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Thread: NEC Table 310.15(B)(16)

  1. #1
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    NEC Table 310.15(B)(16)

    Existing Table Title:

    Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60C Through 90C (140F through 194F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F)

    Proposed Table Title:

    Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60C Through 90C (140F through 194F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F), Where Not Installed in an Underground Bank of Raceways or Underground Multiple Cable Installation.

    Discussion

    The title of Table 310.15(B)(16) implies that the ampacity indicated in the table can be applied to underground duct banks or underground direct-buried cable banks where any number of cables can be installed. The IEEE Standard Power Cable Ampacity Tables, IEEE Std 835-1994, clearly indicates that there is a loss of ampacity when multiple parallel circuits are installed together in a duct bank. This loss of ampacity is comparable, though not identical, to the adjustment factors given in Table 310.15(B)(3)(a).

    In industrial applications, it is not uncommon to parallel four, six, eight or more conduits and sets of power cables to achieve the desired rating. Failing to properly derate cables in these circumstances will lead to overheating of cables and their failure. The correct application of Table 310.15(B)(16) should include the use of adjustment factors in Table 310.15(B)(3)(a) wherever more than 3 current carrying conductors are installed underground (or encased), regardless of the number of encased cables or conduits.
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  2. #2
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    Where is the adjustment requirement now for duct bank installation in the NEC? Would that already cover this?
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  3. #3
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    What is a raceway?

    310.15(B)(3) talks about more than three conductors per raceway or where raceway spacing is not maintained or where no raceway is used, not where raceways are used and the current-carrying conductors per raceway are 3 or less.

    310.15(B)(3)(b) says spacing of raceways is to be maintained.

    Given all of that, if I install a 9-way duct bank, with 9 4-inch conduits, each with four 750kcmil CU XHHW-2, per Table 310.15(B)(16) each duct can support 535A. IEEE 835-1994 (25C not 30C) states that each duct can support 303A @ 100%LF. Heating at 535A is ~300% that at 303A, the cables will overheat.

    If I use the Table 310.15(B)(2)(a) 1.04 adjustment for 25C vs 30C, the IEEE 303A rating becomes 291A.

    If I use the Table 310.15(B)(3)(a) adjustment for 27 conductors, the 535A becomes 240A and overheating will not occur.

    The problem is that the NEC, as written, can lead to an NEC-compliant design that leads to overheating of conductors in duct banks.

    My argument is that thermally an underground duct bank is a "raceway."
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