Results 1 to 8 of 8

Thread: conductor derating based on insulation rating

  1. #1
    Join Date
    Jul 2018
    Location
    Honolulu, HI, USA
    Posts
    2

    conductor derating based on insulation rating

    In conforming to NEC Table 310.15(B)(3)(a) to de-rate more than three THHN current-carrying conductors in raceway, we use the 90 degree C insulation rating in Table 310.15(B0(17). The Contractor is insisting on using the 75 degree C rating for this; we believe the 75 degree C rating only applies to overcurrent protection, not temperature de-rating. Which of us is correct?

  2. #2
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    33,306
    IMO, you are correct. BTW, welcome to the forum

    110.14(C) Temperature Limitations. The temperature rating associatedwith 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.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  3. #3
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    19,003
    Quote Originally Posted by mkhudson View Post
    Which of us is correct?
    Neither.

    110.14(C) says you can use the higher temperature column for both adjustments (meaning more that 3 CCCs in a conduit) and correction (meaning for ambient temperatures higher than 30C). So that makes the contractor wrong. But I don't know what you mean by "the 75 degree C rating only applies to overcurrent protection." So I can't say you are right.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  4. #4
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    33,306
    Quote Originally Posted by charlie b View Post
    Neither.

    110.14(C) says you can use the higher temperature column for both adjustments (meaning more that 3 CCCs in a conduit) and correction (meaning for ambient temperatures higher than 30C). So that makes the contractor wrong. But I don't know what you mean by "the 75 degree C rating only applies to overcurrent protection." So I can't say you are right.

    I assumed he meant the terminals on the overcurrent protective device.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  5. #5
    Join Date
    Jul 2018
    Location
    Honolulu, HI, USA
    Posts
    2
    Thank you very much. We use the 75 degree C column to determine conductor overcurrent protection limitations, per table 310.15.(B)(16). But for de-rating, we use the 90 degree C column which is the actual insulation type.

  6. #6
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    33,306
    Quote Originally Posted by mkhudson View Post
    Thank you very much. We use the 75 degree C column to determine conductor overcurrent protection limitations, per table 310.15.(B)(16). But for de-rating, we use the 90 degree C column which is the actual insulation type.

    You have it correct assuming 90C wire which just about everything is today.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  7. #7
    Join Date
    Jul 2003
    Location
    New Jersey
    Posts
    25,336
    Quote Originally Posted by mkhudson View Post
    In conforming to NEC Table 310.15(B)(3)(a) to de-rate more than three THHN current-carrying conductors in raceway, we use the 90 degree C insulation rating in Table 310.15(B0(17). The Contractor is insisting on using the 75 degree C rating for this; we believe the 75 degree C rating only applies to overcurrent protection, not temperature de-rating. Which of us is correct?
    If the conductors are rated for 90° C then you can use that ampacity for derating purposes but you are not required to. If you use the 75° C ampacity you may end up using larger conductors and wasting some money in the process.

    I believe that the correct table you're referencing is 310.15(B)(16) for conductors in a raceway.

    Welcome to the Forum.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  8. #8
    Join Date
    Dec 2008
    Posts
    520
    There are some differences between art. 110.14(C) and 310.15(B)(3)
    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).
    What “ampacity”?
    110.14(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.
    Because we design power plants we do not use tables but calculate the ampacity.
    In art.310.15(C) Engineering Supervision it is the formula:
    I=√(Tc-Ta)/(Rdc(1+Yc)Rca)
    Following Neher & McGrath and IEEE 835 we start with the rated conductor temperature [let’s say 90oC]
    If we follow IEEE 835/1994 Annex A number of conductors is built-in Rca formula:
    Rca=Ri+n(Rj+Rsd+Rc+Re) where:
    Ri insulation thermal resistance [T.O.F.]
    Rj jacket thermal resistance
    Rsd =Cable to conduit
    Rc=Conduit wall
    Re=is thermal resistance of the earth or external thermal circuit (air, duct bank, etc.)
    n=number of conductors within a stated diameter[Neher & McGrath Nomenclature]
    That means-in my opinion- the derating purpose it is to adjust the ampacity in order to keep conductor temperature at 90oC.[it is not in order to reduce the conductor temperature].

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •