Derating for number of conductors in conduit

Discussion in 'Electrical Forum discussion & Blog' started by cacher_chick, Feb 21, 2010.

  1. cacher_chick

    cacher_chick Test, Don't Guess!

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    Been doing some reading and want to make sure I understand-

    If there are from 7 to 9 current carrying conductors in a raceway, the maximum allowed current is 70% of the rating for the conductor.

    For example, I installed 4- 120v branch circuits (8 conductors) which left the panel in one conduit before splitting off in different directions.

    12 GA THHN/THWN is rated at 25A, & 70% is 17.5A, meaning each branch must be connected to a 15A breaker.

    I found this to be surprising as I had assumed that a larger conduit would circumvent the need for derating.

    Am I applying this correctly?
  2. codeone

    codeone Code Enforcement

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    You use the 90 degree column from table 310.16 for your derating. #12 is good for 30 amp in the 90 degree column.
    So it would be 30 x 70% = 21Amp you can use a 20 a breaker.
    A larger conduit does not circumvent the need for derating.
    Last edited: Feb 21, 2010
  3. cacher_chick

    cacher_chick Test, Don't Guess!

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    Thanks for that codeone....

    Given the above info, what if my new branch connects to old work that is wired with type TW wire?
    Does the whole circuit have to then be derated in the 60 degree column?
    What if this old work is only 2 conductors in a conduit?
  4. codeone

    codeone Code Enforcement

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    TW is rated for 25 A in the 60 degree column. #12 is limited elsewhere in the code to 20 A. You would only have a problem if you had TW running in the pipe with the THHN/THWN conductors due to the fact the TW would have to be derated from the 60 degree colum. The TW insulation cannot handle the heat the THHN/THWN insulation can. Now if you attach the the THHN/THWN to a TW (to extend the circuit) thats not in the same pipe there would be no problem with the installation.
    If they are in the same pipe you would need to derate for the weakest conductor insulation.
  5. cacher_chick

    cacher_chick Test, Don't Guess!

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    Thank you codeone for taking the time to clarify this for me, I really do appreciate it!
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