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Thread: CEC Table 5c vs 310.15(B)(3)(a)

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    CEC Table 5c vs 310.15(B)(3)(a)

    Can anyone explain why the two tables (NEC and CEC) referenced in the title are so drastically different? 1-6 conductors are the same, but after that NEC is significantly more conservative. The tables for a #12 90c THHN (T90 equivalent) in both codes give a 30A starting value. For 20A circuits you need to move to a #10 after the 9th conductor in NEC, but not until the 25th in CEC? What am I looking past? Thanks!


    Table 310.15(B)(3)(a) NEC 2014
    Number of conductors vs derating value
    4-6 = 80%
    7-9 = 70%
    10-20 = 50%
    21-30 = 45%


    Table 5C
    Number of conductors vs derating value
    4-6 = 80%
    7-24 = 70%
    25-42 = 60%

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    I have no idea why but that is a heck of a difference. I can't imagine 24 conductors at only 70%
    They say I shot a man named Gray and took his wife to Italy
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    Quote Originally Posted by Dennis Alwon View Post
    I have no idea why but that is a heck of a difference. I can't imagine 24 conductors at only 70%
    I don't know the reason for the vast difference either, but FWIW Massachusetts has amended the NEC to 70% for 7-24 conductors, same as that CEC chart.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Could it be due to the normal temperatures? Canada does not hit the higher summer temps as hit by parts of the USA, so the wires do not face the higher ambient temperatures??
    Just thinking... know Jamaica is considering smaller groupings due to higher climate temps..
    Student of electrical codes. Please Take others advice first.

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    Quote Originally Posted by Adamjamma View Post
    Could it be due to the normal temperatures? Canada does not hit the higher summer temps as hit by parts of the USA, so the wires do not face the higher ambient temperatures??
    Just thinking... know Jamaica is considering smaller groupings due to higher climate temps..
    I do not know why they are different, but Outside temperatures have nothing to do with it.

    It gets a lot hotter here here than many assume and besides, the insides of houses and building are climate controlled so our wires are at the same ambient temp as yours.

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    But most homes in Jamaica do not have climate control. An AC unit here is too expensive for most who work for the equivalent of 80 canadian dollars a week or less.
    Student of electrical codes. Please Take others advice first.

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    The NEC used that same table for a long time, but it specified that it only applied where the conductors had a 50% load diversity. That was not a defined term and when they attempted to define it, they could not agree on a definition. That table is still in the NEC but not as a code rule, it is in Informative Annex B as Table B.310.15(B)(2)(11).
    I believe the Massachusetts amendment also uses that table.

    Does the Canadian Code say anything about load diversity for the application of that table?
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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    In the 1990 code the ampacity adjustment rule was in Note 8 to the 0-2000 volt ampacity tables. That edition had two columns with differing derating values. Column A was for installations with a 50% load diversity where you had over 9 current carrying conductors. Column B was the same as we have in the code now. In the 1993 code the table only had the column without load diversity in Article 310, and the information from the other column was relocated to Annex B.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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    Quote Originally Posted by don_resqcapt19 View Post
    The NEC used that same table for a long time, but it specified that it only applied where the conductors had a 50% load diversity. That was not a defined term and when they attempted to define it, they could not agree on a definition. That table is still in the NEC but not as a code rule, it is in Informative Annex B as Table B.310.15(B)(2)(11).
    I believe the Massachusetts amendment also uses that table.

    Does the Canadian Code say anything about load diversity for the application of that table?
    What is load diversity?

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    Quote Originally Posted by Eddy_Current View Post
    What is load diversity?
    No one could really define it and that is why that table is no longer a code rule.
    50% load diversity could mean 50% of the conductors in the raceway or cable loaded at 100% of their rating, or could be all of the conductors loaded to 50% of their rating.
    The second one would make more sense as that would be less heat produced from the I²R losses.
    Either way, it would be difficult to enforce.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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