >30% of rated ampacity with good measurements yeilds good results

    you can also remove power and actually measure a loop by shorting one end
    or calc off as-buits
    The difference between genius and stupidity is that genius has its limits.

  2. #12
    Join Date
    Jul 2015
    Location
    New York, NY
    Posts
    177
    Quote Originally Posted by Ingenieur View Post
    you need 2 v readings
    supply and load at the same time (2 guys, do each ph and avg)
    take i readings at same time, should be steady, avg 3 phases
    feeder Z = (v supply - v load) / i Ohms

    max v drop = Z x cond ampacity (x 80% depending on application)

    eg
    v supply 485
    v load 480
    i = 100
    Z = 5/100 = 0.05 Ohm

    if 250 kcmil rated ampacity = 255
    v drop = 0.05 x 255 = 13 or 2.7%
    if loading is 80% (cond sized 1.25 x load)
    v drop = 0.05 x 0.8 x 255 = 10 v or 2.1%

    the closer the i is to the rated ampacity the better
    >30% of rated ampacity with good measurements yeilds good results

    you can also remove power and actually measure a loop by shorting one end
    or calc off as-buits
    Thanks. ..... "the closer the i is to the rated ampacity the better"... why is that? i is your calculated demand amps.

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