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Current Reduction of Multiple Conductors in one Conduit

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    Current Reduction of Multiple Conductors in one Conduit

    A man tried to tell me (of which I knew he was wrong) that You did not have to derate the current capacity of multiple conductors in the same conduit! Don't remember the simple way of figuring the number? (If there was one). The number in question was five (5) different 3-phase combinations plus a ground in one 3/4 rigid. In other words, 15 hots and one ground. What would be the maximum circuit current allowance?
    I forgot to mention it was 15 #12s (hot) with a #12 ground.

    [ May 09, 2003, 03:02 PM: Message edited by: crazytom ]

    #2
    Re: Current Reduction of Multiple Conductors in one Conduit

    15 hots would require a 50% derating (per NEC Table 310.15(B)(2)(a). The actual maximum allowed current would depend on the size of the conductors. You start with the 90C ampacity from Table 310.16, and derate from there.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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      #3
      Re: Current Reduction of Multiple Conductors in one Conduit

      You did not mention whether you had a neutral. It might make a difference. Also if you have harmonic loads it will make a difference.

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        #4
        Re: Current Reduction of Multiple Conductors in one Conduit

        The greatest enemy of conductor insulation deterioration is heat. Each CURRENT-CARRYING CONDUCTOR produces heat that effects all other conductors withing the conduit or cable. Where more than 3 conductors are within a conduit or cable, the adjustment factors listed in Table 310.15(B)(2)(a)must be applied. For example, where a 3-phase, 4-wire system is installed in a raceway and there are 3rd harmonic currents in the neutral conductor, an adjustment factor of 80% is applicable.

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          #5
          Re: Current Reduction of Multiple Conductors in one Conduit

          Well I'll be, I forgot the most inportant part.
          15 number #12 Hots with a #12 ground.

          So in other words it would be under 10 amps per leg on the 15 hots!?!

          [ May 09, 2003, 03:03 PM: Message edited by: crazytom ]

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            #6
            Re: Current Reduction of Multiple Conductors in one Conduit

            No, it would be 15 amps per leg. 15 is 50% of 30, and 30 is the 90C rating for #12 thhn.
            Charles E. Beck, P.E., Seattle
            Comments based on 2017 NEC unless otherwise noted.

            Comment


              #7
              Re: Current Reduction of Multiple Conductors in one Conduit

              Crazytom
              You have 5 conductors per phase for a 3 phase installation.(total 15 phase conductors) Would I be wrong to assume that the voltage is 120/208 or 277/480? If so did you install a neutral along with the grounding conductor?

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