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Thread: Equipment Grounding Conductor Run with 5KV or 15KV Feede

  1. #1
    Join Date
    Sep 2004
    Posts
    9

    Equipment Grounding Conductor Run with 5KV or 15KV Feede

    The NEC doesn't seem to differentiate the minimum size of the equipment grounding conductor run with feeder conductors for LV systems, say 600V and below, vs. medium and high voltage systems, say 5 KV and 15KV. Is the proper size for medium and high voltage systems based on 250.122?

  2. #2
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    19,423

    Re: Equipment Grounding Conductor Run with 5KV or 15KV Feede

    The code does not really address higher voltage systems, but remember that Table 250.122 is the minimum permitted size of the EGC and that 250.4 may require the use of a larger conductor.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
    Join Date
    May 2005
    Location
    Redmond, WA
    Posts
    784

    Re: Equipment Grounding Conductor Run with 5KV or 15KV Feede

    See the thread "melted ground wire" for some discussion on this. NEC required EGC wire size is a minimum. That size should be verified against the expected fault current and the fault clearing time and the cable's short time withstand curve.

    Several guides and cable suppliers' websites have these curves. (IEEE Brown Book, Okonite Technical Manual, Southwire Power Cable Manual)

    Note that the numbers quoted in the other post refer to the fusing limit of bare ground conductors which is around 1000C temperature. We really want to keep the copper temperature under 250C during a fault to prevent insulation damage.
    Bob Wilson

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