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120% Rule with Meter Mains/ MLO Panels (SC12L200F)

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    120% Rule with Meter Mains/ MLO Panels (SC12L200F)

    Hi all,
    I am new to this forum and have plundered throughout the forum looking for a clear answer to this. (If there is a thread that answers please direct me to it)

    So here we go.
    I am working on coming up with a solution for this Main Panel which I see quite frequently in my area. I am attaching a photo of the spec sheet.

    Square D Meter Main (Newer looking) SC12L200F. One note on the panel says "TOTAL CIRCUIT BREAKER HANDLE RATING NOT TO EXCEED 140A PER CIRCUIT CONNECTOR". I have seen that the six handle rule would apply instead of the 120% rule but we keep coming across an issue with how we calculate the maximum solar backfeed on this panel and how the Bus Bar would play into it. My understanding is that if the panel has an up stream OCPD we would use that into the 120% rule calcs. This panel does not (unless the meter counts, hence "Meter Main"?) but we want to make sure that 1. We are not Frying the Busbar and 2. We are following all codes associated with these types of panels.

    So the question is, for this panel "Mains 200A Max" what is the maximum amount of Solar Backfeed can be on this panel that follows NEC 2017 and will not do anything harmful to the main panel.

    #2
    When there is literally no upstream breaker (i.e. the service disconnect panel), the 120% rule simply does not apply. A PV backfeed breaker in this panel would be, by definition, a supply side connection. With all of the baggage that implies.
    The rules which would apply are that the PV backfeed (rated output, not breaker size) must not exceed the nominal service capacity. If the POCO wire does not seem large enough to handle the computed backfeed, that is not a problem for you to worry about. It is an NESC and policy issue for POCO, not a an NEC issue for you. A sadly large number of electricians who do not do solar regularly are likely to be confused about this too.
    Now this setup, as described so far, runs the risk of overloading the bus in the Meter Main if the load on the panel exceeds the bus rating. But with an MLO panel and six hand rule the calculated load cannot be greater than the service size and therefore the bus size.

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      #3
      If I was to provide a electrical load calculation that shows that the loads do not exceed the "Mains 200A Max" and show the jurisdiction that we have a supply side connection, in theory that would suffice? (Always depends on the jurisdiction)

      Comment


        #4
        Originally posted by Tristan_Ambrose View Post
        If I was to provide a electrical load calculation that shows that the loads do not exceed the "Mains 200A Max" and show the jurisdiction that we have a supply side connection, in theory that would suffice? (Always depends on the jurisdiction)
        Yes, realizing that some AHJs and some POCOs ban supply side connections entirely, potentially requiring a panel change.

        Sent from my XT1585 using Tapatalk

        Comment


          #5
          I've installed on this panel numerous times without issue. Never noticed that 140A note, but depending on what that means I guess the max backfeed for a single breaker might be 140A, but the total would be 200A. You shouldn't need to provide a load calculation if the load breakers don't exceed 200A. The solar is not a load. If the load breakers do exceed 200A then I would expect to provide a load calculation to prove that the existing install is compliant.

          Comment


            #6
            Originally posted by jaggedben View Post
            I've installed on this panel numerous times without issue. Never noticed that 140A note, but depending on what that means I guess the max backfeed for a single breaker might be 140A, but the total would be 200A. You shouldn't need to provide a load calculation if the load breakers don't exceed 200A. The solar is not a load. If the load breakers do exceed 200A then I would expect to provide a load calculation to prove that the existing install is compliant.
            This makes sense to me. I like doing load calcs and we see this panel quite often where we show up and they have a 200A max and they have 245A (rating) of breakers on it. I just want to make sure that whenever we do install that we do not damage the panel by adding a lets say 40A solar backfeed ocpd on there.

            Thank you all for your help. It would be awesome if NEC 2020 addressed this for clarity.

            Comment


              #7
              Originally posted by Tristan_Ambrose View Post

              This makes sense to me. I like doing load calcs and we see this panel quite often where we show up and they have a 200A max and they have 245A (rating) of breakers on it. I just want to make sure that whenever we do install that we do not damage the panel by adding a lets say 40A solar backfeed ocpd on there.

              Thank you all for your help. It would be awesome if NEC 2020 addressed this for clarity.
              I submit that with the exception of whether a line side interconnection is or is not a service, the NEC is actually pretty clear. If it's a load side connection there are three ways to qualify a busbar as compliant under 705.12(B) (or D before the 2017 release), and they are pretty straightforward.

              Comment


                #8
                Originally posted by GoldDigger View Post
                The rules which would apply are that the PV backfeed (rated output, not breaker size) must not exceed the nominal service capacity. If the POCO wire does not seem large enough to handle the computed backfeed, that is not a problem for you to worry about. It is an NESC and policy issue for POCO, not a an NEC issue for you. A sadly large number of electricians who do not do solar regularly are likely to be confused about this too.
                .
                On my turf, we're responsible for up to the 'point of connection' ,on a typical OH service.

                I'll be doing a 'line tap' monday , no panel work ....

                The poco is upsizing the xformer to accommodate the PV load , i'll be replacing an old #2 drop with 4/0 , PV meter/disco into poco 200A meter load side w/ milbank taps

                am i also confused? ~RJ~

                Comment


                  #9
                  Originally posted by romex jockey View Post

                  On my turf, we're responsible for up to the 'point of connection' ,on a typical OH service.

                  I'll be doing a 'line tap' monday , no panel work ....

                  The poco is upsizing the xformer to accommodate the PV load , i'll be replacing an old #2 drop with 4/0 , PV meter/disco into poco 200A meter load side w/ milbank taps

                  am i also confused? ~RJ~
                  The definition of the "service point" as used in the NEC is not unambiguously specified, but rather is a local policy by the POCO as recognized by the AHJ. It is not identical to the location of the service disconnect. It is established by local convention and does not necessarily correspond with who is "responsible for" the wiring. Many POCOs require the EC to supply and pull the wire, particularly for underground service, even though it is on the POCO side of the service point. That should mean that the conductor size is whatever is specified by POCO by their engineering rules, rather than the NEC size for the calculated load.

                  Comment


                    #11
                    Originally posted by romex jockey View Post

                    On my turf, we're responsible for up to the 'point of connection' ,on a typical OH service.

                    I'll be doing a 'line tap' monday , no panel work ....

                    The poco is upsizing the xformer to accommodate the PV load , i'll be replacing an old #2 drop with 4/0 , PV meter/disco into poco 200A meter load side w/ milbank taps

                    am i also confused? ~RJ~
                    Why would the POCO need to upsize the transformer and service conductors? The PV system is a source, not a load. Unless it is a bigger source than the building associated with the transformer is a load, no upsizing should be necessary.

                    Comment


                      #12
                      Originally posted by ggunn View Post

                      Why would the POCO need to upsize the transformer and service conductors? The PV system is a source, not a load. Unless it is a bigger source than the building associated with the transformer is a load, no upsizing should be necessary.

                      To my <limited> understanding, they see it as something additional feeding back on their lines ggunn

                      They have a 10 or maybe it's 12 KW cap, at which point they review their end of the service drop

                      ~RJ~

                      Comment


                        #13
                        Originally posted by romex jockey View Post


                        To my <limited> understanding, they see it as something additional feeding back on their lines ggunn

                        They have a 10 or maybe it's 12 KW cap, at which point they review their end of the service drop

                        ~RJ~
                        Line side connections are limited to "the size of the service", which is is subject to interpretation, but normally that means that the PV system can never push more current back through the service conductors than the load for which the service was designed, even if it is running flat out with all the loads off line. If it is running and loads are active the current in the conductors is the difference between (not the sum of) the demand of the load and the supply from the PV. I have, however, encountered inspectors who cannot seem to grasp that current cannot move two directions through a conductor simultaneously.

                        Comment


                          #14
                          Originally posted by Tristan_Ambrose View Post
                          .

                          Thank you all for your help. It would be awesome if NEC 2020 addressed this for clarity.

                          Unfortunately the 2020 NEC is going to make that entire service panel illegal. I'm not looking forward to conversations about whether it can be grandfathered as an existing installation when you just add a PV breaker.

                          Comment


                            #15
                            the 140 amp note on the panel cover normally means the max load on the tab the breaker plugs into, not the actual buss. most loadcenters
                            by design are overated. but most manufactuers would not admit it.

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