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  • Solar PV earthing - cable size?

    What wire size should I use when earthing my solar panel frame?
    The cable run to the house' earth rod would be 20m away.

  • #2
    Usually awg 6 solid copper.

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    • #3
      No questions asked # 4 AWG solid bare tinned copper. 6 AWG may fail inspection. Codes specify minimums, and for a GEC depends on the size of the Service conductors entering the facility. NEC table 250.66 dictates the minimum and goes like this for Copper

      #8 AWG for up to a #2 AWG Service (50 to 100-Amp)
      #6 AWG for up to 2/0 AWG Service (50 to 150-Amps)
      #4 AWG for up to 3/0 AWG Service ( 50 to 300 Amps)

      Use a 4 AWG and you get no questions unless your house has a 300 amp or more service. If you have a service of 300 amps or more, you are the top 1% income earners and don't give a crap. Let the hired help worry about it and take the responsibility.

      Here is how electricians do grounding. Drive 2 Rods in the ground, connect them with #4 AWG, collect a check, and call it a day. They are not going to bother carrying #8 or #6 AWG on the truck when 4 fits everything and is what the Inspector is looking for. If he sees something smaller, he ask questions.
      Last edited by Sunking; 08-02-2016, 07:06 PM.
      MSEE, PE

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      • #4
        I see, thank-you for that explanation.
        However, I don't understand why the amps for the house affects the thickness the earthing cable for the solar panel frames.
        I thought the main reason for earthing PV metal frames, is to prevent lightning strikes.

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        • #5
          Originally posted by somawheels View Post
          I see, thank-you for that explanation.
          However, I don't understand why the amps for the house affects the thickness the earthing cable for the solar panel frames.
          I thought the main reason for earthing PV metal frames, is to prevent lightning strikes.
          It comes down to meeting the National Electric Code for Grounding conductor sizes based on service conductor sizes.

          Sometimes going with a wire size a little bigger will get the AHJ to pass the inspection quicker then going with a wire size that just meets code.

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          • #6
            Originally posted by somawheels View Post
            I see, thank-you for that explanation.
            However, I don't understand why the amps for the house affects the thickness the earthing cable for the solar panel frames.
            I thought the main reason for earthing PV metal frames, is to prevent lightning strikes.
            That is only one function. The main purpose is to protect you from the utility primary to secondary faults.
            MSEE, PE

            Comment


            • #7
              Originally posted by somawheels View Post
              I see, thank-you for that explanation.
              However, I don't understand why the amps for the house affects the thickness the earthing cable for the solar panel frames.
              I thought the main reason for earthing PV metal frames, is to prevent lightning strikes.
              Grounding the PV array has NOTHING to do with lightning. If you get a direct strike, it will still blow something up.
              Grounding the array is done to protect the equipment and personnel from shock. The ground wire provides a short circuit path that would blow the fuse and stop power from flowing.

              If the ground cable is unprotected, it must be at least 6awg. If you run it in conduit, it can be as small as the power wires coming off of the array.
              The house service cable size has nothing to do with the array ground, unless the power company decided the service wires could fall onto the array.
              Last edited by emartin00; 08-03-2016, 12:57 PM.

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              • somawheels
                somawheels commented
                Editing a comment
                I've seen a many websites say otherwise. some also say Lightning strikes some distance away can damage your equipment and this is a reason to earth the PV frame.

            • #8
              Originally posted by Sunking View Post

              That is only one function. The main purpose is to protect you from the utility primary to secondary faults.
              How does Utility power get to the solar panel frames?

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              • #9
                Originally posted by emartin00 View Post
                If the ground cable is unprotected, it must be at least 6awg. If you run it in conduit, it can be as small as the power wires coming off of the array.
                The house service cable size has nothing to do with the array ground, unless the power company decided the service wires could fall onto the array.
                I think the OP is asking about the GEC, what you describe sounds like the equipment bonding conductor which is sized per 250.122.

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                • #10
                  Originally posted by somawheels View Post
                  How does Utility power get to the solar panel frames?
                  Two ways.

                  1. Overhead lines tend to fall.

                  2. Utility Transformer or distribution Primary to Secondary fault.

                  The way to look at this is earth grounding is NOT to protect what happens inside your home as it cannot possible do that. It is to protect your home from what is outside trying to come in. I have taught quite a few Grounding and Earthing classes to contractors and through Mike Holt Higher Education and had a test and told every student if they did not get this question right they would fail. Did not matter if they got all the other 49 right, fail question one and you fail the test and course. No one ever missed it, now your turn.

                  Q Why Ground?

                  a. Provide an effective low impedance Planned Path for external Faults (example utility services like power, CATV, and Telco) to be routed to earth safely.

                  b. In larger commercial and industrial installation short out capacitive coupling between circuit conductors capable of developing 25 amperes of steady-state AC leakage endangering personal. Secondary is leaking current deteriorates cable insulation.

                  c. Minimize touch potential differences.

                  d. Provide a reference point for signals, transients, and return point of low impedance circuit conductors to safely and quickly operate over current protection protection devices through their equipment bonding conductors.

                  e. Minimize static charges

                  f. All the above

                  One last point. Under normal operating conditions Ground Does Nothing, at least not in your homes and electrical systems. Just because it does not normally do anything does NOT mean it is not needed because it is. It is there to minimize your risk of FIRE. Notice I did not say electrocute you? It certainly aids in minimizing the risk of electrocution, the code is there to prevent fire and explosions. Face it most home have two power sources that do not play well together. Bad things happen when you play with electricity and natural gas or fuel oil.

                  So like it or not, we guys who make the code conspired to make one size fits all approach for Article 250 which covers Grounding and Bonding. In some application the minimum requirements may exceed a particular application and be some over kill. So what it is safe. It would be impossible to have different requirements for all the possible applications.
                  Last edited by Sunking; 08-03-2016, 04:32 PM.
                  MSEE, PE

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                  • #11
                    I'll stick my neck out and say "e". I hope I'm right, because I don't see how a connection to ground helps trip overcurrent devices.

                    Edit: If you meant "Why bond", I think it's "f".
                    Last edited by sdold; 08-03-2016, 05:58 PM. Reason: Removed the smiley face, because electrical safety is serious business

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                    • #12
                      Originally posted by sdold View Post
                      I'll stick my neck out and say "e". I hope I'm right, the reason is that I don't see how a ground connection helps trip overcurrent devices.
                      Excellent question. Now you get a shocking answer.

                      No ground is required or needed to operate a fuse or breaker, just a path. Examine the pix below.



                      What you see is an accurate representation of any house hold circuit. It is a properly Earth Grounded System where one of the circuit conductors is bonded to a Reference aka Earth. That is done at the service entrance, Going to your receptacles are 3-wires called Line (Black), Neutral (White), and Ground (Green) . Note the fuse installed on the Line conductor aka hot. Under normal operation load current flows from Line to Neutral conductors only. Ground is doing nothing.

                      Chit happens and inside your gizmo the Line Conductor becomes detached and touches the equipment case energizing your gizmo you are holding in your hands. Sparks and fire fly, then quits. When the Line Conductor in second pix makes contact with the grounded chassis via Green Ground wire bonded to the chassis. You now have a dead Fault of L to G. That results in a 100 or more amps flowing in the Line and Ground conductor. In the Line conductor you have a 20 amp breaker with over 100 amps flowing. So the breaker operates within 6 cycles and opens the circuit removing power.

                      Now let's take out our imaginary eraser and erase that Earth Ground and let it Float. We repeat the same thing above and it still works with no Earth Ground. Why?

                      Because the N & G Conductors are bonded together at the Main Bonding Jumper. Power does not flow through ground. It comes in and leaves on the utility conductors, not earth. In other words current returns to the Source. Does not matter if it is the utility, battery, or generator. Only one power source uses earth as a conductor, lightning and static.
                      Last edited by Sunking; 08-04-2016, 10:25 AM.
                      MSEE, PE

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                      • #13
                        You see this a lot though. This is from a Fluke document that came with our new 1625 ground tester: "If fault currents have no path to the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people."

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                        • #14
                          Originally posted by sdold View Post
                          You see this a lot though. This is from a Fluke document that came with our new 1625 ground tester: "If fault currents have no path to the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people."
                          You are making a mistake even experienced Sparkies make. You assume the word Ground has something to do with Earth. It does not necessarily mean that. .

                          Some Sparkies even have trouble understanding what a Grounded System is. All it means is one circuit conductor is intentionally bonded to Ground or somethin gin place of ground like a car frame. What we are discussing is a Grounded System. One circuit conductor is Grounded aka Neutral. It makes Over Current Circuit Protection real easy by just using one OCPD on the undergrounded hot circuit conductor aka Line.

                          I know how your Ground Tester Works, it requires no connection to earth. All they are doing is impossing a current on L-N and measuring the voltage between N-G. If they see a voltage, the meter knows there is not a proper EBC in place or the third Green Wire.

                          The Planned Fault Path is the third green wire. Both the Grounded Circuit Conductor (improperly used as Neutral), and Green Ground Conductors are bonded together at a common Buss Bar in your panel box. Look at the diagram. At that Ground buss is where the GEC terminates to reference both N & G to Earth Potential. No current flows through the GEC to earth. The green ground wire never carries any current unless you have a fault as shown above. The fault current then uses the green G conductor to return the current to the source.

                          That third wire or Green wire use to be called Equipment Ground Conductor (EGC) . There is that damn word Ground again. So to clarify we changed the name to Equipment Bonding Conductor or EBC. The Planned Path is the Green Wire, not earth or ground.
                          Last edited by Sunking; 08-03-2016, 07:14 PM.
                          MSEE, PE

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                          • #15
                            Originally posted by Sunking View Post
                            You are making a mistake even experienced Sparkies make. You assume the word Ground has something to do with Earth. It does not necessarily mean that
                            I answered your question the way I did because to me, grounding describes a connection to the earth, and bonding describes a connection back to the source for the sole purpose of carrying a fault current. When you asked the question "Why ground?", I thought you meant "Why connect to (earth) ground?" Sorry if I misunderstood. Can I change my answer to "f"? The ground tester is a modern version of the old "Megger", it's the 4-terminal type for 3-point FOP or 4-point tests. It also has a clamp-on feature that relies on the rod under test to be connected to a larger system with other electrodes.
                            Last edited by sdold; 08-03-2016, 08:22 PM. Reason: Added more stuff

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