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Thread: PVC duct bank question.

  1. #1
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    PVC duct bank question.

    PVC duct bank question.

    (I have already been looking but can’t find a reference to this.)

    Question:
    Is there a mandate or a code requirement in NEC 2005 that requires that I put chairs on PVC in a duct bank?

    Specifics
    1. (5) Five feet deep
    2. No concrete
    3. 6” sand envelope (bottom, top and sides).
    4. (8) eight 2” PVC with 3 phase 480 feeders
    5. Located under building pad.
    Type fast before a mod comes along and shuts down the thread...

  2. #2
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    I can only come up with 310.15(B)(2)(b) & annex B. Annex B has separations from 7.5" to 10" center to center of pipe. Hope this helps.
    May your electrons flow forever, and mine, one day longer! Ron

  3. #3
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    I never use them. Here are two 3" and two 2" over four 3". At 480V your 300.5 not 300.50


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  4. #4
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    310.15(B)(2)(b) does require that spacing be maintained. But that is similar to the requirement that for parallel conductors the distances be equal. How equally spaced do they have to be? For my money, if you can find a way to keep the conduits spaced in a reasonably even way through the use of sand or other material, I would be happy.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  5. #5
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    Quote Originally Posted by charlie b
    310.15(B)(2)(b) does require that spacing be maintained. How equally spaced do they have to be?
    I'm thinking more along the lines of what is space. A mm, 4", mabey 10' ?

  6. #6
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    I submitted a proposal to specify the minimum space between conduits 3 or 4 code cycles and the panel rejected it with a comment that if you have enough space to install couplings and locknuts you have spacing.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  7. #7
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    Quote Originally Posted by don_resqcapt19
    I submitted a proposal to specify the minimum space between conduits 3 or 4 code cycles and the panel rejected it with a comment that if you have enough space to install couplings and locknuts you have spacing.
    Don
    Well then Don that means the conduits are touching each other. So you should propose that the term raceway is taken out of 310.15(B)2(B).

    BTW what was the min. spacing you had in mind and why? Have you seen an underground failure due to heating?

  8. #8
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    Chris,
    Well then Don that means the conduits are touching each other.
    No, if the couplings and locknuts are touching, the conduits will have small spaces between them.
    BTW what was the min. spacing you had in mind and why?
    Not sure what spacing I wanted, and the only reason was that the current rule is vague and unenforceable.
    Have you seen an underground failure due to heating?
    I have not, but there are a number of documented cases where they have, but only when the loads were not calculated using Article 220. These cases were the supporting information for what is now Annex B. At one time those tables were going to be in the actual code rule. They were published in Article 310 with an effective date 3 years later, but moved to the annex in the next code cycle.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  9. #9
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    Quote Originally Posted by don_resqcapt19
    Chris,No, if the couplings and locknuts are touching, the conduits will have small spaces between them.
    I beg to differ. Given the inherent flex of RNC conduit I have long runs that are pushed together during backfill. So as far as ITO's OP, we do not worry about or enforce this.

  10. #10
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    Quote Originally Posted by chris kennedy
    I beg to differ. Given the inherent flex of RNC conduit I have long runs that are pushed together during backfill. So as far as ITO's OP, we do not worry about or enforce this.
    That may be one way to look at or I might be inclined to say that the spacing of at least the couplings and connectors must be maintained and that because of PVC flexibility we have to provide something to maintain that spacing.

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