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    EGC sizing

    I have feeder breaker1 that is 1200AF (amps frame)/1000AT (amps trip) and feeder breaker2 that is 1200AF (amps frame)/800AT (amps trip). Question is equipment grounding conductor size is it based on amps frame or amps trip NEC 2014 Article 250? Which code section 250.122?

    #2
    250.122 based on the size of the the overcurrent protection not the frame size.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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      #3
      Yea. What he said.
      Charles E. Beck, P.E., Seattle
      Comments based on 2017 NEC unless otherwise noted.

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        #4
        Lets say somebody changes trip unit setting from 1000AT to 1200AT then would not that be a problem EGC was sized based on 1000AT? Assume phase feeders are sized 1200Amps.
        Last edited by hhsting; 07-10-19, 03:48 PM.

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          #5
          Originally posted by hhsting View Post
          Lets say somebody changes trip unit setting from 100AT to 1200AT then would not that be a problem EGC was sized based on 1000AT? Assume phase feeders are sized 1200Amps.

          Sent from my SM-G935U using Tapatalk
          Depends what the EGC size is based on Table 250.122 for 1000 amps and 1200 amps. If the ungrounded conductors are already sized for 1200 amps then the EGC should already be upsized even with a 1000 amp OCPD.
          Rob

          Moderator

          All responses based on the 2014 NEC unless otherwise noted

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            #6
            Originally posted by infinity View Post
            Depends what the EGC size is based on Table 250.122 for 1000 amps and 1200 amps. If the ungrounded conductors are already sized for 1200 amps then the EGC should already be upsized even with a 1000 amp OCPD.
            To clarify for the poster-- you are saying that if the trip level is set lower than the breaker max then the equipment grounding conductor can be sized to the lower trip ampacity. Correct?
            They say I shot a man named Gray and took his wife to Italy
            She inherited a million bucks and when she died it came to me
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              #7
              Originally posted by hhsting View Post
              Lets say somebody changes trip unit setting from 1000AT to 1200AT then would not that be a problem EGC was sized based on 1000AT?
              The simple answer is that that "problem" belongs to the "somebody" who does the future revision. We don't have to design today for changes that someone else might make later.

              Charles E. Beck, P.E., Seattle
              Comments based on 2017 NEC unless otherwise noted.

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                #8
                Originally posted by Dennis Alwon View Post
                To clarify for the poster-- you are saying that if the trip level is set lower than the breaker max then the equipment grounding conductor can be sized to the lower trip ampacity. Correct?
                That is what Rob said in his post #2. His post #5 is bringing 250.122(B) into play. If the conductors are sized for 1200 amps, and the OCPD is set to 1000 amps, then the conductors must have been "increased in size" beyond what was needed for ampacity purposes. Therefore, the EGC will have also been increased in size proportionally, so that the EGC will already be big enough to support a change to a 1200 amp setting.

                Charles E. Beck, P.E., Seattle
                Comments based on 2017 NEC unless otherwise noted.

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                  #9
                  Originally posted by Dennis Alwon View Post
                  To clarify for the poster-- you are saying that if the trip level is set lower than the breaker max then the equipment grounding conductor can be sized to the lower trip ampacity. Correct?
                  Yes, it's no different than having a 125 amp fuse in a 200 amp disconnect switch you would size the EGC based on the OCPD size, in this example 125 amps. His other hypothetical question regards changing the trip setting later because the conductors are already sized for 1200 amps. In that scenario the EGC would already need to be oversized {250.122(B)} because the conductors have been increased in size from 1000 amps.
                  Rob

                  Moderator

                  All responses based on the 2014 NEC unless otherwise noted

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                    #10
                    Originally posted by hhsting View Post
                    Lets say somebody changes trip unit setting from 1000AT to 1200AT then would not that be a problem EGC was sized based on 1000AT? Assume phase feeders are sized 1200Amps.
                    If you have 1000A OC protection but have 1200 amp conductors, 250.122(B) is going to require an increase in EGC size anyway over what it would be for 1000 amp conductors. (did not check to see if end result is same/greater size EGC than required for a 1200 amp device).
                    I live for today, I'm just a day behind.

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