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Deciding which end to make positive.

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  • Deciding which end to make positive.

    Hey Guys. Trying to be a better engineer here. When it comes to home runs I never considered whether the short home run right next to my j-box would be more beneficial as a positive or a negative. While my early training tells me that current flows from negative to positive, the circuit still makes the same loop in the end. Speaking strictly ungrounded inverters here (cause the grounded conductor would always be my choice for the long run), does anyone have an opinion or engineering fact that would give reason to believe that it is better one way or the other? Since current starts at the negative, would that mean that it is more beneficial to have the negative closest to the inverter?

    Thanks ahead of time!

  • #2
    It will not affect your efficiency. There might be other considerations like standardization. Bruce Roe

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    • #3
      You have 2 wires carrying power - & + but lets cover this first (and Sunking will clear up any mess I make of it)

      There is supposed to be an Earth Ground for the electrical, usually bonded to the white wire in the main electrical AC service panel. Just this one place.

      There is supposed to be an Equipment ground, so the metal parts of the mounting assy, does not become energized.

      Different code rules, at different dates, might apply in your area, so your local building official can update you on that.

      With the DC side of things, the regardless which wire + or - is "possibly" bonded to ground somewhere, you should not assume the grounded wire would have less amps running in it. The same amps in the + wire, HAVE to return thru the - wire. No shortcuts allowed. Again, your local Code will be your rulebook, not what my inspector used 3 years ago.
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

      Comment


      • #4
        Originally posted by Mike90250 View Post
        You have 2 wires carrying power - & + but lets cover this first (and Sunking will clear up any mess I make of it)

        There is supposed to be an Earth Ground for the electrical, usually bonded to the white wire in the main electrical AC service panel. Just this one place.

        There is supposed to be an Equipment ground, so the metal parts of the mounting assy, does not become energized.

        Different code rules, at different dates, might apply in your area, so your local building official can update you on that.

        With the DC side of things, the regardless which wire + or - is "possibly" bonded to ground somewhere, you should not assume the grounded wire would have less amps running in it. The same amps in the + wire, HAVE to return thru the - wire. No shortcuts allowed. Again, your local Code will be your rulebook, not what my inspector used 3 years ago.
        Yes yes, I definitely agree with all of that. My influence to make the grounded conductor the long run and the ungrounded the short, is purely to minimize the chance of ground faults. Make the wire with the highest potential the shortest wire on the rooftop is how I prefer to approach the grounded inverters. It was purely a thought of safety through minimizing risk. The consideration isn't the same. The argument came up at work to label the positive and negative ends of the strings on the physical circuit layout of the design. And I couldn't really think why to worry about it. At least not from an engineering point of view. I don't care which one is 10 feet and which one is 80 ft. If red is more expensive then black, then make the red one the short run, sure. But that's the only reason I could think of why to care in this instance.

        Comment


        • #5
          Look up (google) "skip wiring solar panels"
          https://solarprofessional.com/articl...d#.W9OnOOfPzIU
          Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
          || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
          || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

          solar: http://tinyurl.com/LMR-Solar
          gen: http://tinyurl.com/LMR-Lister

          Comment


          • #6
            Do skip wiring and both ends will be rIght there ....

            https://solarprofessional.com/articl...d#.W9Os3BopChA
            OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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            • #7
              t
              Originally posted by Mike90250 View Post
              Look up (google) "skip wiring solar panels"
              I can't use those schemes on any of my panels, what with landscape mounts and snow gaps. The methods used
              by my original installer were all revised. I have nearly a dozen MC4 extension cords, not a problem. Bruce Roe

              Comment


              • #8
                Hey guys thanks so much. I will definitely have to start using that. Genius

                Comment


                • #9
                  Originally posted by McMac View Post
                  Since current starts at the negative
                  Nonsense, makes no difference. Current can start with negative or positive. You are talking theory of Electron Current Flow vs Conventional Current Flow. Take your pick, whichever you prefer to work with on paper. Work with Conventional Current Flow and you work with positive numbers, work with Electron Current Flow and you work with negative numbers. No one cares and electrons could care less which way you use. Only matters on paper for calculations. In practice absolutely no difference.

                  As for which polarity you bond to earth only matters if you are using lead sheathed underground cables. Telcos bond positive to earth to eliminate electrolysis on lead sheathed cables that cause corrosion and ate through the cable. All that green nasty stuff you find under the hood of any car positive battery term post. However you will find few to no Positive Ground systems used anywhere. In fact most equipment manufactures bond Negative to chassis and makes that decision a no-brainer for the user.

                  If Safety is the concern, then last thing you want to do is ground your system. All that does is make it dangerous and prone to unnecessary outages. That is why Industry prefers not use Grounded Systems and avoid them like the plague.

                  Open any electrical distribution panel and they all have something in common, buses. Example your everyday home electrical panel has 3 copper Buses. L1. L2, and Ground Circuit. On L1 and L2 Buses you install a OCPD, and your black wire to the Load Side of the OCPD. White and Green terminate to the Ground Circuit Bus. So that means all Feeders and Branch circuit conductors are all the same length, Like a round oval race track, make no difference which way you go around in the circle is the exact same difference. Only difference is the Driver. Do you like to turn left or right? Car and track does not care, that is why the driver is in the center unlike passenger cars being on the Left side in the USA, and communist/socialist on the Right Side. Us Army came up with that so Snipers know who to shoot.
                  Last edited by Sunking; 10-29-2018, 12:49 AM.
                  MSEE, PE

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