parallel cables

Discussion in 'Homework Help' started by keith@kbw, Jun 10, 2009.

  1. keith@kbw

    Thread Starter New Member

    Jun 10, 2009
    Hello all,
    can any tell me how to calculate parallel swa cable supply?
    we need to reduce 150mm 4 core swa to 2 smallers ones.
    cct= 180amp 3 phase+n at 150 meters in length. :confused:
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Just find the table that gives current-carrying capacity for various sized conductors. That same table should also give resistance/stated length.
  3. stuartsjg

    New Member

    Jun 16, 2009
    Starting from scratch, A recent cable i done was:
    620A normal load, 3Ph 4wire.

    I started by adding 10%
    Then add a factor for grouping with other cables, here it was 0.85
    Then installation method and temp, here both were 1.00 as they were on tray in free air under 15C
    So: 620A + 62A + 93A + 0A = 775A design capacity
    The solution here was 3 parallel 120mm.sq single core cables per phase + 2 for neutral, which i factored in a further grouping factor of 0.7 adding another 232A.

    My total design load was now 1007A and the 3 cables can take 1020A.
    I then verified this by looking at the voltage drop over the short 30M run which at 620A, 440V was 2.3v.

    Basically what i done was take total current and divide by the number of conductors then factor in the additional grouping factor.

    For you, 150mm.sq at 180A, assuming you already applied derating factors to come to the conclusion of 150mm you can say 75A per cable.

    If touching use 0.85 for two cables, if evenly spaced, use 0.95.
    Say 0.85.
    So you need:
    90A + grouping
    = 90A + 14A
    = 104A

    I come to either a 35 or 50mm.sq cable if V-drop isnt a problem.

    Now check the V-drop. For 35 its nearly 15V, for 50 its just over 12v.

    Your 1 cable solution was 10.3v drop, so design for that:
    180A, 150M, 10.3v => 0.38mV/A/M drop.
    Two cables in parallel => 0.763mV/A/M max drop.

    My tables has 70mm.sq as having 0.64mV/A/M or 8.6V drop.

    After all that, I would go for 70mm.sq cable.

    Its best to do things both ways, ie. by voltage drop and by current rating.

    Take my 620A example, as its only 30M, if i allowed a 5% voltage drop and designed for that then its something silly like a 25mm.sq cable!

    Go both ways, both should give a sensible result!


    ps. a note on the voltage drop, im usnig the Noscab book for offshore and marine cables, different manufacturers and cable types will have different figures even for the same size of cable!
    Last edited: Jun 16, 2009