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  • davidon
    Junior Member
    • Aug 2023
    • 3

    Panels on Barn and backfeed power back to main panel at house. Is this do-able?

    Total Noob to this. In MD. Looking to put Panels on barn which is about 3-350 ft from house. Currently have sub panel at barn with 100A breaker. 200A main breaker at house. There are 4 wires going from house to barn, direct burial, I believe they are 2/0 gauge with neutral a bit higher gauge. Some companies are saying I need to dig a new trench and wiring in order to accomodate the solar. I want to avoid that if possible. Looking to set up a 20kW system. What are my options?
  • bcroe
    Solar Fanatic
    • Jan 2012
    • 5203

    #2
    Originally posted by davidon
    Total Noob to this. In MD. Looking to put Panels on barn which is about 3-350 ft from house. Currently have sub panel at barn with 100A breaker. 200A main breaker at house. There are 4 wires going from house to barn, direct burial, I believe they are 2/0 gauge with neutral a bit higher gauge. Some companies are saying I need to dig a new trench and wiring in order to accomodate the solar. I want to avoid that if possible. Looking to set up a 20kW system. What are my options?
    You need to map it all out, and check against local codes.

    First problem is getting power from the barn to the house. possibly a
    pair of 10KW inverters in the barn produce 20KW, which is 240VAC
    at 83A. The barn could be upgraded to a 200A box to make it easier,
    did that here. The pair of inverters would feed a pair of 60A breakers
    located at the opposite end of this box, from the main house input.
    Other circuits in the middle. The 100A feed from the house and the 83A
    possible from the inverters will add up to less than the capacity of a 200A box.

    Next question is voltage loss in a 700 foot loop of 2/0 gauge copper. Its
    resistance is about .0779 ohms per 1000 feet, or about 0.055 ohms for
    a 700 ft loop. At 83A the voltage lost will be about 4.5V which will be added
    to the house line voltage. Power lost would I squared R, or 375W.

    At the house box, the 100A breaker feeding the barn must be located at
    the opposite end of the bus bar, from the Power Co input breaker. This
    will assure that current from the PoCo and solar are flowing in opposite
    directions to any loads, so they will cancel, not add up in the busbar. Some
    codes have a belt and suspenders requirement that the sum of all currents
    supplying the busbar cannot exceed 240A for your 200A box. In that case
    the PoCo feed breaker may be changed to 150A, which when added to
    the 83A solar is only 233A.

    Having matched arrays and inverters makes it very easy to check for faults.
    As long as outputs are a close match, all is well. good luck, Bruce Roe

    Comment

    • davidon
      Junior Member
      • Aug 2023
      • 3

      #3
      Company I am thinking of using says they will tap into supply side of barn power so no panel upgrade needed. Is this kosher?

      Comment

      • bcroe
        Solar Fanatic
        • Jan 2012
        • 5203

        #4
        Originally posted by davidon
        Company I am thinking of using says they will tap into supply side of barn power so no panel upgrade needed. Is this kosher?
        Check your local codes. Make sure the long run is really 2/0 copper, I would consider
        the minimum for acceptable losses. It might be aluminum, in that case you would need
        to go at least 2 gauges heavier. I use 4/0 aluminum here for 1% loss with 600 ft loop
        and 15KW, but you are running 700 ft at 20KW. Bruce Roe
        Last edited by bcroe; 10-06-2023, 06:03 PM.

        Comment

        • davidon
          Junior Member
          • Aug 2023
          • 3

          #5
          Haven't checked codes but work will be inspected so I assume it's acceptable. Wire IS Alum 2/0. What would loss be with this? I am putting up some extra panels (20.7kW). Extra trenching/wiring would be a big additional cost. Using IQ8Ms and they're also estimating a 1% loss with what's there now.

          Comment

          • SunEagle
            Super Moderator
            • Oct 2012
            • 15126

            #6
            Originally posted by davidon
            Haven't checked codes but work will be inspected so I assume it's acceptable. Wire IS Alum 2/0. What would loss be with this? I am putting up some extra panels (20.7kW). Extra trenching/wiring would be a big additional cost. Using IQ8Ms and they're also estimating a 1% loss with what's there now.
            If the run is a higher voltage AC from the IQ8,s you may be ok. If was a lower DC voltage you may exceed the 2% VD requirement.

            Comment

            • bcroe
              Solar Fanatic
              • Jan 2012
              • 5203

              #7
              Originally posted by davidon
              Haven't checked codes but work will be inspected so I assume it's acceptable. Wire IS Alum 2/0. What would loss be with this? I am putting up some extra panels (20.7kW). Extra trenching/wiring would be a big additional cost. Using IQ8Ms and they're also estimating a 1% loss with what's there now.
              Do the arithmetic, aluminum will be around 0.124 ohms/1000 ft, times 700 ft
              is .087 ohms, that times 83A is 7.2V or near 3% loss. You can run that way,
              esp if your peaks are largely limited by clouds or orientation, but you might
              also have trouble with inverters tripping out on high line voltage.

              Another method is use staggered array orientation, to spread energy at
              lower levels over longer time. This will cost some panel output, I just
              bought enough panels to compensate, also handles clouds better.

              After 7 years of high AC losses, I buried aluminum triplex direct burial, must
              be 24 inches deep. Not that expensive for me since I have a trencher, much
              more if you hire someone to install copper. good luck, Bruce Roe

              Comment

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