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Smallish solar on 100A existing main panel.

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  • Smallish solar on 100A existing main panel.

    I'm looking to install a small solar system on my garage. Current main panel is a 100A service, with what i believe to be an Eaton/Westinghouse/Cutler Hammer/whatever brand they are now 643532 BR 100A panel. Unfortunately, there is no label-plate/sticker in existence on the panel (circa 1994), so I have no definite way of knowing if it has a 100A bus, or 125A bus. Either way, the way I understand the NEC regarding multiple feeders, I should be able to run 120% of the bus in total feeders into the panel, so that would allow a 20A solar feeder, which would provide for up to 16A of 240V inverted power. With this in mind, I'm scheming on building a system using

    16 - SolarWorld 275 Plus Black
    16 - Enphase M250 MicroInverters

    The M250's max out at 1.0A a piece, which brings my array to a max of 16A, feeding through a 20A breaker, into my main panel (I may opt to install a 50A subpanel closer to the array, to break out a couple breakers from the main, which is pretty congested, and leave some room for expansion for some landscaping lights and general outlet use).

    Anyone see any problems with this setup, from a practicality, and/or code side?

  • #2
    Hmmm... so I just realized that my panelboard is a partially center tapped board (4 slots above main, 8 slots below). I know the 2011 code has caused a number of discrepancies in interpretation here and whether 'opposite end' is possible on a center tapped bus. I know I can properly configure breakers to ensure no bus section is over rating, but what has been the experience of those here regarding selling that idea to an inspector? I am a bit spoiled doing mostly commercial (hospital) work, where we can do detailed bus current flow analyses to show worst-case bus loading at every segment, but is that something that flies in the residential world (where I have little to no experience)?

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    • #3
      Originally posted by CraziFuzzy View Post
      Hmmm... so I just realized that my panelboard is a partially center tapped board (4 slots above main, 8 slots below). I know the 2011 code has caused a number of discrepancies in interpretation here and whether 'opposite end' is possible on a center tapped bus. I know I can properly configure breakers to ensure no bus section is over rating, but what has been the experience of those here regarding selling that idea to an inspector? I am a bit spoiled doing mostly commercial (hospital) work, where we can do detailed bus current flow analyses to show worst-case bus loading at every segment, but is that something that flies in the residential world (where I have little to no experience)?
      The National Electrical Code (and therefore inspectors) are very clear and firm that with a center fed main into the panel, the 100% rule must be applied rather than the more flexible 120% rule.
      Any steps you take to assure that no excess current appear at any point along the bus would have to prevent (somehow) the possibility that some later owner might install new breakers or move old ones rather than just looking carefully at the existing size breakers and replacing those as needed.)
      SunnyBoy 3000 US, 18 BP Solar 175B panels.

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      • #4
        Originally posted by inetdog View Post
        The National Electrical Code (and therefore inspectors) are very clear and firm that with a center fed main into the panel, the 100% rule must be applied rather than the more flexible 120% rule.
        Any steps you take to assure that no excess current appear at any point along the bus would have to prevent (somehow) the possibility that some later owner might install new breakers or move old ones rather than just looking carefully at the existing size breakers and replacing those as needed.)
        That I understand. In my particular case, the best solution appears to be to add a subpanel, and move some of my loads onto it. The subpanel would receive the solar feed (opposite it's feeder), and be tied to the main at the main's bottom position. The 4 spaces above the main's feeder would be left empty, and marked appropriately. Typically, at least in my commercial work, that simply involves blanking off the unusable breaker slots and labeling the unusable positions with long-life labeling. My concern wasn't whether it could be done, but whether an inspector would approve of it.

        Short of that method, the other option is to actually derate my panel down by replacing the 100A feeder breaker with an 80A feeder breaker, and simply working within the 100%. The existing 100A feeder breaker, unfortunately, is the only thing I have on the panel that even close to identifies the bus bar's capacity to begin with.

        It's hard for me to justify an entire meter panel replacement and an upgrade in service as part of my method to reduce electric grid use, but that will always remain the last chance option. Just have to send the family away for a couple days while I do the work.

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