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Thread: De-rating conductors

  1. #1
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    De-rating conductors

    I know that the 90 deg C column can be used for calculating adjustment factors when 4 or more current carrying conductors are used in a conduit. But where is it specifically noted in the code. Chapter and verse.
    Thank you

  2. #2
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    Look at 110.14(C).
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  3. #3
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    Quote Originally Posted by infinity View Post
    Look at 110.14(C).
    That's the right spot. But you might consider waiting until you are off work and have a beer in your hands, before trying to figure out exactly what that article is telling you.

    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  4. #4
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    The article 114(C) (C) Temperature Limitations.it is a little confusing:
    The temperature rating associated with the ampacity of a conductor shall be selected
    (C) Temperature Limitations. The temperature rating associated
    with the ampacity of a conductor shall be selected
    and coordinated so as not to exceed the lowest temperature
    rating of any connected termination, conductor, or device.
    Conductors with temperature ratings higher than specified
    for terminations shall be permitted to be used for ampacity
    adjustment, correction, or both.
    310.15 Ampacities for Conductors Rated 0–2000 Volts (B) Tables:
    The temperature correction and adjustment factors shall
    be permitted to be applied to the ampacity for the temperature
    rating of the conductor, if the corrected and adjusted
    ampacity does not exceed the ampacity for the temperature
    rating of the termination in accordance with the provisions
    of 110.14(C).
    For instance let's take 1/0 copper conductor 90oC insulated
    From Table 310.15(B)(16) I=170 A for 3 conductor per position
    Now let's take 6 conductors from Table 310.15(B)(3)(a)
    I=170*.8=136 A
    From Table 310.15(B)(16) I=150 A for 75oC
    That means since 136<150 A it is permitted.

  5. #5
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    Quote Originally Posted by Julius Right View Post
    For instance let's take 1/0 copper conductor 90oC insulated
    From Table 310.15(B)(16) I=170 A for 3 conductor per position
    Now let's take 6 conductors from Table 310.15(B)(3)(a)
    I=170*.8=136 A
    From Table 310.15(B)(16) I=150 A for 75oC
    That means since 136<150 A it is permitted.
    Since your example has 6 CCC's you're derating will be 80% as you stated. However after the calculation those conductors are only 136 amp conductors and that is their maximum ampacity. Depending on the particulars you may be able to use a 150 amp OCPD with the 136 amp conductors via the next size up rule.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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