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GFCI and direct burial wire depth question for my solar panel install

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  • GFCI and direct burial wire depth question for my solar panel install

    My system begins with 4 x 260 Watt panels wired 2S/2P, and in this configuration they will generate (at Vmp/STC) 62 volts and 16.8 amps.

    I have a measured 138 feet that I must traverse underground to bring this into the home and into my charge controller (using #4 direct burial wire). Normally this would require trenching to 24" depth, but if I add a GFCI breaker box fused at 20 amps at the above ground juncture where the #10 panel wiring will transition to #4, can I by so doing get the burial depth down to only 12"?

  • #2
    Using GFCI does not change burial requirements. Sorry I know your back does not want to hear that.
    MSEE, PE

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    • #3
      Originally posted by Sunking View Post
      Using GFCI does not change burial requirements. Sorry I know your back does not want to hear that.
      But I came across this:

      NEC 300-5 GFCI When a GFCI protected residential branch circuit is rated 20 amps or less and 120 volts or less, the minimum cover can be reduced to 12 inches.
      At this website (about 1/3 of the way down the web page, and under the header: Residential Electrical Code Examples for Underground):

      http://www.ask-the-electrician.com/e...derground.html

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      • #4
        You are not talking about a Branch Circuit. You are talking about a Feeder Circuit.

        However because it is solar and so low power, that is one part of the code you can safely ignore unless it is being inspected. I am not suggesting that though. The point or reason for the code is if you were digging at 2 feet is considered deep enough so normal digging would not go that deep. It is there to keep you the person with a shovel in your hand from being electrocuted. 62 volts is not lethal.
        MSEE, PE

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        • #5
          If I go "overhead" with the wiring from the panels to the charge controller instead of burying it, it would traverse only about 37 feet outside. Would that be an option? That would probably get me down to #8 wire.

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          • #6
            Originally posted by Silver_Is_Money View Post
            If I go "overhead" with the wiring from the panels to the charge controller instead of burying it, it would traverse only about 37 feet outside. Would that be an option? That would probably get me down to #8 wire.
            Absolutely or use your plumber skills and run it in pipe gets you out of the minimum burial depth. You have options.

            Keep in Mind as far as code goes you are only required to use minimum #10 AWG to meet safety requirements. But the code is not a design or performance guide. At 62 volts, 140 feet, with 17 amps of current it will require a:

            4 AWG @ 1.9% loss
            6 AWG @ 3.03% loss
            8 AWG @ 4.82% loss

            Makes no difference if it is buried, over head, or ran in pipe because it is DC and not AC. With that said I would not use 8 AWG as you would incur about 5% loss. My calculations are based on 75 degree Celsius, so it meets or exceeds any engineering principles. 4, 6, or 8 AWG exceeds any short circuit calculations the NEC would impose on derating conductors. So what it comes down to is how much loss are you willing to give up and how much money are you willing to spend to minimize losses.

            You can use NEC minimum requirement of 10 AWG to pinch pennies, but you give up 8% power loss doing so. It is all about trade-offs and how much pain you are willing to bare. Either pain in the wallet, or pain from power loss. You decide as you now have all the information needed to make your own decision.

            Hope that helps, clear things up, and answers your questions.
            MSEE, PE

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            • #7
              By running the wires overhead instead of underground I would be taking it from 138 feet if wire set down to only about 37 feet. Wouldn't that reduce the required gauge of the wire?

              Is it legal to run overhead wiring outside, or does it have to go underground?

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              • #8
                Originally posted by Silver_Is_Money View Post
                By running the wires overhead instead of underground I would be taking it from 138 feet if wire set down to only about 37 feet. Wouldn't that reduce the required gauge of the wire?
                Yes, but how do you cut 100 feet off? At 40 feet you can now use NEC minimum of 10 AWG and incur 2.03% loss

                Originally posted by Silver_Is_Money View Post
                Is it legal to run overhead wiring outside, or does it have to go underground?
                Whatever floats your boat. NEC allows either.
                MSEE, PE

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                • #9
                  Originally posted by Silver_Is_Money View Post
                  By running the wires overhead instead of underground I would be taking it from 138 feet if wire set down to only about 37 feet. Wouldn't that reduce the required gauge of the wire?

                  Is it legal to run overhead wiring outside, or does it have to go underground?
                  You can just adjust down all the quoted losses in proportion to the length ratio. So the quote loss at 137 is reduced by 37/138 going to overhead

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                  • #10
                    The back yard of my house has a huge concrete pad and my shed with solar panels is on the far side of it. To go around the concrete pad and trench through the dirt without cutting the concrete it takes 138 feet (due partly also to the need to go around a second out building), but to go straight across the pad from my shed to my house overhead requires only about 37 feet.

                    How high up does the wire have to be in order to travel overhead? I will need to also factor the rise in my length.

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                    • #11
                      Originally posted by Silver_Is_Money View Post
                      How high up does the wire have to be in order to travel overhead? I will need to also factor the rise in my length.
                      Per Code 150 volts and less is 10 feet. But that assumes utility service, feeders, and branch circuits. What I am trying to say is how every high you are comfortable with. 62 volts is just not much of a hazard. And if none of the polarities are grounded, no chance of electrocution. But the code answer is 10 feet.
                      MSEE, PE

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