Electrical Panel Sizing

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  • muna
    Junior Member
    • Feb 2024
    • 7
    #1

    Electrical Panel Sizing

    We have a 200A main electrical panel with a 100A subpanel in the garage. I talked to a few solar companies and only one of them mentioned that I might need to upgrade the electrical panel depending on my desired solar system size. Considering that comment, I started reading about electrical panel sizing and solar system size. It seems I need to add the "existing load" and the maximum solar panel amperage together and then add 20% to comply with NEC (and California?) code. But what is the existing load? From what I read, it is the max that all connected devices might draw at any time. That seems VERY vague because it's something that can change as you add power hungry devices. It's also something that is hard/impossible to dictate by code, exactly because it's something that can vary so I am not sure how it is determined if an electrical panel complies with the code. What am I missing?

    Now that one company said that I could go to a 200/225 panel which would add about $5k. I guess the 200A is the max amperage that can come from the grid. Then 225A is the maximum load on the bus. So basically it adds another 25A to support the additional current from the solar panels. I assume the work involved is "just" taking the old panel out and putting a new panel in with a beefed up bus. Why does something like that add $5k? That seems a lot.

    That same company also said that as an alternative the subpanel could be changed which is a lot cheaper. Does that mean the solar system would be connected to the subpanel? I assume that also means the power from the solar system can only supply things connected to the subpanel but not the main panel. That in return would completely oversize my solar system as many of the power hungry appliances (2x HAVC, 240V oven, 240V induction, 240V microwave, etc) are on the main panel.
    Tags: None
    • Mike 134
      Solar Fanatic
      • Jan 2022
      • 393
      #2
      They are taking the $$$$ way to satisfy the 120% rule. That rule states that your main breaker (200amps) plus the breaker feeding solar from the panels (perhaps 60amps) cannot add up to be more than 120% of the buss rating of the panel.
      What I did is replace the main 200amp breaker with a 150Amp breaker and now I'm compliant. I could have everything in the house on and I'd never come close to 150amps much less the 200.

        Comment

        • muna
          Junior Member
          • Feb 2024
          • 7
          #3
          So you just switched the 200A main breaker in the main panel with 150A main breakers?

          Do you happen to know what they mean with modifying the subpanel? Also, if I should decide with upgrading the panel (probably not), are there and federal (or other) tax incentives? Our local utility company has some incentives to upgrade to a SPAN smart panel (they don't mention SPAN but the requirements are tailored to SPAN) but their smart panels are prohibitively expensive for retrofits. And it might not make sense for a home with a 200A panel, anyways.

          The 120% Solar Rule Explained & When to Derate Your Breaker
          https://blog.gogreensolar.com/120-percent-rule-derate-breaker-for-solar
          What is the 120% solar rule, and what should you do to meet this criteria? Learn how to calculate and derate your breaker to accommodate your system.

            Comment

            • Mike 134
              Solar Fanatic
              • Jan 2022
              • 393
              #4
              Originally posted by muna
              So you just switched the 200A main breaker in the main panel with 150A main breakers? Yes

              Do you happen to know what they mean with modifying the subpanel? without more information I do not Also, if I should decide with upgrading the panel (probably not), are there and federal (or other) tax incentives? My understanding any of the work related to putting solar on your house is eligible for the 30% tax credit. Our local utility company has some incentives to upgrade to a SPAN smart panel (they don't mention SPAN but the requirements are tailored to SPAN) but their smart panels are prohibitively expensive for retrofits. And it might not make sense for a home with a 200A panel, anyways.

              https://blog.gogreensolar.com/120-pe...aker-for-solar
              See above in red

                Comment

                • muna
                  Junior Member
                  • Feb 2024
                  • 7
                  #5
                  Thanks you for your input.

                    Comment

                    • scrambler
                      Solar Fanatic
                      • Mar 2019
                      • 500
                      #6
                      The 20% rule is really about the panel bus bar rating.

                      If the busbar of the panel is rated for 200Amps, and you add a solar breaker in there that could inject 60Amps, you now have the potential to overload the panel busbar with 260A..

                      Below is the actual text of that "20% rule" which list the various way to look at it (b) is usually the one applying

                      ACTUAL 2014 705.12(D)(2) TEXT:
                      (3) Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards.
                      (a) The sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.
                      Informational Note: This general rule assumes no limitation in the number of the loads or sources applied to busbars or their locations.
                      (b) Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back-fed breaker from the inverter that displays the following or equivalent wording:
                      WARNING: INVERTER OUTPUT CONNECTION; DO NOT RELOCATE THIS OVERCURRENT DEVICE.
                      The warning sign(s) or label (s) shall comply with 110.21(B).
                      (c) The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar. The rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar. Permanent warning labels shall be applied to distribution equipment that displays the following or equivalent wording:
                      WARNING: THIS EQUIPMENT FED BY MULTIPLE SOURCES. TOTAL RATING OF ALL OVERCURRENT DEVICES EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE, SHALL NOT EXCEED AMPACITY OF BUSBAR.

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