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Thread: derating/parallel conductors

  1. #1
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    derating/parallel conductors

    For a 400A service with all service entrance conductors paralled and in the same stick of RMC, and taking into account derating according to Table 310.15(B)(2)(a), Table 310.16 shows using at least 350 kcmil cables. With 6 total cables in the the RMC, it should be 3 to 3.5" based on Table C9.

    So, why do I see so many paralleled 4/0 conductors in 3" RMC for a 400A service?

  2. #2
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    Re: derating/parallel conductors

    Originally posted by jemsvcs:
    So, why do I see so many paralleled 4/0 conductors in 3" RMC for a 400A service?
    Most people will be using 4/0 90 C conductors but they are also in a wet location which means in most cases they will need to be figured at 75 C.

    4/0 @ 75 C is rated 230 amps * 2 runs * .8 adjustment factor resulting in an ampacity of 368 amps.

    This is fine for a 400 amp service if the calculated load is under 368 amps.

    See 240.4(B) and 240.6

    [ September 07, 2005, 04:40 PM: Message edited by: iwire ]

  3. #3
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    Re: derating/parallel conductors

    For what it is worth any 400 amp service I have built used one set of 600 Kcmil CU or two sets of 4/0s in separate raceways minimum.

    [ September 07, 2005, 04:49 PM: Message edited by: iwire ]

  4. #4
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    Re: derating/parallel conductors

    For what it is worth, the Residential Building
    Code permits paralleled sets of 4/0 for 400 amp services in single or separate raceways - Table E3503.1

    In NYS, this supercedes the NEC

  5. #5

    Re: derating/parallel conductors

    4/0 @ 75 C is rated 230 amps * 2 runs * .8 adjustment factor resulting in an ampacity of 368 amps.
    Cant the 90 degree column always be used for derating conductors?

  6. #6
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    Re: derating/parallel conductors

    Earlier I meant to reference Table C8 in reference to RMC.

    The way I see it is this:

    1. Table 310.15(B)(6) allows 4/0 Al for a 200A Service.
    2. 4/0 Al @ 75C is only rated for 180A.
    3. Taking into account the derating factor, 300 kcmil Al @ 75C would give us 230 x 0.8 = 184A per conductor.
    4. So two paralleled 300 kcmil conductors should be permissible - and 3" RMC would also be acceptable.

    Any comments on the reasoning here?

    Thanks,

    Jemsvcs

  7. #7
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    Re: derating/parallel conductors

    Cant the 90 degree column always be used for derating conductors?
    Only if the conductors are rated for 90°C. If you are using THHN/THHW it is only rated for 75°C in wet locations and you start with the 75°C rating for the derating. Note if the wire type has a "-2" suffix, it can be used at the 90°C rating in both wet or dry locations.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  8. #8
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    Re: derating/parallel conductors

    Jemsvcs, 310.15(B)(6) and Table 310.15(B)(6) are specific in text and there is no allowance to parallel any conductors using the tables derated ampacities.

    Although, I guess if you were to utilize 310.15(C) (pay an engineer) you could do it.

    Roger
    Moderator

  9. #9
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    Re: derating/parallel conductors

    Roger,

    If i'm going to pay those kind of rates to get the same answer I might as well send my lawyer some money too.

    Jemsvcs

  10. #10
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    Re: derating/parallel conductors



    Roger
    Moderator

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