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  • #16
    Originally posted by JSchnee21 View Post
    Alternatively, if you put the inverters proximal to the array. You'd be looking at ~225 feet of 120V AC @ 47 amps (times 2 for 240V split phase) for which Southwire's tool recommends 3 AWG (4.13% voltage drop -- 5V). Or 2.62% drop (3.1V) with 1 AWG (both copper in this case).

    This will cost you more in wire, but if you can re-use the existing conduit, it may be cheaper overall. You could also use a separate inverter per string (3) or split into two strings, and run multiple sets of smaller gauge wiring but this will likely cost even more (multiple inverters to buy, many more runs of wire).

    I still agree running the DC would be best, but I don't know how much retrenching will cost. Can you rent a ditch witch?

    One other aspect to consider (though electrical code won't cut you any slack) is that for 20+ hours per day, your currents (regardless of voltage) won't be anywhere near maximal. So the voltage drop will be pretty minimal during all but peak sun hours.

    3 AWG copper is currently about ~$0.99 per foot (L1, L2 and ?neutral? = 675 feet, smaller or reuse existing ground) per inverter Not sure of these are 3-wire (HHG) or 4 wire (HHNG).
    1 AWG is ~$1.65 per foot. 6 AWG is ~$0.57 / ft.

    Pricing from www dot wireandcableyourway dot com

    "4) If I have an existing subpanel near the ground mount (as opposed to going into the main panel 250ft away) can the AC output of the inverter be simply wired to a 30amp circuit breaker in that subpanel where can back feed into the house? ( subpanel is in a shed 200ft away from the meter - but only about 40ft from where the panels would be)"

    Unlikely to pass inspection. But what size sub panel? How big is the breaker feeding it? Wire gauge from house? Could be feasible if panel, breaker, wire are big enough -- but unlikely for such a large PV array. I believe this would generally follow the same 120% rule for a load side tap -- I believe this is permissible on a branch, but I'm not an electrician.
    Something that is needed to be looked at is the de-rating calculation (NEC table) due to putting more than 4 energized wires in the same conduit. That means the wires will be allowed carry less amps then they were originally allowed to run probably requiring bigger wires for the same amp draw.

    You must also look at the "fill %" of the conduit to make sure new wires does not exceed the maximum allowed by the NEC.

    And finally if you have ever tried to use an existing conduit (overhead or underground) you will find it is almost impossible to pull in more wires unless the conduit is very big and you already have a spare wire or "pull cord" in the conduit.

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    • #17
      Originally posted by JSchnee21 View Post
      Alternatively, if you put the inverters proximal to the array. You'd be looking at ~225 feet of 120V AC @ 47 amps (times 2 for 240V split phase) for which Southwire's tool recommends 3 AWG (4.13% voltage drop -- 5V). Or 2.62% drop (3.1V) with 1 AWG (both copper in this case).

      This will cost you more in wire, but if you can re-use the existing conduit, it may be cheaper overall. You could also use a separate inverter per string (3) or split into two strings, and run multiple sets of smaller gauge wiring but this will likely cost even more (multiple inverters to buy, many more runs of wire).

      I still agree running the DC would be best, but I don't know how much retrenching will cost. Can you rent a ditch witch?

      One other aspect to consider (though electrical code won't cut you any slack) is that for 20+ hours per day, your currents (regardless of voltage) won't be anywhere near maximal. So the voltage drop will be pretty minimal during all but peak sun hours.

      3 AWG copper is currently about ~$0.99 per foot (L1, L2 and ?neutral? = 675 feet, smaller or reuse existing ground) per inverter Not sure of these are 3-wire (HHG) or 4 wire (HHNG).
      1 AWG is ~$1.65 per foot. 6 AWG is ~$0.57 / ft.

      Pricing from www dot wireandcableyourway dot com

      "4) If I have an existing subpanel near the ground mount (as opposed to going into the main panel 250ft away) can the AC output of the inverter be simply wired to a 30amp circuit breaker in that subpanel where can back feed into the house? ( subpanel is in a shed 200ft away from the meter - but only about 40ft from where the panels would be)"

      Unlikely to pass inspection. But what size sub panel? How big is the breaker feeding it? Wire gauge from house? Could be feasible if panel, breaker, wire are big enough -- but unlikely for such a large PV array. I believe this would generally follow the same 120% rule for a load side tap -- I believe this is permissible on a branch, but I'm not an electrician.
      trenching in south florida is difficult as its 100% coral rock and only a minuscule layer of dirt

      But what size sub panel? = > the subpanel is on a 50amp circuit breaker (i think) ill check again later

      Comment


      • #18
        In the past it has been possible to rent a trencher. Lately you can buy a low end unit for $3K (including shipping)
        for a narrow (3 or 4 inch) wiring trench. These I found work for me, but have no transmission to propel them. I
        attached a hand cranked winch to pull it along, works just fine. Bruce Roe
        Last edited by bcroe; 04-25-2019, 01:43 PM.

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