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Conductor size and Voltage Drop for Design

  1. Mar 9, 2012 #1


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    If specs on a 300Hp electric motor;
    480 Volts
    3 Phase

    The full load Amps of the motor is 361A, which I found in the NEC table. My distance is 700ft to a control center and I can't seem to figure out what copper conductor size to use. In order to find the Voltage drop I have to know the CM, I assume. Can any one help me with my design?
  2. jcsd
  3. Mar 9, 2012 #2
    Time for an electrician - the conductor (cable) selection depends on, location, temp rise, how is it run (Conduit, Raceway, underground - etc). This is in the NEC - but available from a number of places online ( http://sparkyjohn.com/ampacity/ampacity.pdf )

    The cable is selected to ensure it does not overheat.

    Then once the cable is selected you can find the V drop along A length V = Res ( Ohm/foot * 700Ft) * I current. Noting that the voltage at the load has 2 x the voltage drop of on length ( in 3 wire this is not totally accurate but will suffice)

    I would also say - the Generator should have a Circut Breaker - at the generator. The cable really needs to be sized based on the breaker rating and or setting - not the generator.

    If the load is known, for example a 200A panel, you may also be able to economized by using a smaller breaker at the generator and running a smaller cable.

    Since an error in this system can cause a fire (that is why the NEC in part of the Nat Fire Protection Assn) - I refer back to my first line.
  4. Mar 9, 2012 #3


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    Ok, from some Voltage Drop online calculators said 400kcmil would be the minimum conductor size to use in a conduit. Would that be the right ideal? The environment near saltwater, so I'm using Aluminum conduit for no corrosion.

  5. Mar 9, 2012 #4
    Umm - Aluminum and Salt water not a good combo, PVC would be better if you can get it to withstand the cable pull -

    Per the NEC - 3 conductors in a conduit, 400MCM is an odd size, you may have better luck with the 500MCM (kcmil) - if you can afford the wire a size larger is often a good idea, espically if you will be starting large motors / high load.

    Pulling 4 x 500MCM - in anything - is HARD - again, I refer back to a pro. I am EE with 15 years hands on service of electrical switchgear (480VAC up) - and I would PAY for a union electrician to do this out of my own pocket before I would try to tackle the job.
  6. Mar 9, 2012 #5

    jim hardy

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    should be plenty of Ampacity charts from wire suppliers.

    On a run that long check voltage drop it may be more limiting than thermal.


    just a tip -
    Beware of iron conduit fittings around high curent AC conductors.
    If you use any be sure all 3 phases go through same fitting.
    Magnetic field around a single phase carrying hundreds of amps can heat a closed iron loop that surrounds the conductor. It makes a 60 hz induction heater.
    If all 3 phases go through, currents add to zero of course.
    We had trouble with iron conduit elbows in a big run buried under concrete... contractor had run out of PVC ells.
  7. Mar 9, 2012 #6


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    Yes, I do agree that Al is not a good combo, put it's at least can withstand the cable pull. Or maybe steel could be better, but corrosion is worse in that matter. I'm thinking 500MCM could be a better choice. Do you think it wouldn’t make any much difference though between the two sizes? I mean you have lesser Voltage load at the end of the circuit or wire. Heck, will the voltage drop for 400MCM-500MCM or above still meet NEC? You're right though; regardless to pull any wire size like that is unimaginable lol....

    Thanks for pdf Jim H, helped me get a better idea.
  8. Mar 9, 2012 #7

    jim hardy

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    http://www.pearen.ca/Reference/wire.htm [Broken]

    shows 0.00002074 ohms/ft for 500mcm

    which X 700 ft X 360 amps = 5.24 volts drop, just over 1%
    Last edited by a moderator: May 5, 2017
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