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    Medium Voltage feeder and Ground conductor

    Am reviewing a project with medium voltage feeders (4160V), multiple conductors per phase, each set in it's own conduit and each conduit containing 3 conductors only. i.e. no equipment ground. I gather they are relying on the shield for grounding. Is that permissible OR should I add a ground conductor to each conduit. If the latter, and given that this is a service, my inclination would be to go with a 3/0 bare copper ground conductor in accordance with Table 310.66. What is the correct approach. Obviously it can't hurt to add additional grounding but is it a waste? and what is the letter of the code where this is concerned?

    Thanks,

    Mike
    Mike Shields, PE
    Boston, MA

    #2
    Does the raceway qualify as an EGC? Is the 4160 ungrounded.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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      #3
      Answer to your question

      It is underground but run in sched 40 PVC, concrete encased. So no.
      Mike Shields, PE
      Boston, MA

      Comment


        #4
        Is the 4160 high resistance grounded, low resistance grounded, ungrounded, or solidly grounded? High resistance is most common and doesn't require much ampacity since 5-25 A is a typical maximum ground fault and shielding has plenty of ampacity. Solidly grounded is where heavy grounds are needed. IEEE Red Book among other sources is pretty clear that the most economical and best performance ground for 4160 is high resistance. Also you can get 3 conductor MV-105 with grounds. If it's ungrounded they have to put fillers in the voids anyways.

        Sent from my SM-T350 using Tapatalk

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          #5
          Originally posted by mshields View Post
          Am reviewing a project with medium voltage feeders (4160V), multiple conductors per phase, each set in it's own conduit and each conduit containing 3 conductors only. i.e. no equipment ground. I gather they are relying on the shield for grounding. Is that permissible OR should I add a ground conductor to each conduit. If the latter, and given that this is a service, my inclination would be to go with a 3/0 bare copper ground conductor in accordance with Table 310.66. What is the correct approach. Obviously it can't hurt to add additional grounding but is it a waste? and what is the letter of the code where this is concerned?

          Thanks,

          Mike
          There seems to be some confusion here in the responses given. You indicate that these are service conductors. There is no EGC in that case. Review Part X of Article 250, especially 250.186.

          Comment


            #6
            Originally posted by texie View Post
            There seems to be some confusion here in the responses given. You indicate that these are service conductors. There is no EGC in that case. Review Part X of Article 250, especially 250.186.
            I'm confused. I see feeders and the word service. Are they service conductors or feeders?


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              #7
              Originally posted by paulengr View Post
              I'm confused. I see feeders and the word service. Are they service conductors or feeders?


              Sent from my SM-T350 using Tapatalk
              Perhaps it's the mythical "service feeder"

              On a serious note, not that I do much MV, but I have also always been unclear on if or when a shield can be used as an EGC. Hoping we get some responses on this.
              Ethan Brush - East West Electric. NY, WA. MA

              "You can't generalize"

              Comment


                #8
                clarification

                It is actually a service. And it is solidly grounded off a 24.9 to 4160 delta wye transformer. So ok, it's service neutral conductor and not an EGC. Do I still need a dedicated conductor to that end. And would be it therefore be based on table 250.102?
                Mike Shields, PE
                Boston, MA

                Comment


                  #9
                  Originally posted by mshields View Post
                  It is actually a service. And it is solidly grounded off a 24.9 to 4160 delta wye transformer. So ok, it's service neutral conductor and not an EGC. Do I still need a dedicated conductor to that end. And would be it therefore be based on table 250.102?
                  What kind of cable is it?
                  Ethan Brush - East West Electric. NY, WA. MA

                  "You can't generalize"

                  Comment


                    #10
                    Cable type

                    single conductor Okonite Okcoguard MV105 EPR, 15kV

                    And we are running 9 - 750kcmil for 3000A
                    Mike Shields, PE
                    Boston, MA

                    Comment


                      #11
                      Originally posted by electrofelon View Post
                      Perhaps it's the mythical "service feeder"

                      On a serious note, not that I do much MV, but I have also always been unclear on if or when a shield can be used as an EGC. Hoping we get some responses on this.

                      Given that NEC 250.118 does not specifically list a cable shield, and Table 250.112 lists the minimum sizes of Equipment Grounding Conductors(with which your shield probably does not comply), along with common sense and a knowledge of how an inspector would interpret the NEC, I would say that a MV cable shield could not be used.
                      Do not design a system based on what the NEC doesn't directly prohibit, rather, it is up to you to find an article allowing what you want to design (especially if it isn't in common usage).
                      Cable shields do NOT have sufficient ampacity or low enough impedance to be considered an equipment grounding conductor.
                      https://www.eng-tips.com/viewthread.cfm?qid=102131


                      I have seen installations where they don't pull a ground(whether intentionally or unintentionally the shield(s) was the equipment grounding conductor), I don't believe it is usually NEC compliant, the concentric stranding has to plenty big and there is still a concern about damage if there is ever a ground fault on the system. I have seen installations like this with foil tape and concentric wire shield.


                      https://iaeimagazine.org/magazine/20...ver-600-volts/
                      It should be noted that the metal shield of most medium voltage cables is not suitable to be used as an equipment grounding conductor.
                      it appears some may be able to be used
                      Last edited by Wire-Smith; 07-09-19, 12:29 PM.

                      Comment


                        #12
                        Originally posted by Wire-Smith View Post
                        https://www.eng-tips.com/viewthread.cfm?qid=102131


                        I have seen installations where they don't pull a ground(whether intentionally or unintentionally the shield(s) was the equipment grounding conductor), I don't believe it is usually NEC compliant, the concentric stranding has to plenty big and there is still a concern about damage if there is ever a ground fault on the system. I have seen installations like this with foil tape and concentric wire shield.


                        https://iaeimagazine.org/magazine/20...ver-600-volts/

                        it appears some may be able to be used
                        A couple cases. First example is SHD-GC which is a mining duty cable. In this cable each conductor has a shield, and there is another overall shield, and 2 or 3 grounds in it. Obviously qualifies with the integral grounds. Lots of others have similar designs with a ground in contact with the shields.

                        Second is concentric wound neutrals. It has kind of fallen out of favor because it’s fairly expensive. But you essentially have a fully rated neutral where the strands are spread out and a power conductor run inside the neutral. It’s stiffer and more work both pulling and terminating but the “shield” has a much higher ampacity. This is a classic Okonite product and they are very helpful about working with it.


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                          #13
                          Originally posted by mshields View Post
                          It is actually a service. And it is solidly grounded off a 24.9 to 4160 delta wye transformer. So ok, it's service neutral conductor and not an EGC. Do I still need a dedicated conductor to that end. And would be it therefore be based on table 250.102?
                          Is the xfmr customer owned or utility owned? Where is the service point? If the xfmr is customer owned, it would be a separately derived system and the cable would be a feeder not a service. The location of the service point is critical to answering whether it is a service or feeder.

                          Comment


                            #14
                            I think this is the cable:
                            https://www.okonite.com/media/catalo.../files/2-8.pdf
                            According to ICEA P-45-482 the maximum permissible short-circuit current for 1 sec it is 3000 A
                            [conductor maximum temperature 90oC] -in my opinion the minimum length has to be 2200 ft.
                            for 0.1 sec fault clearing time the shield could withstand 9500 A- that means minimum 675 fts.

                            .

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