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  • System Grounding plan questions

    As I am currently installing my first system, I have been doing a ton of reading. At this stage, I have my ironridge rails up, conduit in place, and I'm double checking my plans, rereading manuals and such.. Let me say that the rules for DC system grounding are not clearly laid out. I'm going to describe my plan, and ask that if any of you sees an error please correct me.
    3 strings on the barn roof, that feed two inverters in the house through a single buried pvc conduit. I have three strings (six 10awg wires) and two grounds (also 10awg) going through the conduit. The grounds are tied into the lugs at each ironridge rail, and a grounded bushing on the emt conduit coming of the roof. I think I could use a single ground as it's all in one conduit, but two can't hurt. I will connect each ground to one inverter, and then run #8 bare copper to the water service where my main panel is grounded.
    FWIW, these are older transformer type inverters. It sounds like some of the rules are a little different for transformerless inverters.
    I 'think' this is correct. Though I admit I've read enough about it that I'm dizzy.
    Many thanks. SD

  • #2
    You want the wire from rails to earth direct, with nothing else tied in. By earth, I mean ground rod or metal water pipe or similar. Then you want a second wire from earth to your inverter. Don't use the same wire for inverter ground and rail ground.

    Let's assume that there is a lightning strike near the barn, and some energy is coupled into things on the roof, like the rails. Your ground wire is the way for that energy to reach earth. The closer the strike, the higher the energy, and the more important it is to have a direct path to earth. During that brief instant when 1000+ amps flows through your ground, everything else coupled to that wire will spike up in voltage. For the best chance of surviving that spike, you want the least voltage jump on anything that can be damaged, like your inverter or things connected to your service panel.

    I hope this helps.
    7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

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    • #3
      bob-n, your point is exactly what makes this confusing
      I agree with what you say, in fact it was my first plan. But after reading more: NEC section 690.42 says that transformer isolated systems need a single point of grounding, and that point must be through the inverters internal GFCI. A secondary ground path defeats the GFCI.
      Am I reading it incorrectly?

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      • #4
        Moderator please move to solar panel installation, didn’t realize I posted in the wrong forum

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        • #5
          Originally posted by badwithusernames View Post
          But after reading more: NEC section 690.42 says that transformer isolated systems need a single point of grounding, and that point must be through the inverters internal GFCI. A secondary ground path defeats the GFCI.
          Am I reading it incorrectly?
          Say what? How did you get that from?

          690.42 690.42 Point of System Grounding Connection.Systems with a ground-fault protective device in accordance with 690.41(B) shall have any current-carrying conductor-to-ground connection made by the ground-fault protective device. For solidly grounded PV systems, the dc circuit grounding connection shall be made at any single point on the PV output circuit.

          You bond the system at any single point of the PV output circuit.Strictly for Lightning Protection. You would not direct lightning into your house before you bond it. You want it going straight to earth and the code gives you that ability so you can bond it anywhere to accomplish that. All that is done where th epanels are. Not after going inside inviting lightning inside. You wanna keep it outside.
          Last edited by Sunking; 09-23-2020, 08:36 PM.
          MSEE, PE

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          • #6
            Without knowing the details here is the best tip I can give you. Brand it in your brain SINGLE POINT. It is the key for maximum electrical protection and equipment operation. It does not matter if we are talking radio towers, data centers, wind farms, or your house electrical system.

            Your house has an AC Service Entrance where they put the meter can and disconnect. Code requires it to be Bonded, and the conductor is easy to find. It is that wire that goes straight down to dirt and connected to a Ground Rod. That is called Earth Ground Electrode System and your SINGLE POINT. All equipment grounds originate right there on that wire that runs from your meter can to dirt. Dang Simple huh?
            MSEE, PE

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