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Thread: Equipment Ground conductor size for MV Feeder

  1. #1
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    Oct 2008
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    Equipment Ground conductor size for MV Feeder

    I am looking at a inside of an industrial facility where we are replacing the main feeders to a 2000A 12.47kV Switchgear lineup. The new feeders will be (3) sets of 750MCM cables. It is my understanding that.

    Per 250.190(C)(3) table 250.122 shall be used to size the EGC based on the setting of the fuse or the setting of the overcurrent relay. This particular feeder is fed from a breaker which will have a 2000A pickup setting on the protective relay.

    Based on this 250.122 requires a 250MCM cable based on the 2000A setting of the feeder relay. Since these cables will be run in (3) separate conduits as part of a parallel feeder it is required per 250.122(F) that each parallel feeder and conduit have a 250MCM ground run with it.

    Am I understanding this right? Something seems odd that this large of an EGC would be required in relation to phase conductors. We are also considering running (4) sets of 500MCM instead for phase conductors which would still require 250MCM EGC due to 2000A setting. If 500MCM cables are use the 250MCM EGC in each conduit would be almost as large as the phase conductors?!?!

  2. #2
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    Yep, full sized EGC in each raceway per 250.122 based on the OCPD.
    "Electricity is really just organized lightning." George Carlin


    Derek

  3. #3
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    right. The EGC just never needs to be bigger than the phase conductors.
    Ron

  4. #4
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    Oct 2008
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    This particular feeder runs overhead with Ariel cable for a while before it transitions on a bus structure to underground insulated cable. We are only replacing the insulated cable portion. Does this change anything?

    Does EGC need to be insulated or can it be bare copper?

  5. #5
    Join Date
    Oct 2008
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    I know also that the EGC on MV systems is typically only a 600V rated cable but I cannot find specifically in the NEC where it states this only has to be 600V?

    The only reference I see if 250.184(A) however this refers to a "neutral conductor" and not specifically and equipment ground conductor.

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