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    PV breaker size.

    When I was first inspecting PV I was told that the total of the PV breaker + the main breaker could not equal more than 120% of the bus rating of the panel

    I have one now where they have a 200 amp bus, derating the main to 175 and installing a 70 amp PV breaker and I said that was not allowed. His argument is that he only has 63 amps of solar back feed.

    Opinions.
    I can build anything you want if you draw a picture of it on the back of a big enough check.

    [COLOR=red]There's no substitute for hard work....but that doesn't mean I'm going to give up trying to find one.[/COLOR]

    John Childress
    Electrical Inspector
    IAEI / CEI / C10
    Certified Electrical Inspector

    #2
    Originally posted by cowboyjwc View Post
    When I was first inspecting PV I was told that the total of the PV breaker + the main breaker could not equal more than 120% of the bus rating of the panel

    I have one now where they have a 200 amp bus, derating the main to 175 and installing a 70 amp PV breaker and I said that was not allowed. His argument is that he only has 63 amps of solar back feed.

    Opinions.
    What you were told is correct. In general, PV breaker sum total + main breaker, cannot exceed 120% of the busbar rating. So this means for a 200A with a 175A main breaker, you'd be limited to 65A worth of interconnection breaker. Observe that 65A+175A = 240A, and 240A is 120% of 200A. Good luck finding a 65A breaker, because that is not a standard size. And this is the crux of what I will discuss in the next paragraph. So in a practical sense, for NEC2011 and earlier, you'd be limited to 60A worth of interconnection breaker, which is 48A of total inverter current. That's an 11.4kW single phase inverter on a standard 240V single phase system.

    HOWEVER, in NEC2014, the code has changed ever so slightly on this. They've rewritten the language so that rounding errors like the above, are no longer a show stopper. Instead of using the total of interconnection breakers, you now use the factor that drives their sizes instead. So 1.25*total inverter rated current, rather than this value rounded to the breaker that you use.

    So in this example, you are no longer limited to 48A of inverter current. You are now limited to 52A of inverter current, which is 12.48 kWac worth of inverter on a standard 240V single phase system.


    The big question for people in places where NEC2014 does not apply is, "can I take advantage of a favorable future rule?".

    I'm not an inspector, and I certainly cannot speak for any jurisdiction, so the following is entirely my opinion. If it were my judgement call, I would allow it, provided that all related future rules are also followed, and the installation meets the intent of the substantiation for the future rule. I don't see a need to follow unrelated future rules (such as rapid shutdown), simply because you intend to take advantage of the 2014 rule for 705.12(D). Although I can understand if an AHJ requires you to follow the entire NEC2014 when taking advantage of a future rule, even if it requires a stricter installation for complying with unrelated rules.
    Last edited by Carultch; 09-29-15, 06:52 PM.

    Comment


      #3
      Originally posted by Carultch View Post
      What you were told is correct. In general, PV breaker sum total + main breaker, cannot exceed 120% of the busbar rating. So this means for a 200A with a 175A main breaker, you'd be limited to 65A worth of interconnection breaker. Observe that 65A+175A = 240A, and 240A is 120% of 200A. Good luck finding a 65A breaker, because that is not a standard size. And this is the crux of what I will discuss in the next paragraph. So in a practical sense, for NEC2011 and earlier, you'd be limited to 60A worth of interconnection breaker, which is 48A of total inverter current. That's an 11.4kW single phase inverter on a standard 240V single phase system.

      HOWEVER, in NEC2014, the code has changed ever so slightly on this. They've rewritten the language so that rounding errors like the above, are no longer a show stopper. Instead of using the total of interconnection breakers, you now use the factor that drives their sizes instead. So 1.25*total inverter rated current, rather than this value rounded to the breaker that you use.

      So in this example, you are no longer limited to 48A of inverter current. You are now limited to 52A of inverter current, which is 12.48 kWac worth of inverter on a standard 240V single phase system.


      The big question for people in places where NEC2014 does not apply is, "can I take advantage of a favorable future rule?".

      I'm not an inspector, and I certainly cannot speak for any jurisdiction, so the following is entirely my opinion. If it were my judgement call, I would allow it, provided that all related future rules are also followed, and the installation meets the intent of the substantiation for the future rule. I don't see a need to follow unrelated future rules (such as rapid shutdown), simply because you intend to take advantage of the 2014 rule for 705.12(D). Although I can understand if an AHJ requires you to follow the entire NEC2014 when taking advantage of a future rule, even if it requires a stricter installation for complying with unrelated rules.
      Very well answered and so I can answer one of your questions or comments. We are on the 2011 here and because of that we cannot enforce or allow the use of the 2014 unless we had adopted parts of it by ordinance which we could have. People are always trying to use the less restrictive codes.

      So in your opinion do you think that the 5 amps difference really matter? Of course if I do my job and inspect to the code I cannot allow it, but my job does allow me some leeway.
      I can build anything you want if you draw a picture of it on the back of a big enough check.

      [COLOR=red]There's no substitute for hard work....but that doesn't mean I'm going to give up trying to find one.[/COLOR]

      John Childress
      Electrical Inspector
      IAEI / CEI / C10
      Certified Electrical Inspector

      Comment


        #4
        I don't think 5 amps makes a difference.

        That said, if I was an EC that lost this job due to bidding it to code I would expect you to make the EC that got the job meet the current code.

        Comment


          #5
          Originally posted by cowboyjwc View Post
          When I was first inspecting PV I was told that the total of the PV breaker + the main breaker could not equal more than 120% of the bus rating of the panel

          I have one now where they have a 200 amp bus, derating the main to 175 and installing a 70 amp PV breaker and I said that was not allowed. His argument is that he only has 63 amps of solar back feed.

          Opinions.
          It depends on your code cycle; that rule changed in 2014. The 120% rule used to be as you say but in 2014 it changed to the sum of the main breaker and 125% of the maximum rated current from the inverter.

          Comment


            #6
            Originally posted by cowboyjwc View Post
            Very well answered and so I can answer one of your questions or comments. We are on the 2011 here and because of that we cannot enforce or allow the use of the 2014 unless we had adopted parts of it by ordinance which we could have. People are always trying to use the less restrictive codes.

            So in your opinion do you think that the 5 amps difference really matter? Of course if I do my job and inspect to the code I cannot allow it, but my job does allow me some leeway.
            Does the 5A matter for safety? No, else the rule could not have changed. But it is a code violation if you are still on the 2011 NEC.

            Comment


              #7
              Originally posted by cowboyjwc View Post
              Very well answered and so I can answer one of your questions or comments. We are on the 2011 here and because of that we cannot enforce or allow the use of the 2014 unless we had adopted parts of it by ordinance which we could have. People are always trying to use the less restrictive codes.

              So in your opinion do you think that the 5 amps difference really matter? Of course if I do my job and inspect to the code I cannot allow it, but my job does allow me some leeway.
              I'm glad you appreciate my answer.

              I'm not sure where the 120% rule even came from. I haven't seen 120% elsewhere in the code, so it sounds like a specific number that was decided for a reason. From a Kirchhoff law perspective, I don't see how you would overload a bus if fed at full rating from opposite ends (a 200% rule). When feeding it from opposite ends, eventually there is a point where it diminishes to zero. Provided fed from opposite ends and that branch breakers are well distributed, no part of the busbar would be over the busbar rating.

              From what I understand, the intent of this rule is to curtail the overheating of breakers within the panelboard. The panelboard is tested for the breakers in a 100A panelboard to only receive a combined 100A per line among them all. Not 200A. So 120% seems to be a compromise. Worst case scenario, the load breakers would nuisance trip, given thermal components to their operation.
              Last edited by Carultch; 09-30-15, 01:39 AM.

              Comment


                #8
                Originally posted by cowboyjwc View Post
                When I was first inspecting PV I was told that the total of the PV breaker + the main breaker could not equal more than 120% of the bus rating of the panel

                I have one now where they have a 200 amp bus, derating the main to 175 and installing a 70 amp PV breaker and I said that was not allowed. His argument is that he only has 63 amps of solar back feed.

                Opinions.
                You can let it slide because it's safe, or you can tell the installer to put the job on hold for 15 months until we're on the 2016 California Electric Code, then pull a new permit with the same plans and call for inspection.

                Comment


                  #9
                  Incidentally, I understand there's code making panel task force discussing whether to raise 120% to 150% for the 2017 NEC. But it's still 120% in the First Draft.

                  Comment


                    #10
                    Well since the "code says what it says" I kicked out the plan check and see what happens. I'll keep you all posted.
                    I can build anything you want if you draw a picture of it on the back of a big enough check.

                    [COLOR=red]There's no substitute for hard work....but that doesn't mean I'm going to give up trying to find one.[/COLOR]

                    John Childress
                    Electrical Inspector
                    IAEI / CEI / C10
                    Certified Electrical Inspector

                    Comment

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