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Clarification open the 120% rule

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  • Clarification open the 120% rule

    I understand that if you have a 100 amps main breaker, and a main panel bus bar rated 125Amps, you should be allowed to have up to 50 amps for the inverter breaker as per the calculation

    1.2 x 125 amps = 150 amps minus 100amps main = 50 Amps

    If that is correct my question is as follow.

    My main breaker is outside the house all by itself. From it a line goes to the garage where the breaker panel is. That panel is where the Inverter breaker would go, and that panel has a bus bar rated 125 amps.

    For the above calculation, which bus bar amp rating matters, the one outside where the Main 100 amps breaker is, or the one in the garage breaker panel?

    Because if it is the outside, there is no label on that one.

    Attached picture of the outside breaker and the inside panel.

    Note:The breaker panel is full, but we will be creating a Critical load sub panel, so that will free space for the inverter breaker.

    Attached Files
    Last edited by scrambler; 04-23-2019, 08:05 PM.

  • #2
    Both bus bars have to be 125A rating as well as the conductors connecting them. And the solar breaker has to be at the tail end of the bus. Your inspector may or may not make you prove that the subpanel is 125A. I've had to replace panels in this same situation because it can't be proven to an inspector what something is rated. My usual advice to customers is that they should think about replacing their electrical panels every 50 years or so anyway - I mean how many electrical appliances last that long? A lot of houses burn down due to electrical panel fires too. When installing solar is the time to do it as the tax credits will cover the panel change as well. Don't be cheap when it comes to electrical safety. Upgrade that little old 100A panel to 200A while you are at it. Your house's value will be greatly increased as well.
    BSEE, R11, NABCEP, Chevy BoltEV, >2500kW installed


    • #3
      If I understand what you are saying, if I cant find out the bus bar rating of the outside main breaker box as well as the wire rating between the outside main breaker box and the indoor panel, I could have to upgrade all of it.

      This would mean at a minimum, new outside breaker panel, new wiring between the outside breaker panel and the indoor panel.
      The wiring would be the killer as there is no way to pass wires where the original ones are (without opening up walls and ceilings across the whole house) so I would have to install an outside line between the outdoor panel and the garage, which in itself would make the whole thing very costly and probably render the project not worth it...
      And that is if I keep the indoor panel at 100 amps, or else I also need to change the indoor panel for even more cost.

      My solar project may be taking a wrong turn...

      I wonder if I could speak with an inspector and get pre evaluation of the situation from him.

      I also don't quite understand why the outdoor bus bar and wiring between outdoor breaker and indoor panel needs to be 125amps.

      I understand that in the indoor panel, it is possible that the bus bar would see current coming from the grid and from the inverter, therefore we need the bus bar to allow for that extra load (the 120% rule).

      But that should never be the case for the wring between the indoor panel and the outdoor breaker.
      I cant think of a scenario where I would be feeding current to the grid at the same time as importing current from it. SO that line and outdoor breaker should never see more than 100 amps when importing from grid, and 50 amps when exporting to the grid.

      Am I missing something here ?
      Last edited by scrambler; 04-23-2019, 11:45 PM.


      • #4
        The rule interpretation is that it must be satisfied at each bus and conductor all the way back to the service entrance conductors (just upstream of the service disconnect.)
        So technically the 100A main and the wiring from it to the indoor panel must also satisfy the 120% rule. That means that IF the outside main is in a panel in which you could mount an additional breaker, then the bus would have to also be 125A or else you would have to reduce the size of that main breaker. And that actually makes some sense,
        However, some inspectors would see the absurdity of applying the 120% rule to a conductor which is all supply at one end and all load and PV at the other end. Same reasoning applies to the outside main breaker if it is not in a multi-breaker panel.
        Best to run it by your AHJ before spending money to upgrade.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.


        • #5
          That is indeed what makes sense to me.
          There is no actual panel where you can add breakers outside.
          Just a feeding breaker hardwired to the main line.

          I guess a visit to the permit office is in order.


          • #6
            I would replace that box also. You said you would have to pull drywall up in order to wire a replacement box but you are also saying you want to do a critical loads panel which would likely include drywall work?

            They may make you fix the current wiring problems in the box upon inspection. FYI, there are loose wires in the box that should at least have a wire nut on the end of them, the wires coming into/out of the box need to all go through some type of strain relief(some currently aren't), and also the strain reliefs are only UL listed/rated for so many wires going through them(the one on the upper right looks overloaded). When you buy a strain relief (cord grip, cable clamp, wire gland....there are all different types) they will have a UL listing of the size and number of wires that can go through them, same for wire nuts, outlet boxes, and conduit. This is a easy fix and common stuff....

            Good luck with your project.


            • #7
              Originally posted by NewBostonConst View Post
              I would replace that box also. You said you would have to pull drywall up in order to wire a replacement box but you are also saying you want to do a critical loads panel which would likely include drywall work?
              The critical load panel will be in the garage next to the existing breaker panel, so minimal work and included in the base quote.
              Wiring between the outside breaker and the garage panel is a lot longer (opposite ends of the house) and a lot more complicated (Vaulted ceilings in between).

              I will still review what it would cost...