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Sizing ground conductor for fire pump

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    Sizing ground conductor for fire pump

    If using a 208/480V step up transformer, can you adjust the ground conductor based on what the secondary OCPD would have been? If not, you would have to size the ground conductor on the secondary based on the primary OCPD right?

    For instance, a 1600A OCPD on the primary would require a #4/0 ground. For the secondary, a 600A OCPD would allow #1 ground (although secondary OCPD not allowed for fire pumps) but can you size the secondary ground conductor based on this 600A OCPD or do you have go off of the 1600A OCPD?

    #2
    According to article 250.66 states 3/0 wire on the primary and the secondary

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      #3
      250.66 is for grounding electrode conductors. I'm talking about sizing the EGC for fire pump feeders.

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        #4
        Originally posted by cppoly View Post
        If using a 208/480V step up transformer, can you adjust the ground conductor based on what the secondary OCPD would have been? If not, you would have to size the ground conductor on the secondary based on the primary OCPD right?

        For instance, a 1600A OCPD on the primary would require a #4/0 ground. For the secondary, a 600A OCPD would allow #1 ground (although secondary OCPD not allowed for fire pumps) but can you size the secondary ground conductor based on this 600A OCPD or do you have go off of the 1600A OCPD?
        Interesting question, so you have a step-up transformer feeding the fire pump controller with no secondary OCPD and you're looking to size the EGC?
        Rob

        Moderator

        All responses based on the 2017 NEC unless otherwise noted

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          #5
          Yup, correct! Secondary OCPD not allowed as per article 695.5. But we always size EGC as per the OCPD, but if OCPD is not allowed on secondary, you would have a way oversized EGC based on the primary OCPD.

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            #6
            Originally posted by cppoly View Post
            Yup, correct! Secondary OCPD not allowed as per article 695.5. But we always size EGC as per the OCPD, but if OCPD is not allowed on secondary, you would have a way oversized EGC based on the primary OCPD.
            I don't believe that you would have an EGC on the secondary, at least until you reach the Fire Pump Controller. I think it would be a supply side bonding jumper per 250.30(A)(2). Perhaps someone with more experience with that type of installation will chime in.

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              #7
              I agree with David. From the secondary terminals of a transformer to the first overcurrent device (in this instance, it is at the FP controller), you have an SSBJ. For a 600 amp service I would use (2) sets of 350 KCMIL copper conductors. Per Table 250.102(C), that would call for a 2/0 SSBJ.
              Charles E. Beck, P.E., Seattle
              Comments based on 2017 NEC unless otherwise noted.

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