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Thread: Underground Feeder Ampacity

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    Underground Feeder Ampacity

    I have 4000A and 2500A underground feeders from two main services to the building disconnects. A code official has concerns with the heat effects of the cables in conduit due their proximity to eachother. It has been stated that the cable ampacities in NEC table 310.15 do not apply because "underground conduit" is not one of the scenerios listed on the table (i.e "Raceway, Cable, or Earth - direct buried).


    I have been referring to articles 310.15c - Engineering Supervision and Annex B in an attempt to determine the required ampacity of conductors in this scenerio, however I am coming up unsuccessful.

    Does anyone have experience with this and how to find or calculate the adjusted ampacities?


    Thanks.

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    Those calculations are normally done by the use of electrical engineering software programs.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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    Quote Originally Posted by Scooter250 View Post
    It has been stated that the cable ampacities in NEC table 310.15 do not apply because "underground conduit" is not one of the scenerios listed on the table (i.e "Raceway, Cable, or Earth - direct buried).
    Are the cables in a raceway?

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    Quote Originally Posted by Scooter250 View Post
    I have 4000A and 2500A underground feeders from two main services to the building disconnects. A code official has concerns with the heat effects of the cables in conduit due their proximity to eachother. It has been stated that the cable ampacities in NEC table 310.15 do not apply because "underground conduit" is not one of the scenerios listed on the table (i.e "Raceway, Cable, or Earth - direct buried).


    I have been referring to articles 310.15c - Engineering Supervision and Annex B in an attempt to determine the required ampacity of conductors in this scenerio, however I am coming up unsuccessful.

    Does anyone have experience with this and how to find or calculate the adjusted ampacities?


    Thanks.
    Unless you are an engineer forget about 310.15c.

    I think you are referring to the title of the table:

    Table 310.15(B)(16) (formerly Table 310.16) 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)*
    "Underground conduit" is a raceway. The point of the wording here is that the values in the table are for three current carrying conductors or less that occupy the same raceway, cable or other space, if there is more than three current carrying conductors then you must derate the ampacities in the table. Note it also says "based on ambient temperature of 30C. I know it has been a hot summer for most of us, but conductors buried at least 18" deep or more probably are not in a 30C ambient very often, That means you can adjust ampacity upward. Keep in mind the exception to 310.15(A)(2) when doing so:

    Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.
    If above ground portion of the run is limited in length you can probably actually increase ampacity because of the underground ambient temperature.

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    Quote Originally Posted by Scooter250 View Post
    ... Does anyone have experience with this and how to find or calculate the adjusted ampacities?
    Don is correct. If you want to know more of what is involved in the calculation, google "Neher-McGrath" (link provided below)...

    http://www.google.com/search?sourcei...=g3g-C1g-CK1s2
    I will have achieved my life's goal if I die with a smile on my face.

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    I have done the Neher McGrath calculation many times, but always with a software package. The variables are too numerous and too complex to handle in a manual calculation. It should be noted that I have never come up with a calculated result that was higher than the values shown in table 310.16.

    This subject was discussed at a state-wide meeting of state inspectors about a decade ago. They agreed amongst themselves that Table 310.16 values could be used for underground installations, provided that the ampacity requirements were based on an article 220 load calculation. Their reasoning appeared to me to be that the calculation is conservative, and that the mutual heating effects would not be so severe as to outweigh that conservatism.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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