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Sizing Electrical Cables

  1. Sep 15, 2009 #1

    I am trying to work out what cable to supply a 3 phase motor with. I am using data from the manual that tells me:

    for 34A max load current use 6mm2

    The drive output I am using has an Imax(cont) = 34A and an Imax = 52A.

    Is ir correct to use 6mm cable, i.e. should I be using the CONTINUOUS RMS MAX to calculate current sizes or the ABSOLUTE MAX?
  2. jcsd
  3. Sep 15, 2009 #2
    I'm a physicist, so take this for what its worth, but I think the issue is that a small cable diameter means more resistance, which means more power lost in the cable, which means less power getting to the motor.

    Thinking this way, if you want to loose no more than a certain amount of power in the cable, then you can say I1^2 R1 = I2^2 R2, and using the fact that R ~ 1/A, A2 = A1 I2^2/I1^2 = 2.3 * 6 mm^2 = 14 mm^2

    I'd try something slightly larger than this, just to be safe...
  4. Sep 15, 2009 #3


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    The iMax from the supply is the max it can provide, not necessarily the max the motor will take. You need to rate the cable for the max the motor will take PLUS any safety margin required by the wiring codes in your area.
    You also have to take into account where the cable is - the max cable current assumes a certain room temperature and amount of cooling. If you are running this cable inside a metal conduit in a hot engine room you will have to allow for larger cables.

    ps. pastro - the issue is that small cable is more resistance which means more energy lost in the cable, which means heating and more resistance, ... etc.. which is a fire risk.
  5. Sep 16, 2009 #4
    Ok thanks for your help guys.

    I'm gussing that the maximum value you are talking about is the continuous maximum value of current. e.g. a 22kW motor may draw 40A under normal operation but at start-up it can draw nearly 8 times that, just over 300A.

    If I was sizing cables to deal with 300A they would be huge. So cables must have to be sized according to their continuous max rather than instantaneous max (e.g. start-up currents).

    I am driving the 22kW motor (as a generator) with a smaller motor (7.5kW) so the currents outputs in steady-state should be low.
  6. Sep 16, 2009 #5
    They wouldn't be that huge, not even three times as thick. The problem boils down to heat. If it is for a few seconds then the thinner cable will be ok. But the power dissipated in the cable is quadratic in the current. So if your cable will normally heat up 1 degree under normal operation it would heat up 64 degrees with 8 times the current (steady state, no run away resistance considered) If you have to worry that your motor might get blocked, then it will be in the starting configuration again for a longer time drawing a lot of current. If your cable is mounted on some metal to dissipate the heat it can withstand more current btw.
    I had a quick check on wikipedia, and some other sites, and they seem to use the continuous current for starter cables, so I guess sou should do ok. And I also think some pages give you some type of fuse or burst rating for the cables.
  7. Sep 16, 2009 #6


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  8. Sep 17, 2009 #7
    "I am driving the 22kW motor (as a generator) with a smaller motor (7.5kW) so the currents outputs in steady-state should be low."

    The reasons for this are that I am researching motor faults and need to create a test rig. The smaller motor will be the one under test, the larger motor plus drive is a dynamometer which will allow me to apply different loads to the motor under test.

    Thanks everyone for your help, I think I understand the issues here much better.
  9. Sep 19, 2009 #8
    Just use a 10mm cable the next size up from 6mm on a 40A type C breaker thats what i would use
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