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Grounding Electrode Conductor size on SDS

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    Grounding Electrode Conductor size on SDS

    I've got a customer that just purchased a trailer-mounted generator for use on a construction site to run some equipment. The generator is a 600KW, 3-phase, 4-wire, 480 VAC unit. The generator connections are wired to a circuit breaker (trip unit long-time set to 800A) mounted inside of the generator enclosure. The customer does not have any equipment that will require the neutral. We will be bonding the neutral to the ground in the generator enclosure and then running a grounding electrode conductor to a ground rod.

    I know we should size the grounding electrode conductor based on 250.66 but this allows us to use a #6 copper for this purpose (assuming we use a rod electrode and it meets 250.52A(5)). Is that correct?

    Also, with regards to the bonding jumper between the Neutral and the ground, it should be sized based on 250.66 as well, right? Since we will have 3 parallel conductors per phase (each is 350 kcmil), this would put the size for the bonding jumper at 2/0 copper. The two parallel conductors would be treated as one 1050 kcmil and thus you'd use that size for table 250.66, right? Thanks in advance.

    #2
    Originally posted by RollTideYall View Post

    Also, with regards to the bonding jumper between the Neutral and the ground, it should be sized based on 250.66 as well, right? Since we will have 3 parallel conductors per phase (each is 350 kcmil), this would put the size for the bonding jumper at 2/0 copper. The two parallel conductors would be treated as one 1050 kcmil and thus you'd use that size for table 250.66, right? Thanks in advance.
    You're looking for the system bonding jumper size. Take a look at 250.102(C).

    Rob

    Moderator

    All responses based on the 2017 NEC unless otherwise noted

    Comment


      #3
      Originally posted by infinity View Post

      You're looking for the system bonding jumper size. Take a look at 250.102(C).
      Yea, so the size is correct, then. I was using a 2011 copy of the NEC without that table. Didn't notice til you mentioned it. Thanks. How about the grounding electrode conductor? Any thoughts there? #6 just seems really small in this instance. Thanks.

      Comment


        #4
        #6 to the ground rod is fine. No matter how huge you go with the conductor to the ground rod said ground rod is only going to dissipate a tiny amount of electrical energy into the dirt.
        If Billy Idol is on your playlist go reevaluate your life.

        Comment


          #5
          Originally posted by ActionDave View Post
          #6 to the ground rod is fine. No matter how huge you go with the conductor to the ground rod said ground rod is only going to dissipate a tiny amount of electrical energy into the dirt.
          Yea, sometimes you just question yourself when it comes to these things. Thanks for the feedback.

          Comment


            #6
            In your installation the ground rod will be doing next to nothing anyway so a #6 is sufficient.
            Rob

            Moderator

            All responses based on the 2017 NEC unless otherwise noted

            Comment


              #7
              My question: Shouldn't the neutral be run, perhaps as an EGC, to the equipment?
              Master Electrician
              Electrical Contractor
              Richmond, VA

              Comment


                #8
                Originally posted by LarryFine View Post
                My question: Shouldn't the neutral be run, perhaps as an EGC, to the equipment?
                590.2 & 250.110 agrees with you, an 800A breaker would be a at least a 1/0 CU as per T250.122. Probably one for each set.
                Comments based on 2017 NEC unless otherwise noted.

                Comment


                  #9
                  Originally posted by LarryFine View Post
                  My question: Shouldn't the neutral be run, perhaps as an EGC, to the equipment?
                  Your answer is yes.
                  If Billy Idol is on your playlist go reevaluate your life.

                  Comment


                    #10
                    Originally posted by ActionDave View Post

                    Your answer is yes.
                    So you're saying we do need to run a neutral with the phase conductors? Let me explain more of what's happening here. There is a lowboy trailer with this large generator on it. We're going to be mounting a 600A disconnect and two 100A disconnects on the trailer as well. So, the leads from the circuit breaker on the generator are going to be put in conduit to a distribution box and will be split off to the disconnects. Are you saying we should run a neutral to each of the disconnects? We will be running an EGC to each disconnect from the generator.

                    The neutral and ground are going to be bonded before they leave the generator so the neutral will have no purpose (all of the loads are 3-phase, 480vac) if we run it outside the generator because we can't ground it again (parallel paths). Am I wrong here? If so, why? Thanks.
                    Last edited by RollTideYall; 09-23-19, 09:49 AM.

                    Comment


                      #11
                      Originally posted by RollTideYall View Post

                      So you're saying we do need to run a neutral with the phase conductors? Let me explain more of what's happening here. There is a lowboy trailer with this large generator on it. We're going to be mounting a 600A disconnect and two 100A disconnects on the trailer as well. So, the leads from the circuit breaker on the generator are going to be put in conduit to a distribution box and will be split off to the disconnects. Are you saying we should run a neutral to each of the disconnects? We will be running an EGC to each disconnect from the generator.

                      The neutral and ground are going to be bonded before they leave the generator so the neutral will have no purpose (all of the loads are 3-phase, 480vac) if we run it outside the generator because we can't ground it again (parallel paths). Am I wrong here? If so, why? Thanks.
                      No, if the load doesn't require a neutral you don't need to run one. You still need an equipment grounding conductor, that is kind of what was said but in a somewhat confusing manner IMO.
                      I live for today, I'm just a day behind.

                      Comment


                        #12
                        Originally posted by kwired View Post
                        No, if the load doesn't require a neutral you don't need to run one. You still need an equipment grounding conductor, that is kind of what was said but in a somewhat confusing manner IMO.
                        Ok, that's kind of what I thought but I was just clearing the air for anyone that reads this in the future. Thanks.

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