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  • off grid setup questions related to grounding and awg

    I am putting in a small solar setup at a remote building and have a few questions related to grounding and what gauge wire to use. I attached a diagram of some of the equipment I have and the gauge wire.

    My questions are below:

    1. Do I need to run a ground off of the battery bank?

    2. Does the gauge of the ground wire matter (for example: should the ground wire be larger in diameter than the + and - wires). I was told that the larger ground wire will push the current to the grounding rod due to it being the least resistant.

    3. Can the DC and AC portions of the wiring be grounded to the same location as shown in the picture.

    Also, if there is anything else you can see wrong with the setup I have in the picture.

    Thanks!


    solarpicforsite.jpg

  • #2
    No requirement for a 12 volt system to be grounded. All depends on how you intend to implement over current protection.
    MSEE, PE

    Comment


    • #3
      Although NEC doesn't require 12V systems to be grounded, it's still good practice. The ground must be of equal or greater size as the current carrying conductors (+ and -), 6AWG bare copper is commonly used. There is no place to ground the Morningstar ProStar 30, but at one place in your DC breaker box (like the Midnite Big Baby Box), you should bond (wire) the negative to ground, thus grounding the negative line, turning it into a grounded current carrying conductor. That will in essence, ground your battery bank. If lightning is a concern, you should put a lightning arrestor in the system, like Midnite's MNSPD-115.

      Read the install manual for the Morningstar inverter, it shows you connect the DC ground to the ground terminal, and you also want to have the AC neutral line connected to earth ground. The manual says to use 12AWG.
      Solar Queen
      altE Store

      Comment


      • #4
        Originally posted by arnold_ky View Post
        I am putting in a small solar setup at a remote building and have a few questions related to grounding and what gauge wire to use. I attached a diagram of some of the equipment I have and the gauge wire.

        My questions are below:

        1. Do I need to run a ground off of the battery bank?

        2. Does the gauge of the ground wire matter (for example: should the ground wire be larger in diameter than the + and - wires). I was told that the larger ground wire will push the current to the grounding rod due to it being the least resistant.

        3. Can the DC and AC portions of the wiring be grounded to the same location as shown in the picture.

        Also, if there is anything else you can see wrong with the setup I have in the picture.

        Thanks!


        [ATTACH=CONFIG]5214[/ATTACH]
        I know your question is in regard to the ground wire but I see a couple of issues with your wiring diagram.

        The 100amp fuse is too much for that #6 wire between the inverter and battery. That #6 is good for about 75Amps max so the fuse should be at or below what the wire can handle.

        The other issue (or what I perceive) is the way you are wiring your AC power from that 300watt inverter to the Electric Panel. If that Panel is connected to the Utility Grid you can't use a non grid tie inverter.

        Comment


        • #5
          Good point, I had assumed it was being correctly used as an off-grid inverter and it was going to a stand alone AC box they are putting in.

          Reading more of the inverter install manual, it does recommend a 100A fuse and 6AWG wire, and 4AWG wire for ground. Not sure why it wants 100A fuse, the inverter is rated for 300W continuous, 600W surge, 10V minimum voltage, so even if it is figuring for the surge at low voltage, 600W / 10V = 60A x 1.25 continuous = 75A fuse. So the 6AWG wire is appropriate, but the 100A fuse is too high. We tell people to use the continuous wattage, so technically, you only need a 40A fuse. I'll ask Morningstar about it, what they were thinking.
          Solar Queen
          altE Store

          Comment


          • #6
            Thanks Amy@alte, now that I look at the charge controller again it doesn't have the ground, using the breaker box makes sense.

            Suneagle, On the diagram it says electric box but it is really just a breaker box. The system is completely off grid. Also, I thought the invertor called for the 100 amp fuse. I will double check that.

            I will go through the suggestions and update the picture.


            Thanks again for all the quick replies.

            Comment


            • #7
              I just double checked my friendly NEC 2014 Handbook. Table 310.15(B)(17) lists wire for 30C (86F). Assuming the inverter will be inside and not exposed to temperatures over 86F, 90C 6AWG wire is good for up to 105A. The inverter itself can handle up to 2AWG, which is overkill, but the manual says 6AWG or bigger. So if you want to use a 100A fuse, that'll work. Personally, I'd use no bigger than a 60A fuse and 6AWG. The nice thing with using 60A, is you can just get a DIN rail breaker that would fit in the Big Baby Box, so use the breaker box for the 20A breakers in and out of the charge controller, and the 60A (or go wild, go up to a 63A) breaker for the inverter.
              Solar Queen
              altE Store

              Comment


              • #8
                Here is an updated pic of the layout with the DC breaker box. I haven't had a chance to look at the inverter ground just yet so that may be off.

                Also, is it okay to have the DC and AC grounds connecting together?


                pic02.jpg


                Thanks!

                Comment


                • #9
                  Something very wrong here. A 300 watt inverter 12 volt DC input will never operate a 100 amp fuse any day of the week. Second point as shown does not matter because if you were to have a fault on either polarity of the battery nothing happens. Second point is the Inverter has GFCI built into it and a ground reference does nothing. The whole deign is flawed from the get go.

                  That 3 amp fuse you show on the Ac line will never operate, the inverter does not have enough power to operate it.
                  MSEE, PE

                  Comment


                  • #10
                    I just sent a note to Morningstar asking why they are recommending a 100A fuse on the DC side, I should have an answer next week, they tend to be very responsive. They also spec'ed the 3A fuse on the AC side, 300W / 120V = 2.5A x 1.25 = 3A, so that's right.
                    Solar Queen
                    altE Store

                    Comment


                    • #11
                      Originally posted by Amy@altE View Post
                      I just sent a note to Morningstar asking why they are recommending a 100A fuse on the DC side, I should have an answer next week, they tend to be very responsive. They also spec'ed the 3A fuse on the AC side, 300W / 120V = 2.5A x 1.25 = 3A, so that's right.
                      The 3 amp fuse will never blow. There is no justification for a 100 AMP fuse feeding a 300 watt 12 volt inverter. A first year student can tell you that. Maybe for a 1000 watt inverter, but not a 300 watt unit. Wired the way illustrated would never blow the fuse if a fault occurred unless it was a dead bolt fault line to line. Not to mention all that unprotected wire between the battery and fuse.
                      MSEE, PE

                      Comment


                      • #12
                        Outside of the inverter fuse issues (all were per Morningstar's documentation and as Amy stated was a possible issue), is everything else in the second picture looking as it should be or any other suggestions?

                        Thanks.

                        Comment


                        • #13
                          Fuses can be used to protect the wire from overheating and melting the insulation. It doesn't matter what the normal load of the equipment hooked up to it might be, except in that the load sets the minimum fuse size (not the maximum).

                          I agree with the others.. 100A fuse on 6 AWG does look oversized. Per 2011 NEC Table 310.15(B)(16), 90 deg THHN should be 75A if no other derates are necessary. However, if the terminals on the breaker box are not marked, 60 deg C has to be used per 110.14(C), even if the wire insulation had a higher rating. That would limit the fuse to 55 A max.

                          If wire with single layer insulation is being used, Per NEC Table 310.15(B)(17), the 90 deg does allow 105 A, but that isn't useful if it being connected to an unmarked (60 deg) terminal. The 310.15(b)(16) rating for the terminal would apply, 55 A max if it is unmarked (probably use 50 A), or 65 A max if marked 75 deg (probably use 60 A).
                          Last edited by sensij; 11-21-2014, 09:06 PM. Reason: Edit - dusted off the 2011 NEC Tables
                          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                          Comment


                          • #14
                            Let's start with the batteries and work our way out. You show some section of wire of some distance from the battery term post to the OCPD's (over current protection device) going to the inverter and controller. Extremely dangerous and violates all code and engineering practices. If a fault were to occur on the wiring between the battery and OCPD the batteries would explode and the wire would vaporize instantly. If you do not know why you have no biz answering any questions and should have known better to begin with. Even if the OCPD's were in the correct location would not work because they have no return path back to the source which is the batteries making the whole system dangerous.

                            Second point there is absolutely no way to make the inverter code compliant to use in premises wiring application because it is a mobile inverter, and mobile inverters are not made to supply premises wiring systems thus not listed for the intended purpose. It could be made safe and work, but would never pass any inspection. FWIW the way detailed in the schematic is not safe, and unnecessary expense which adds no function of safety. .

                            The problem with the 100 fuse is it does not protect the inverter or its output wiring. It is a mobile inverter and fuses on the output are USELESS, even a 3 amp fuse. The inverter is a very soft source. It cannot produce enough output current to trip a 3 amp fuse. It couldn't even if it tried. Being a mobile inverter it does not rely on a output fuse to protect the output wiring. It uses its own built in breaker and GFCI. That is why it has a Plug receptacle instead of a wire terminal. IT also relies on the input fuse size fuse. Not to protect the output wring, but to help minimize internal circuit damage. Instead of being limited to 30 or 40 amps, you just allowed 100 amps to pass and fry it to a crisp. Lastly with a 100 amp OCPD if a fault were to occur, would cause currents in excess of what the battery can supply and would likely result in damage to the batteries like melting the term post, warping the plates/grids, or God forbid a battery explosion.

                            Lastly being 12 volts means there is no requirement to ground the system. If given the choice you would always go with a floating system because they are safer and not prone unnecessary outages. Over Current Protection cost a bit more with a Floating System, but the savings gained in not have to bond the system more than make up for the cost of two extra OCPD's. Not to mention the added safety and reliability of a floating system.
                            MSEE, PE

                            Comment


                            • #15
                              Originally posted by Amy@altE View Post
                              I just sent a note to Morningstar asking why they are recommending a 100A fuse on the DC side, I should have an answer next week, they tend to be very responsive. They also spec'ed the 3A fuse on the AC side, 300W / 120V = 2.5A x 1.25 = 3A, so that's right.
                              But if that 300w inverter has a surge rating of 600 watts then at 120v it could see a jump of 5 amps which may eventually blow that 3 amp fuse. Just a thought.

                              As for the fuse on the #6 wire. I think 100amp is overkill I would rather go smaller to make sure I protect the wire. I rather have the fuse go then my wire melt.

                              Comment

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