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    Service Tap???

    An existing building has a 37.5 KVA, 120/240V, single phase service transformer, and a 200 amp MCB panel. I am reviewing a design that intercepts the service conductors just inside the building wall, installs what they are calling a "pull box," connects a set of #1 wires to the existing service conductors, and runs them to a new 100 amp MCB panel. This seems wrong on many levels. But I will start with, "are we allowed to tap a service"? I know that if they made the connection at the transformer secondary, kept the existing 200 amps worth of wire to the 200 amp MCB panel, and ran 100 amps worth of wire to the new 100 amp MCB panel, they would not be overloading any wires. They might have an issue with two services to the same building, but that's another matter. As shown, they appear to want the existing 200 amps worth of wire to supply both panels, at least up to the location of the pull box.

    What do you think?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

    #2
    Originally posted by charlie b View Post
    An existing building has a 37.5 KVA, 120/240V, single phase service transformer, and a 200 amp MCB panel. I am reviewing a design that intercepts the service conductors just inside the building wall, installs what they are calling a "pull box," connects a set of #1 wires to the existing service conductors, and runs them to a new 100 amp MCB panel. This seems wrong on many levels. But I will start with, "are we allowed to tap a service"? I know that if they made the connection at the transformer secondary, kept the existing 200 amps worth of wire to the 200 amp MCB panel, and ran 100 amps worth of wire to the new 100 amp MCB panel, they would not be overloading any wires. They might have an issue with two services to the same building, but that's another matter. As shown, they appear to want the existing 200 amps worth of wire to supply both panels, at least up to the location of the pull box.

    What do you think?
    As long as the service disconnects are grouped and the calculated load for the building does not exceed the conductor ampacity (200 amps?) as you already know the location of the grouped disconnects must be as close as possible entering the building

    Comment


      #3
      I also don't see any issue. Once service conductors supply more than one disconnect, their overload protection is permitted to be met by multiple ocpd's whose sum exceeds the ampacity.
      Ethan Brush - East West Electric. NY, WA. MA

      "You can't generalize"

      Comment


        #4
        I don't know the rating of the service conductors. I think it safe to believe they are good for the existing 200 amp panel's MCB. In the attached image, the new and existing panels are side-by-side, apparently in an indoor corridor. So they meet the "grouped" requirement. It appears that the existing service conductors travel some distance before they connect to the existing panel. By putting a pull point just inside the building's exterior wall, you will now have two sets of service conductors travelling an unknown distance before hitting their respective service disconnecting means (which double as the panel's OCPD).
        Attached Files
        Charles E. Beck, P.E., Seattle
        Comments based on 2017 NEC unless otherwise noted.

        Comment


          #5
          Originally posted by charlie b View Post
          I don't know the rating of the service conductors. I think it safe to believe they are good for the existing 200 amp panel's MCB. In the attached image, the new and existing panels are side-by-side, apparently in an indoor corridor. So they meet the "grouped" requirement. It appears that the existing service conductors travel some distance before they connect to the existing panel. By putting a pull point just inside the building's exterior wall, you will now have two sets of service conductors travelling an unknown distance before hitting their respective service disconnecting means (which double as the panel's OCPD).
          Charlie,

          As you know, Washington gives you 15' of SEC inside a building. What is a bit of a grey area however is how this is treated when multiple sets are run per 230.40 exception 2.

          Edit to add:

          For the other code references for this installation, see 230.33, and 230.90(A) exception 3.
          Ethan Brush - East West Electric. NY, WA. MA

          "You can't generalize"

          Comment


            #6
            Originally posted by charlie b View Post
            I don't know the rating of the service conductors. I think it safe to believe they are good for the existing 200 amp panel's MCB
            The additional panel if adding additional load may exceed the original calculated load the 200 amp service was based on

            Originally posted by charlie b View Post
            It appears that the existing service conductors travel some distance before they connect to the existing panel. By putting a pull point just inside the building's exterior wall, you will now have two sets of service conductors travelling an unknown distance before hitting their respective service disconnecting means
            If this is an existing violation you may need to add a 200 amp service disconnect with 200 amp feed through lugs that can except an additional 100 amp breaker

            The existing 200 amp feed (service entrance) may need reconfigured to be a feeder to a sub feed 200 amp panel.

            It may become necessary to upgrade the 200 amp service to account for the increase in load

            Comment


              #7
              Originally posted by electrofelon View Post
              I also don't see any issue. Once service conductors supply more than one disconnect, their overload protection is permitted to be met by multiple ocpd's whose sum exceeds the ampacity.
              I agree with this statement, but I can't find the code reference to back it up. Little help?

              Comment


                #8
                The only potential issue I see is the possible overloading of the existing conductors. If a load calculation proves that they're sufficient then there is no issue. Also there should not be a "#6 ground" in the raceway with the new SEC's.
                Rob

                Moderator

                All responses based on the 2017 NEC unless otherwise noted

                Comment


                  #9
                  Originally posted by Leespark57 View Post
                  I agree with this statement, but I can't find the code reference to back it up. Little help?
                  I believe 230.90 Exception 3 covers that,

                  I share charlie b's concern but this situation is not much different from having a MLO service panelboard with six overcurrent devices.
                  At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

                  Comment


                    #10
                    From and installation perspective, if there is sufficient room in the existing enclosure, could feed though lugs be installed on the bus? That would keep the existing OC protection in place and remove the need to intercept the service conductors.

                    Comment


                      #11
                      Originally posted by oldsparky52 View Post
                      From and installation perspective, if there is sufficient room in the existing enclosure, could feed though lugs be installed on the bus? That would keep the existing OC protection in place and remove the need to intercept the service conductors.
                      Certainly could. Or just install a 100 amp breaker to a sub even - move a couple circuits over if you need to free up spaces for it. U don't really gain anything by tapping the SEC bc you don't get any more load capability. Presumably this is about a lack of circuit spaces
                      Ethan Brush - East West Electric. NY, WA. MA

                      "You can't generalize"

                      Comment


                        #12
                        I don't see anything wrong with this type of install either, as long as others have said as far as grouping disconnects, and, making sure the overall load doesn't exceed the rating of the Service Conductors.

                        However,

                        There is actually a significant difference in putting a subfeed breaker in the main panel to feed a sub panel versus tapping the service conductors and going directly to the 2nd panel.

                        There is much more diversity in tapping the service conductors.

                        It's not likely both panels would be fully loaded at all times. In the service tap scenario you're not choking the 2nd panel by making it dependant on the load that's already on the 200 amp main breaker.

                        For instance,

                        I could see where the main panel could be drawing nearly full load and the 100 amp subpanel drawing only a fraction of it, and tripping out the 200 amp main if you do the subfeed thing.

                        Where as if you had tapped the service conductors in this same scenario, you would still be within the limits of the main, and the Service Conducors, and, no problems would occur.

                        Much like using a single 400 amp main breaker panel to feed all the loads rather than to choke a 400 amp service by installing (2) 200 amp main breaker panels which is usually only done because the 400 amp load centers aren't something that are readily available to pick up over the counter, and, most don't want to pay the price or wait on a loadcenter with a 400 amp main breaker.

                        Even though it would be much more diverse in the long run.

                        JAP>

                        Comment


                          #13
                          Where are the electric meters in this scenario? Would these two panels be separately metered requiring a connection ahead of the existing meter?
                          Rob

                          Moderator

                          All responses based on the 2017 NEC unless otherwise noted

                          Comment


                            #14
                            I know nothing that does not appear in the sketch I provided in post #4. I know nothing about metering, about load calculations, or about the size of the service conductors. Fortunately, in my role of design reviewer, I have the option of posting my comments in the form of questions (e.g., How do you intend to address . . . ) as opposed to statements (e.g., You have a code violation here). I will give the original design engineers the chance to explain their intent.
                            Charles E. Beck, P.E., Seattle
                            Comments based on 2017 NEC unless otherwise noted.

                            Comment


                              #15
                              Let me bring the discussion back to my original question:
                              Originally posted by charlie b View Post
                              "are we allowed to tap a service"?
                              240.21(B) speaks of feeder taps. 240.21(C) does not quite fit the sketch in post #4, as we have neither "a set of conductors feeding a single load" nor "each set of conductors feeding separate loads." Instead, we have one set of conductors that is tapped in order to supply a second load. 240.21(D) mentions service conductors, but does not describe a tap situation.

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

                              Comment

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