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parralel power cables

Discussion in 'Electrical Engineering' started by Dave, Dec 22, 2003.

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  1. Dave

    Dave Guest


    I have a question about paralleling cables.
    I know the U.S. NATIONAL ELECTRICAL CODE states parrallel cables must all be
    the same length.
    however in practice it is not always practical to have them all exactly the
    same. what is an acceptable allowance on the difference?

    the situation I have is 12 parrallel cables per phase 12.5 kv 2410 amps
    3 phase 60 cycle.
    36 cables total, 12 per conduit. quanity of six, six inch ridgid galvenized
    2 A, 2 B , 2 C cables in each conduit.
    With a 330 foot length will a difference in length of 6 feet in cables of the
    same phase cause an unacceptable inbalance of current??

    I am not an enginer, but an electrican. I have seen people spend an extra 50
    or more hours in a situation like this to get the cables all the exact length.
    Is this nessasary??
    From what I read and calculate, using the impedance of the cables at 2410
    amps/12 = 200.8 amps per conductor.
    effective impedance of 500 MCM cable in steel conduit shortest 330 ft. long =
    effective impedance of 500 MCM cable in steel conduit longest 336 ft. long =
    the rest of the cables are all between this in proportional lenghts 300' 8",
    301' 16" 302'4", ect due to the difference in length of the bends in conduit.

    the formula I have found is

    I long cable = I total x Z short cable/(Z long cable + Z short cable)
    I short cable = I total x Z long cable/(Z long cable + Z short cable)
    The shortest cable would carry a current of 202.6 amps
    the longet acurrent of 199 amps
    Is this the correct way to figure the current inbalance???

    I understand all cables must be the same insulaton type, conductor type, same
    type of conduit ect.
    If you assume equal temperatures, you will be conservative in the current
    split. That is, you will calculate more current in the shorter conductor than
    will actually be the case. The shorter conductors will carry more current
    because the resistance and inductance will be less. With higher temperature,
    the resistance will increase, reducing the difference between the short and
    long conductors.
  2. B J Conner

    B J Conner Guest

    There should be an engineer that designed that job. Find him or her and
    have he or she give you the answer. If he says they all have to be the
    exact same length then ask for chapter and verse.
    It dosen't sound well engineered to me.
    The length can vary but duct configuration, fill arounf the duct ( dirt,
    sand, concrete etc.) and the loading cycle are things that need to be
    He's using 12 cables at 200 amps each when he could have use 6 rated at 400.
    Six phase conductors in one conduit have to be derated to 80 % of of that
    with 3 conductors.
    If he does want the cables to be the exact same length then start what ever
    paperwork you have to get more money for the job.
  3. Ben Miller

    Ben Miller Guest

    That depends on the engineering calculations, how much safety factor is in
    the cable sizes, etc. If the worst-case cable is within it's allowable
    ampacity after derating (and hopefully with some allowance for future
    increase), and the available fault current was calculated based on the
    shortest cable, etc. then there shouldn't be problems. If the cables are
    sized to the limit, then you need to find room for an extra 6 feet
    somewhere! In any case, you should follow the approved plans or have them
    changed to reflect the actual situation.

    Ben Miller
  4. Brian

    Brian Guest

    No accecptable difference. Here is what you are really saying: "I am too
    lazy to measure 36 cables the same length". The Code is there for a reason.
    Being lazy is no excuse!!!
  5. John Gilmer

    John Gilmer Guest

    Seems to me that it would be better to bundle conductors in groups of 3 or 4
    (if neutral is carried) and then parallel the three phase "feeders."

    Each feeder would with be twisted (random) or have the conductors
    quasi-randomly transposed.

    This would cancel out most inductive coupling effects and make the current
    division in reverse proportion to length.

    In the original post, if each feeder were in its own conduit, one feeder
    being a few percent longer than other would reduce its current by only a few
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