Forums » Electrical Engineering

Pvc underground conduit

    • 4 posts
    August 25, 2019 1:36 AM PDT

    does pvc underground conduit need to be spaced apart. If it does what is the code article. 2) what is the meaning of 310.15(3)(b)

    • 160 posts
    August 25, 2019 4:42 AM PDT

    310.15(B)(3)(b) Raceway Spacing. Spacing between raceways shall be maintained.

    Well, according to this section then the raceways would have to be spaced.  The unfortunate part is that it does not say how far apart.  If conduits are laying atop of one another then, IMO, de-rating would have to take place. Space them 1/4" apart or less and you're good.  Obviously you may want to leave more than that if possible.

    Remember in most cases using #12 or 14 gauge 90C wire the derating won't affect anything until you get to 10 conductors.

    #12 @ 90C is rated 30 amps--9 current carrying conductor's requires a 70% de-rate.  30 x .7= 21 amps so you are still good for 20 amps.

    #14 @ 90 C is rated 25 amps- 9 current carrying conductor's require a 70% de-rate.  25 x .7 = 17.5 amps which is good for 15 amps

    • 75 posts
    August 29, 2019 1:59 PM PDT

    Ed -

    Have you looked at Informative Annex B.  It discusses Duct Bank orientation, spacing, and ampacity.

    It is not part of the NEC.  It is an engineering supplement.


    And, No, I don't understand the meaning of 310.15.3.b.  What is the point: 

    Slide a sheet of paper between the conduits and you are good.  And if they touch, you're not?

    Let the the spacing wobble around from 1" to 2" and you're not?  That doesn't make much sense.


    Look at annex B



    • 103 posts
    August 29, 2019 6:39 PM PDT

    One possible reason for spacing:

    Suppose the conductors in one conduit have an insulation failure & a ground fault results.  If the conduit is conductive & touching another conduit the fault current will use the second conduit as well as its own to return to the source. Possibly damaging a second unrelated circuit. 

    If the conduits are PVC & a fault burns through the wall having an inch or two of cement between the hot spot & a second conduit could protect the second conduit from damage.

    Another reason for spacing could be heat dissipation.