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Unsure about wire gauge/breaker amperage from Battery to DC breaker box

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  • Unsure about wire gauge/breaker amperage from Battery to DC breaker box

    400W of solar
    150V/35amp charge controller
    2000W inverter/charger
    300ah battery bank

    This install is in a 19ft travel trailer

    I'm pretty much following the wiring diagram from explorist.life, which can be seen here https://www.explorist.life/?da_image=51400
    *EDIT: I plan to wire my panels in 2x "series" groups then these 2 groups in parallel

    However, install videos I've watched show folks using 6 gauge wire to connect battery positive to the DC part of the factory breaker box. I know Will Prowse suggests a 6 gauge wire/50-100 amp fuse for this purpose in his book.

    One youtube guy ran a 6 gauge wire straight from his battery (after the big fuse between battery and positive bus bar) to the factory positive terminal on the front of the trailer where the old, removed at this point, factory battery connected. I'm not sure what the gauge of the wire from this terminal to the DC breaker box was, but this guy in the video is pretty switched on and I imagine it was adequate.

    So my dilemma is do I follow the interactive wiring diagram above (which is essentially for a van) and run positive and negative 2 gauge wire with a 100 amp breaker on positive from my battery bank to the trailer's DC breaker box and just discard the factory wire/wires coming from the terminal at the front of the trailer where the removed factory battery was, or do I run the positive 2 gauge wire with the 100 amp fuse to the positive terminal at the front of the RV where the old battery used to be connected and just ground to chassis from the negative bus bar and assume that the factory wire from the factory battery terminal to the DC breaker box is adequate?

    I'm all about overkill for safety. However, I don't think I'm gonna be able to wire a 2 gauge wire into the DC breaker box, as it will probably be too big in physical size. I don't think you can suddenly "size-down" the 2 gauge wire just before it connects to the DC breaker box.

    All we will be running off the DC breaker box is the lights, a vent fan, the little fan for the composting toilet, and then whatever gets used by the furnace and the fridge while they run on propane (I understand they both use a small amount of power even when running on propane).

    As you can see, I am a total novice. Any advice would be appreciated.
    Last edited by Skwidward; 04-08-2019, 10:52 AM.

  • #2
    I can't imagine my 12V DC breaker box requiring any more than 50 amps.

    I mean it's just a few lights and what I mentioned above.

    I'm thinking 6 gauge wire with a 50 amp breaker should do the trick.

    Also, I'm thinking of just wiring from my new battery bank to the factory positive terminal on the front of the trailer, as that is already wired to the RV's DC breaker box.

    Can anyone imagine why I would need more than 6 gauge wire with a 50 amp fuse?

    Comment


    • #3
      The issue is your 2000 watt inverter. At 12V you can technically draw 170 amps. While your loads may never get that high it is still safer to run wire that can handle what the inverter can handle and fuse it properly.

      Running a smaller wire and fuse may lead to a hot wire & melted insulation or a blown fuse. Which is something most people do not want to deal with.

      Comment


      • #4
        Originally posted by SunEagle View Post
        The issue is your 2000 watt inverter. At 12V you can technically draw 170 amps. While your loads may never get that high it is still safer to run wire that can handle what the inverter can handle and fuse it properly.

        Running a smaller wire and fuse may lead to a hot wire & melted insulation or a blown fuse. Which is something most people do not want to deal with.
        Thank you. Better to err on the side of safety.

        I think I will stick with the suggested 2 gauge wire and 100 amp fuse.

        I have another question if you have a minute... can I use 8 gauge PV wire with regular MC4 connectors? Like, will the 8 gauge wire fit?

        Also, if the 12V DC breaker box can potentially draw 170 amps, how can I get away running 12/2 Romex with a 15 amp breaker from the AC out of the inverter to the AC breaker box of the trailer? I'm guessing (I am a total novice with all this) that's because this will be a 110 volt system. Is that correct?

        Thank you so much for helping me along
        Last edited by Skwidward; 04-09-2019, 03:52 PM.

        Comment


        • #5
          Originally posted by Skwidward View Post

          Thank you. Better to err on the side of safety.

          I think I will stick with the suggested 2 gauge wire and 100 amp fuse.

          I have another question if you have a minute... can I use 8 gauge PV wire with regular MC4 connectors? Like, will the 8 gauge wire fit?

          Also, if the 12V DC breaker box can potentially draw 170 amps, how can I get away running 12/2 Romex with a 15 amp breaker from the AC out of the inverter to the AC breaker box of the trailer? I'm guessing (I am a total novice with all this) that's because this will be a 110 volt system. Is that correct?

          Thank you so much for helping me along
          I am not sure of the maximum wire size for an MC4 connector. I think #8 may be a little too big but I could be wrong. You have to check the requirements for those MC4 connectors.

          You are correct about only requiring a #12 wire on a 15A breaker. It is because of the higher AC voltage of 110 - 120V. That is why running any wire at 12V is a killer because it requires much bigger wire for the same size wattage load. Basic electrical formula Watts = Amps x Volts.

          Comment


          • #6
            Remember there are 2 main factors in wire gauge (conductor cross section). The wire needs
            protection (such as a fuse) so that a continuous current cannot be high enough to overheat
            the insulation.

            Then there is the effect on the circuit, of voltage lost in wire resistance. A long run of 120V
            wiring might accept a loss of 2V, less than 2%. But upping the wire size to get 2V loss at
            10 times the current and 12V (same power) is going to be 17% loss, probably unacceptable.
            To get the same percentage loss at 12V, the conductor cross section must increase as the
            SQUARE of 10 times. That gets to be a lot of copper over any distance, which is why best
            practice is very short runs at low voltages, and higher voltages for long runs. Bruce Roe

            Comment

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