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  • Upgrade suggestions

    Hi, I have had Solar since 2018. I found a good deal on a second inverter and would like to use some stimulus money to upgrade. My installer has gone out of business so I am looking at doing a lot of this myself. I have the following:

    Fronius Primo 6.0
    22 - 345w Panels
    MPPT1 - 12 - 4140w
    MPPT2 - 10 - 3450w
    Total - 7590w

    I clip in the spring quite a bit. I have room on my Pergola to mount 6 panels in Portrait. Trying to come up with the best configuration. My plan would be to reconfigure the strings to keep panels that are close to each other on their own string. My thought was the following:

    Fronius Primo 6.0 - Inverter 1
    13 - 345w Panels
    MPPT1 - 2415w (existing)
    MPPT2 - 2070w (existing)
    Total - 4485w

    Fronius Primo 6.0 - Inverter 2
    7 - 345w Panels
    6 - 395w Panels
    MPPT1 - 2415w (existing)
    MPPT2 - 2370w (new)
    Total - 4785w
    Grand Total - 9270w

    I was talking to someone online that supposedly knows solar and they told me that it is not a good idea to mix wattage even if they are on their own strings. He is saying I should stick with 345w across both MPPT. He said that it could cause premature inverter failure. Is this accurate advise? I want to get the most out of the space I have but don't want to cause problems for myself.

    Next question is the solar disconnect. I was thinking the best way to wire this would be to replace my 30amp disconnect with a 60 along with replacing the wire to the panel with appropriate gauge wire and 60 amp in the main panel. Then run each inverter to small 60 amp sub panel with each inverter having a 30amp breaker and the sub panel connected to the disconnect. Does this sound correct or should I have a disconnect for each inverter?

    Any suggestions for racking on top of a Pergola? I was hoping to have something I can change the angle but I don't want to break the bank.

    Thanks for the help!

  • #2
    You need help from a local professional.
    - a 6kW inverter is too big for a 30amp disconnect
    - adding a 2nd inverter will be likely be too big for a 60amp disconnect and too big for sure to backfeed a 200amp breaker panel under the 20% NEC backfeed rule. This will necessitate modifying or upgrading the service.
    - might be possible to replace the inverter with a 7.7kW (max for a 200amp service) and then add the 2nd array on at a different orientation to avoid overloading the inverter.
    BSEE, R11, NABCEP, Chevy BoltEV, >2500kW installed


    • #3
      If you want to avoid clipping, you could just swing roughly half the panels somewhat
      east, and the other half somewhat west. I have been doing that here since 2013. I
      do not agree that all strings in parallel must be identical panels. All cells generate
      close to the same voltage, so I make sure each string has the same number of cells.
      Panels with 60 or 72 cells, many different brands play together nicely here.

      Racking can cost more than the panels, esp if you ground mount for optimal performance
      with seasonable changeable tilt. It is worth it here, huge reduction in snow removal.

      I have been absolutely beating the sh... out of my FRONIUS inverters for nearly 8
      years, no problem with them. good luck, Bruce Roe
      Last edited by bcroe; 02-26-2021, 05:03 PM.


      • #4
        Current setup was installed by a professional with a 30amp disconnect feeding a 30 amp breaker all passed inspection.


        • #5
          Solarix is correct. The Fronius Primo 6.0 spec sheet says AC breaker should be 35A for 240VAC and 40A for 208VAC. Guessing you are 240VAC split phase so you should have 35A breaker.

          There are other aspects of the current and proposed set ups that deal with total string voltage and MPPT voltage range of 240 to 480 VDC. I think the current 12 panel string might be above max MPPT voltage range but under max voltage; proposed 6 panel string might be at or just slightly above minimum MPPT voltage. If this is something that doesn't need to be highlighted, then sorry for bringing it up.
          Last edited by oregon_phil; 02-26-2021, 04:34 PM.


          • #6
            Ok, I'll have to find someone locally to figure it out. Clearly not as "simple" as I thought.