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Thread: Underground cable derating

  1. #1
    Join Date
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    Underground cable derating

    I have a situation where we have a 3000A overcurrent protection (main service) and the calculated load at 2100A. I'm running the conduit in a duct bank, do I need to derate the cables based on the calculated load or the overcurrent device? In another word, when I do the calculations, should the total ampacity of the cables in the ductbank be at minimum of 2100A or 3000A?

    Thanks for thoughts in advance

  2. #2
    Join Date
    Jul 2003
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    My opinion.

    You size the cable per the OC device that's protecting the cable at it's supply end (240.21), unless it's a tap and complies with 240.21(B).

    steve

  3. #3
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    Derate means you are going to install a larger cable to provide the same ampacity. You are hoping to under size your cables. Your overcurrent protection must protect your wire and equipment, so you would need a minumum wire ampacity of 3000A after any derating. Sounds like this one might be over you head if you are doing this install yourself. I would recommend haven't someone design the service for you.

  4. #4
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    See 230.90(A)- "Such protection shall be required by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor..."

    Now look at exception number 2. This exception allows the service conductors to be smaller than the ampacity of two to six service disconnects provided that the service conductors are large enough for the total calculated load. In short, when there is only one service disconnect, the service conductors must be fully sized based on the service overcurrent device.

    Marty
    Marty Schumacher
    Electrical Inspector, Plans Examiner, Code Instructor


    "All men by nature desire to know"

  5. #5
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    I don’t understand your use of the word “derate.” What makes you think that the ampacity of the conductors might have been one value, but now must be given a lower rating because of something about the installation? If you put more than three current-carrying conductors in a conduit, then you reduce (derate) the ampacity of the conductors. If the ambient temperature is over 86F, then you reduce (derate) the ampacity of the conductors. Does one of these situations apply?

    As to making selections of conductors and overcurrent settings, the process goes as follows:

    · Step 1, calculate the load. You have done that, with the result of 2100 amps.
    · Step 2, select a conductor that has an ampacity, under the installed conditions, of at least 2100 amps.
    · Step 3, provide an overcurrent protection device that will protect the selected conductors.

    So if you select conductors with an ampacity of only 2100 (e.g., 5 sets of 600 MCM THHN), then you must set the OCPD to no higher than 2100 amps. If you wish to use an OCPD setting of 3000 amps, then you must select conductors that have an ampacity of at least 3000 (e.g., 8 sets of 500 MCM THHN).
    Charles E. Beck, P.E., Seattle
    Comments based on 2008 NEC unless otherwise noted.

  6. #6
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    Since this installation is underground, table 310-16 won't apply. The IEEE "brown book" section 13.4 discusses this. Appendix B in the NEC can provide you with some helpful information. Earth is a good insulator and will retain the heat generated by the current through the cable. The ampacity of the cable in an underground installation MUST be reduced

  7. #7
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    Quote Originally Posted by bsh View Post
    Since this installation is underground, table 310-16 won't apply. The IEEE "brown book" section 13.4 discusses this. Appendix B in the NEC can provide you with some helpful information. Earth is a good insulator and will retain the heat generated by the current through the cable. The ampacity of the cable in an underground installation MUST be reduced
    ??? Are you sure ???

  8. #8
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    The description for table 310-16 says "not more than 3 current carrying conductors" so it won't apply to your installation.

  9. #9
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    Quote Originally Posted by bsh View Post
    The description for table 310-16 says "not more than 3 current carrying conductors" so it won't apply to your installation.
    That table shows the ampacity based on up to 3 conductors, at ambient temperatures. This is the baseline condition.

    You must use this table. If there are more then 3 current carrying conductors then you have to derate based on Table 310.15(B)(2)(a). If your temperatures are other then ambient the you must derate based on correction factors at the bottom of table 310.16.

    You can't disregard the NEC. I don't know about the brown book, but I don't believe it has any regulation of this service. Sounds like it is a design reference, but at the end of the day you must meet NEC requirements. Somebody correct me if wrong.

  10. #10
    Join Date
    Oct 2006
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    Years ago (1986) table 310-16 included "in free air". At that time Appendix B (now called Annex B) gave guidlines on the ampacity of underground installations of cable. This annex is not an NEC requirement and is included only as information.
    Logically, if the ampacity of a cable in conduit is "X" then when that same cable is put in the ground the current carrying capacity is reduced because the earth acts as a barrier to heat dissipation. Annex "B" refers to Neher and McGrath (as does IEEE "Brown Book")and discusses underground cable ampacity.
    Table 310-16 is an instance where the NEC is not perfectly clear and could be applied for underground installations
    I believe that most systems may not load the service continuously and therefore few problems may be encountered for underground installations. If the actual load is not known then the worst case must be used (3000 amps in this case) and the feeder must be sized to carry that load continuosly.

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