Running Conduit?

Discussion in 'Unique Lighting' started by JoeyD, Mar 13, 2008.

  1. JoeyD

    JoeyD LawnSite Silver Member
    Messages: 2,933

    Did you know you can not run more than 3 current carrying cables in one piece of conduit? I just learned this as we were looking to increase our knockout size on our new top secret transformer. No point in having a 2" knockout if you can only legally pull 3 wires through it!! Just thought I would share to all you information junkies!!
  2. Mike M

    Mike M LawnSite Bronze Member
    from usa
    Messages: 1,988

    Holy crap. I was just completely impressed with the conduit I saw at FOLD in Orlando, that came with the new CAST transformers. I figured, great, just put all the wires up one big conduit.

    Great info, Joey, thanks.

    Is that an NEC thing? Must be. Not a manu or product issue, but an installer issue.
  3. Supper Grassy

    Supper Grassy LawnSite Bronze Member
    Messages: 1,582

    Joey nice pic in yor sig.
  4. JoeyD

    JoeyD LawnSite Silver Member
    Messages: 2,933

    Mike, yes it is an NEC thing, I can dig it up if others want to dispute it.....

    Thanks Supper Grassy, we're going to shoot this job one more time and work out some of the hot spots but yeah its coming out nice!! Believe it or not the lighting isnt done!!
  5. stumpjumper

    stumpjumper LawnSite Member
    Messages: 183

    The only stipulations I'm aware of concerning the number of current carrying conductors in conduit is that if you have more than 3 then you have to derate your wire ampacity according to the charts in the code book. Without dragging out my code book I believe the charts are in section 319.
  6. JoeyD

    JoeyD LawnSite Silver Member
    Messages: 2,933

    Alright stumpjumper, your gonna make me get that damn NEC book out again...First off you are correct about having to deregulate the wire if you run multiple current carrying cables in one piece of conduit. Thanks for helping me clear this up!!

    So here it is......

    NEC Article 310.15 2 (a)?
    .........More than 3 current carrying conductors in a raceway or cable. Where the number of C-C Conductors in a raceway or cable exceeds 3, or where single conductors or multiconductor cables are stacked or bundles longer than 24" without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in table 310.15(B)(2)(a). Ea. C-C conductor of a paralleled set of conductors shall be counted as a c-c conductor.

    So in a nut shell table 310.15(B)(2)(a) says......

    if you have 4-6 C-C Cond. you can only use 80% of that cables rated if using 12ga that would be 12amps I believe.

    7-9 C-C Cond. you can only use 70% of the cables rated ampacity....

    and so on and so forth.......So you can run more but now you have to de regulate the cable quite a bit! In our opinion it is better to just use 2 or 3 smaller conduits and run no more than 3 in ea. of them.
  7. stumpjumper

    stumpjumper LawnSite Member
    Messages: 183

    My calculation on no. 12 comes out to 16 amps. at 80%. It's a question of which is the most economic on whether to use more runs of smaller pipe or bigger pipe and derate the wire. If the individual loads are less than the derated wire ampacities than bigger pipe is usually cheaper.
  8. JoeyD

    JoeyD LawnSite Silver Member
    Messages: 2,933

    You have to de regulate the wire to 80% anyway even if you run only 1 wire, I think once you run it into a conduit with 4-6 you have to deregulate it again which would bring it to 12amps. This seems stupid but I think it is correct because as you run more wires in one conduit the heat builds. In most cases I think we are just fine with the way it has been done, but as many of you know at any time an inspector can call you out on the stupidest of things, and it goes by their interpetation which this is exactly how it was interpeted to me by the NEC expert I spoke with.
  9. stumpjumper

    stumpjumper LawnSite Member
    Messages: 183

    No you don't derate the wire until you get above the 3 energized wires, and nuetrals are counted as current carrying conducters. You also must take into account the different temperture ratings of the wire type. Such as THN rated as a 70 degree C. wire and THHN rated as a 90 degree C. wire. The derating table columns are titled "More than 3 current carrying conductors in conduit". And you're right on about the inspectors, I argued interpertations of the code book with them for 25 years, it's like mud wrestling with a pig. There is I believe a paragraph that states branch circuits should not be loaded at more than 80%, but to my knowledge this doesn't affect the wire ampacity rating and should not fiqure into the calculation.
  10. Lite4

    Lite4 LawnSite Gold Member
    Messages: 3,187

    I hate a lot of small conduits running into my trannys, I like one big one, it is so much cleaner. Thanks a lot, you just ruined my day. Why did I have to look at this thread, now I will have to do it correctly.:cry:

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