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310.15(B)(3)(a) Meaning of "maintaining spacing"

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    310.15(B)(3)(a) Meaning of "maintaining spacing"

    Hey All

    Im a little confused about some language in section 310.15(B)(3)(a).

    The code reads:

    (a) More than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.

    does the code ever define what "maintaining spacing" is? Is it a 1 cable diameter spacing? something else?

    My company would like to bundle some type NM multiconductor cables (romex) to create a harness for faster install in residential dwelling units. I have advised that derating factors will be applicable to these cables if we do not "maintain spacing" but I can't define what the required spacing is.

    Any help would be greatly appreciated!

    Thanks

    #2
    Welcome to the forum. The nec does not define how much space is needed just that there needs to be space. Technically a 1/100th inch or less is sufficient just as long as they are not touching
    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

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      #3
      Originally posted by Dennis Alwon View Post
      Welcome to the forum. The nec does not define how much space is needed just that there needs to be space. Technically a 1/100th inch or less is sufficient just as long as they are not touching
      Thanks Dennis!

      Comment


        #4
        I think what they want it to mean is to be intentionally arranged in a way to allow air to flow around conductors/cables to assist with heat dissipation. Casual or inadvertent spacing isn't good enough they want intentional spacing so things pretty much need to be secured to assure maintenance of such spacing.

        I don't know they are that successful at saying it in a way it is clearly understood though.
        I live for today, I'm just a day behind.

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          #5
          I interpret that statement as meaning that you install spacers every so often to force the cables to be at approximately the same distance from each other for the entire run. But then, I don't install cables, so I might be way off base here. I do know that the total impedance of the run will be influenced by the mutual inductance and the mutual capacitance between any two cables. Those values will be constant throughout the run if the spacing is the same throughout the run. But if the cables are sometimes closer to each other and sometimes farther from each other, the mutual impedance effect will vary as you progress down the run. That can cause uneven heating along the run. I would cite that as the reason for derating, if the spacing is not maintained.
          Charles E. Beck, P.E., Seattle
          Comments based on 2017 NEC unless otherwise noted.

          Comment


            #6
            While it is not done, I agree that the word "maintained" requires some physical method be used to maintain the space between the conductors or cables. Just the fact that there is space is not enough to eliminate derating.
            Don, Illinois
            (All code citations are 2017 unless otherwise noted)

            Comment


              #7
              Simple example: if you bore one set of holes through three studs in a row and run two cable through, you are not maintaining spacing. If your bore two sets of holes, you are.

              With what the OP describes about pre-bundling cables into a harness, I think he's exactly right about derating. That harness would have to be assembled with some kind of special parts to maintain separation, or derating would be required.

              Comment


                #8
                I always thought the phrase "maintained spacing" was to be interpreted as the distance of one cable diameter.
                But, I am pulling that from Art 392. This might be a new one for Art 100 in the future.

                Comment


                  #9
                  Cables through a series of holes are static and not going to move. If there is a space between them I don't see how that's not maintaining spacing. If a physical spacing device is required then the NEC should say that so. We've debated this for years with varying opinions and what we're left with is anyone's guess to what maintaining spacing actually means.

                  I would agree that the OP's idea of physically bundling a bunch of NM cables together would make derating mandatory.
                  Rob

                  Moderator

                  All responses based on the 2014 NEC unless otherwise noted

                  Comment


                    #10
                    Originally posted by infinity View Post
                    Cables through a series of holes are static and not going to move. If there is a space between them I don't see how that's not maintaining spacing. If a physical spacing device is required then the NEC should say that so. We've debated this for years with varying opinions and what we're left with is anyone's guess to what maintaining spacing actually means.

                    I would agree that the OP's idea of physically bundling a bunch of NM cables together would make derating mandatory.
                    Unless somehow you include some sort of spacers in the bundling method.
                    I live for today, I'm just a day behind.

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