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Solar Panel and Grid Tie inverter installation on subpanel in large shop

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  • Solar Panel and Grid Tie inverter installation on subpanel in large shop

    I'm looking to help friends install a 3000w array On/in their large shop (Garage) which is roughly 30 yards from the main home. The shop has 200amp service which comes from the main panel on the home. Is there any reason we can't install the array and inverter in/on the shop to provide power for both the shop and home? I'm familiar with grid tie systems that connect directly into the main panel but have no knowledge of any there are installed on a sub panel.

    Also, this is a metal structure in AZ. Anything else we should consider?

    thanks,

    kirk
    Last edited by KirkB; 08-18-2019, 01:22 AM.

  • #2
    Questions ..
    If the garage has a 200 amp Sub-Panel and you say, "it comes from the Main Panel in the home" then what size panel is installed the home ?
    it is possible that the garage sub-panel actually has its own 200 amp own service feed wires from the transformer ?
    If the Garage Panel is a sub-panel from the Main Panel then what size breaker was installed in the Main Panel for the garage sub-panel?
    What size wire was used between the home and the garage ?

    You could add, up to, a 40 amp breaker for PV into a true / real 200 Amp sub-panel - if it is properly "wired".
    But you may need only a 20 amp breaker for a 3000 watt Grid-tie Inverter
    Last edited by NEOH; 08-18-2019, 08:45 AM.

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    • #3
      Originally posted by NEOH View Post
      Questions ..
      If the garage has a 200 amp Sub-Panel and you say, "it comes from the Main Panel in the home" then what size panel is installed the home ?
      it is possible that the garage sub-panel actually has its own 200 amp own service feed wires from the transformer ?
      If the Garage Panel is a sub-panel from the Main Panel then what size breaker was installed in the Main Panel for the garage sub-panel?
      What size wire was used between the home and the garage ?

      You could add, up to, a 40 amp breaker for PV into a true / real 200 Amp sub-panel - if it is properly "wired".
      But you may need only a 20 amp breaker for a 3000 watt Grid-tie Inverter
      Thanks Neoh. This is the dialog I'm hoping for. I haven't had a chance to inspect the main panel as I'm currently out of state. The homeowner had the main panel upgraded when the shop was built to accommodate the sub panel in the shop. That's the info I have been given. As soon as I can, I will confirm this to be the case.

      my biggest concern with a grid tie system on a subpanel at this distance is voltage drop. Say the main panel read 118v per leg and the subpanel reads 114v per leg. The grid tie system would match the 114v which by the time it gets back to the main panel would likely be around 110-111v. Is that a concern?

      Also, what algorithm are you using to derive 40a on a 200a panel and is that 40a per leg or total?

      thanks boss!

      Comment


      • #4
        Originally posted by KirkB

        my biggest concern with a grid tie system on a subpanel at this distance is voltage drop. Say the main panel read 118v per leg and the subpanel reads 114v per leg. The grid tie system would match the 114v which by the time it gets back to the main panel would likely be around 110-111v. Is that a concern?
        Goes the other way. Since you are feeding energy from inverters to your main panel, voltage
        will increase along the way to the inverters, or decrease from the inverters to the panel. At
        30 yards you had best calculate the voltage difference, and consider if over size wire is needed
        to limit losses around that 180 foot loop, and voltage shift. Bruce Roe

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        • #5
          Originally posted by KirkB View Post
          Also, what algorithm are you using to derive 40a on a 200a panel and is that 40a per leg or total?
          Google "solar 120% rule."

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          • #6
            Originally posted by jflorey2 View Post
            Google "solar 120% rule."
            Yes sir Got it. Essentially overloading the panel. I'd like to say that would be common sense but we know where that gets us

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            • #7
              Originally posted by KirkB View Post

              Thanks Neoh. This is the dialog I'm hoping for. I haven't had a chance to inspect the main panel as I'm currently out of state. The homeowner had the main panel upgraded when the shop was built to accommodate the sub panel in the shop. That's the info I have been given. As soon as I can, I will confirm this to be the case.

              my biggest concern with a grid tie system on a subpanel at this distance is voltage drop. Say the main panel read 118v per leg and the subpanel reads 114v per leg. The grid tie system would match the 114v which by the time it gets back to the main panel would likely be around 110-111v. Is that a concern?

              Also, what algorithm are you using to derive 40a on a 200a panel and is that 40a per leg or total?

              thanks boss!
              Typically, you can apply the 20% PV Back-Feed rule to a Breaker Panel
              The simple rule is ... 40 Amps of PV Breaker = 200 Amp Panel x 20%
              I assumed that your 3000 Watt Inverter was 240 Volts, therefore a dual-pole 20 Amp Breaker.
              The PV Inverter will ( slightly ) raise the voltage in your the shop.
              If you you have fat wires, between the house and the shop, that support a 200 amp sub-panel, then I am not concerned about a few volts.

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