Results 1 to 9 of 9

Thread: Derating the ampacity of wires in a conduit 310.15(specifically table 310.15(B)(3)(a)

  1. #1
    Join Date
    Jan 2011
    Location
    Ft Collins, CO
    Posts
    30

    Derating the ampacity of wires in a conduit 310.15(specifically table 310.15(B)(3)(a)

    Setting ambient temps aside, I would like to know if a boat(three conductors and a neutral) should be counted as 3 or 4 current carrying conductors. According to 310.15(B)(5)(a) the neutral does not need to be counted. Can someone give me an example of (b) in the same section. If I read that correctly it is saying if there are more than one phase conductors(i.e boat) then the neutral does need to be counted when derating the wire ampacity. Also, when derating which temp column can I use to derate(60, 75, or 90)? Of course all this aside, we are now required to simultaneosly turn off all three circuits if we share that neutral. Thanks for your thoughts

  2. #2
    Join Date
    Jul 2003
    Location
    New Jersey
    Posts
    18,929
    The shared neutral would only count as a CCC if more than 50% of your load were non-linear. Not very likely IMO. You would use the ampacity at the temperature rating of the conductor for derating purposes. i.e.- THHN 90° C.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  3. #3
    Join Date
    Jan 2011
    Location
    Ft Collins, CO
    Posts
    30

    Lug rating

    I was under the impression that the lug rating was also used in determing the ampacity rating. If the lug is only rated fro 75 degrees then it didn't matter that the THHN was 90 degree rated. I read the definition of a non-linear load and I must admit I don't understand. How would I determine if over 50% of my loads were non-linear. Can you give me an example of a non-linear load?

  4. #4
    Join Date
    Apr 2008
    Location
    Washington State
    Posts
    14

    johngary

    Since you are questioning whether the natural should be counted 310.15 (B)4 clarifies when the Neutral Conductor is to be counted.

    Note not everyone know what a boat is -- it is a slang term for running a 3 Phase circuit using One Neutral -- to supply 120Volt loads in Wye configuration.

    In this case a two wire circuit using the neutral or a 3 wire circuit carries the approximately same current as the line-to-neutral load currents and shall be counted when applying 310(15)(2)(a)

    In most cases -- circuits up to 100amp must use the 60deg column for protection (breaker) -- depending on the wire type -- for the most part all new circuit size wire 14, 12, 10 are THHN and are rated for 90 deg in a dry location. so you can use the 90deg column amperage for de-rating purposes, but you must fuse the protection base on the 60deg column.

    Hopes this helps

    JG

  5. #5
    Join Date
    Jul 2003
    Location
    New Jersey
    Posts
    18,929
    Quote Originally Posted by Guyute View Post
    I was under the impression that the lug rating was also used in determing the ampacity rating. If the lug is only rated fro 75 degrees then it didn't matter that the THHN was 90 degree rated. I read the definition of a non-linear load and I must admit I don't understand. How would I determine if over 50% of my loads were non-linear. Can you give me an example of a non-linear load?
    Some ballasts or power supplies both of which are undergoing design changes to limit their non-linear traits. You can take a look at this thread:

    http://forums.mikeholt.com/showthrea...nic-Distortion

    If the terminals are rated for 75° C then you can start with the 90° C conductor rating and perform your derating calculation. Your usable adjusted ampacity cannot exceed the 75° temperature rating of the terminal.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  6. #6
    Join Date
    Jan 2011
    Location
    Ft Collins, CO
    Posts
    30
    Given that 310.15(B)(5)(b) does require me to count the neutral when using multiwire circuits, You could then pull seperate neutrals to each circuit thus not needing to count the neutral because (5)(a) would then apply. Thanks for the help.

  7. #7
    Join Date
    Jul 2003
    Location
    New Jersey
    Posts
    18,929
    Quote Originally Posted by Guyute View Post
    Given that 310.15(B)(5)(b) does require me to count the neutral when using multiwire circuits, You could then pull seperate neutrals to each circuit thus not needing to count the neutral because (5)(a) would then apply. Thanks for the help.

    Certain MWBC's do not require you to count the neutral as a CCC. In a two wire circuit the neutral is always a CCC. In a three phase, 4 wire MWBC from a WYE system the neutral is not considered a CCC.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  8. #8
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    16,910
    Quote Originally Posted by Guyute View Post
    You could then pull seperate neutrals to each circuit thus not needing to count the neutral because (5)(a) would then apply.
    Not true at all! If instead of pulling three hots and one neutral (total of 3 current-carrying conductors) you pull three hots and three neutrals, you wind up with three circuits, each being a 2-wire circuit, and you have to count all 6 wires as being current-carrying. In this case, 5(a) (2011 NEC) does not apply, because each of the three neutral wires will carry the same current as the hot wire with which it is associated, and not just an unbalanced current.

    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  9. #9
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    16,910
    Quote Originally Posted by Guyute View Post
    I was under the impression that the lug rating was also used in determing the ampacity rating. If the lug is only rated fro 75 degrees then it didn't matter that the THHN was 90 degree rated.
    The lug rating is used, in that you can't have a "final calculated value" of ampacity that exceeds that shown in the column (i.e., 60, 75, or 90) that matches the lug's temperature rating. But you can start the derating process with the column associated with the wire's temperature rating. See 110.14(C) for the rule allowing this.

    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

Posting Permissions

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