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Planning a 8.58 kW grid tie system

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  • Planning a 8.58 kW grid tie system

    I am looking at installing 26 Hanwha L-G4.2 330 Watt cells on a ground mount system. Right now it looks like I will probably use the Unirac GFT in a 2 x 13 configuration (portrait panel layout). Also looked at going with an Iron ridge system but the Unirac is edging ahead. I plan on using the SMA Sunny Boy 7.7 US-40 with 11 panels to input A, 11 panels to input B and 4 panels to input C. My utility does net metering and will roll any excess generation over to the next month. When the net metering contract is terminated you forfeit all credits to the utility. I would like some input on the infrastructure I would need.

    My house is on the northwest side of the property and I have a large field to the south of my property that I plan to have the array face 15 ft or so off the property line. There should be no issues with shading besides really early in the morning and towards sunset. I have 2 barns that I call the North and South barn. The house is farthest north then North Barn then the South barn. The house and South barn have 200 amp service. I asked the utility if I could hook the solar into the south barn and get the credits on the house meter as the rack would just be 50 feet southeast of the South barn. They of course won't let me do that even though the two meters/bills go to the same address. The house averages about 1000 kWh a month for the last 3 years. ~600 kWh or so in the spring or fall and usually > 1500 kWh in Jan and Feb since I heat with geo thermal. The panels for my area should put out a little over 12,000 kWh a year.

    What I plan on doing is putting a new 100 amp sub-panel in the north barn that goes to the house 200 amp main panel. That distance is close to 150 ft. 120 ft in 1 1/2 pvc conduit from the corner of the barn to basement wall of the house then another 20 ft or so to the panel. I would use a 100 amp breaker at the main panel and 3 Al #1 XHHW-2 with a Cu #4 THWN-2 ground. The Sunny Boy would be mounted on the opposite corner of the barn from the sub panel inside as that is the direction of the racks. There would be a 40 amp circuit breaker in the subpanel and it would supply ~ 85 ft (probably less) of #6 Cu THHN-2 in 3/4" conduit inside the barn to the inverter. I assume I would use a #8 neutral and #8 ground. From the inverter out to the rack is another 160 ft so I would need 3 paired DC lines of #10 for the 3 strings in 1" pvc conduit plus a #10 ground.

    In the Sunny Web program that brings my total relative power loss to 2.3%. It might be a little less than that because I was pretty conservative on the distances between the runs. Sunny web projects 12,455.20 kWh of yield per year. PVWatts has it at 11,908 kWh. Still putting the system together but I do have the panels. One thing I noticed since reading these forums is it looks like I probably need to get a load center rated at 125 amps and put a 100 amp main breaker in for a 40 amp backfeed breaker for the north barn. The 40 amp backfeed should be okay at the 200 amp panel unless I am missing something.

    Any other suggestions as I am sure I am missing stuff? To me this seemed like the most logical way but I am also new to this stuff. Open to any suggestions if you see a better way.

  • #2
    Originally posted by wienerdog View Post
    I am looking at installing 26 Hanwha L-G4.2 330 Watt cells on a ground mount system. Right now it looks like I will probably use the Unirac GFT in a 2 x 13 configuration (portrait panel layout). Also looked at going with an Iron ridge system but the Unirac is edging ahead. I plan on using the SMA Sunny Boy 7.7 US-40 with 11 panels to input A, 11 panels to input B and 4 panels to input C. My utility does net metering and will roll any excess generation over to the next month. When the net metering contract is terminated you forfeit all credits to the utility. I would like some input on the infrastructure I would need.

    My house is on the northwest side of the property and I have a large field to the south of my property that I plan to have the array face 15 ft or so off the property line. There should be no issues with shading besides really early in the morning and towards sunset. I have 2 barns that I call the North and South barn. The house is farthest north then North Barn then the South barn. The house and South barn have 200 amp service. I asked the utility if I could hook the solar into the south barn and get the credits on the house meter as the rack would just be 50 feet southeast of the South barn. They of course won't let me do that even though the two meters/bills go to the same address. The house averages about 1000 kWh a month for the last 3 years. ~600 kWh or so in the spring or fall and usually > 1500 kWh in Jan and Feb since I heat with geo thermal. The panels for my area should put out a little over 12,000 kWh a year.

    What I plan on doing is putting a new 100 amp sub-panel in the north barn that goes to the house 200 amp main panel. That distance is close to 150 ft. 120 ft in 1 1/2 pvc conduit from the corner of the barn to basement wall of the house then another 20 ft or so to the panel. I would use a 100 amp breaker at the main panel and 3 Al #1 XHHW-2 with a Cu #4 THWN-2 ground. The Sunny Boy would be mounted on the opposite corner of the barn from the sub panel inside as that is the direction of the racks. There would be a 40 amp circuit breaker in the subpanel and it would supply ~ 85 ft (probably less) of #6 Cu THHN-2 in 3/4" conduit inside the barn to the inverter. I assume I would use a #8 neutral and #8 ground. From the inverter out to the rack is another 160 ft so I would need 3 paired DC lines of #10 for the 3 strings in 1" pvc conduit plus a #10 ground.

    In the Sunny Web program that brings my total relative power loss to 2.3%. It might be a little less than that because I was pretty conservative on the distances between the runs. Sunny web projects 12,455.20 kWh of yield per year. PVWatts has it at 11,908 kWh. Still putting the system together but I do have the panels. One thing I noticed since reading these forums is it looks like I probably need to get a load center rated at 125 amps and put a 100 amp main breaker in for a 40 amp backfeed breaker for the north barn. The 40 amp backfeed should be okay at the 200 amp panel unless I am missing something.

    Any other suggestions as I am sure I am missing stuff? To me this seemed like the most logical way but I am also new to this stuff. Open to any suggestions if you see a better way.
    it seems to me you might have a problem with strings breakdown: 11 + 11 + 4 which would result in DC Vmpp 37 x 11 = 407V / 407V / 148V at 25C of panel temp. Please note it's not ambient temperature but the panel itself which is normally significantly higher. At conservative 35 C the same voltages would look like: 390V / 390V / 142V. This inverter has MPPT range of 270V-480V and starting voltage of 125 V so your 3rd string will be barely making starting voltage. Any chance you can change it to 9 + 9 + 8 panels to get 319V / 319V / 284V? Should be possible as it doesn't affect wiring from the panels as all of them produce the same current anyway.

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    • #3
      Originally posted by max2k View Post

      it seems to me you might have a problem with strings breakdown: 11 + 11 + 4 which would result in DC Vmpp 37 x 11 = 407V / 407V / 148V at 25C of panel temp. Please note it's not ambient temperature but the panel itself which is normally significantly higher. At conservative 35 C the same voltages would look like: 390V / 390V / 142V. This inverter has MPPT range of 270V-480V and starting voltage of 125 V so your 3rd string will be barely making starting voltage. Any chance you can change it to 9 + 9 + 8 panels to get 319V / 319V / 284V? Should be possible as it doesn't affect wiring from the panels as all of them produce the same current anyway.
      I thought it was odd also but that is what their Sunny Web Design tool came up with. The specs have the MPPT listed as 100 - 550 VDC. Minimum voltage is 100 and minimum start is 125 VDC. I assume they do that as I think switching losses are less with a higher voltage so they max each channel.

      I looked at a 25 panel system and they had a string of 3 panels which I thought would be pushing the start voltage all the time. I haven't done anything yet besides having 27 panels. I can change inverters if I want.

      Comment


      • #4
        The *mppt* minimum is 270 V, as max2k wrote. The minimum operating voltage is 100. Outside the mppt range, the efficiency doesn't need to be counted in the efficiency listed on the data sheet, and drops a small amount below the weighted average.

        I don't know why SMA's design tool defaults to a short string, but I would make string lengths as balanced as possible if I could.
        CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

        Comment


        • #5
          I see now. The rated MPPT range is 270 - 480. The MPPT operating range is 100 - 550. So to get the rated 97% you need to stay in the rated range.

          Also kicking around the racking. I see if you get snow mounting in landscape mode is better as the 3 strings run that way. You can cover up to 2/3 with snow and still generate some. Has anyone seen a real life difference between landscape and portrait in snow?

          Comment


          • #6
            Originally posted by sensij View Post
            I don't know why SMA's design tool defaults to a short string, but I would make string lengths as balanced as possible if I could.
            I think I found why the tool says to use a 11 - 11 - 4 configuration instead of a 9 - 9 - 8. The overall efficiency is actually higher with the 11 - 11 - 4. It is 96.8% compared to 96.6% with the more equal length strings. It looks like they always try to maximize string voltage for a larger percentage of the overall power. I might give SMA tech support a call to see how dealing with them is before I buy.

            Comment


            • #7
              Originally posted by wienerdog View Post
              I see now. The rated MPPT range is 270 - 480. The MPPT operating range is 100 - 550. So to get the rated 97% you need to stay in the rated range.

              Also kicking around the racking. I see if you get snow mounting in landscape mode is better as the 3 strings run that way. You can cover up to 2/3 with snow and still generate some. Has anyone seen a real life difference between landscape and portrait in snow?
              In a typical 60 cell or 72 cell panel, each pair of rows of 10 or 12 cells as you work across the panel is bypassable through a diode. So yes, in landscape, if the bottom two rows of cells are covered, the remaining 4 rows will continue to produce. In portrait, if the bottom of the panel is covered, the entire panel output will drop.

              However, try to put this in perspective. How many days of the year are the panels likely to be partially blocked? In winter, with short days, production is already relatively low, so in the big picture the difference between 0, 1/3rd, or 2/3rds production may not impact the overall annual production that much.

              If you have a lot of snow, there is a bigger benefit to going with landscape orientation, if you put some space in between the rows. In that design, when the snow starts to melt, it has more places to drop through the array instead of sliding down and collecting at the bottom of a portrait panel (@bruceroe can explain this better if you don't understand).

              CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

              Comment


              • #8
                Spoke to the AHJ today. It seems they are still on NEC 2008. He said the sub panel can be wired 4 wire or 3 wire and use a ground rod. Any one better than the other besides the 4 wire being the code now?

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