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Thread: NEC 210.122 @ 15kA 34.5kV

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    NEC 210.122 @ 15kA 34.5kV

    According to NEC 250.122, a #4 copper wire is an adequate EGC for a 300A circuit. However, I was reviewing a design where this #4 AWG copper is proposed for a 34.5kV underground line where the available fault current exceeds 15,000A. Per my Southwire Power Cable manual, #4 AWG Copper will be damaged in about 1 cycle at 15kA. Since even the fastest 34.5kV circuit breaker takes 3 cycles, I would expect this #4 ground to be damaged on the first through fault. However, the designer is saying "per NEC 250.122." From what I can find NEC 250.122 applies at all voltages and all AFC. Is there some other clause in NEC that would tell the designer not to use a #4 cu EGC in this application?
    e^(i pi) = -1

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    250.122 gives the minimum size of the EGC. You can use 110.10 to require one suitable for the available fault current.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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