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Help PLEASE sizing up my new solar system for my camper

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  • #16
    Thank you all for your advice

    I've gone with 3 x 100 watt solar panels
    30amp MPPT charger controller
    2 x 6 volt Trojan T105 batteries.

    NOW I have been reading about the wiring.

    From my understanding because I have a small system and it is on a camper it would be better for me to wire the solar panels in parallel.
    Any advice would be appreciated

    Cheers
    Sami!

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    • #17
      the only reason to go parallel over series on a camper is if there is shading issues that affect one or more of your panels differently. For example one panel is partially shaded by your ac unit at a certain time and the others are not. In a series wired array the one shaded panel will bring down the output of the whole array. In a parallel array only the output of the shaded panel is affected.

      Just remember that in a parallel wired array you need to add combiners and fuses since there are 3 strings going into the charge controller.

      personally I would wire in series if possible. It simplifies the wiring. But the reality is that most campers/rvs have limited roof real estate.

      Hope that helps
      Supra
      Last edited by Suprasoup; 01-12-2018, 07:14 PM.

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      • #18
        Originally posted by sami_h23 View Post
        Thank you all for your advice

        I've gone with 3 x 100 watt solar panels
        30amp MPPT charger controller
        2 x 6 volt Trojan T105 batteries.

        NOW I have been reading about the wiring.

        From my understanding because I have a small system and it is on a camper it would be better for me to wire the solar panels in parallel.
        Any advice would be appreciated

        Cheers
        Sami!
        Then you throw away most of the advantages of using a MPPT Controller if you wire the panels in parallel. One word of caution here. Using prime number of panels is a really bad practice because it only gives you two choices to configure them. Either all in series or all in parallel. You can get away with 3, but not 5.7, 11...

        In fact you might not get away with 3 in series. Depends on what the Voc max input is on your controller. Using very expensive low power battery panels like you have most likely have a Voc of 22 volts, so 3 in series is 66 volts and once you make temp correction puts you up around 85 volts. Small Controllers typically have low Voc spec and may force you to put all panels in parallel which is going to require a lot more expense with larger wiring, combiners, and fuses. It will also hurt your efficiency, and a lot more holes in your RV.

        MSEE, PE

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        • #19
          Would two of those panels in series be about the same as three in parallel with an MPPT controller?
          Steve

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          • #20
            Originally posted by sdold View Post
            Would two of those panels in series be about the same as three in parallel with an MPPT controller?
            No... with an mppt controller, and no shade, the 1st order performance of three panels in parallel or in series should be the same. The main losses to consider are transmission (higher current for panels in parallel) and controller efficiency (usually higher losses for the higher voltage of panels in series). The most cost effective and productive design will need to consider the wiring costs (including OCPD for the parallel configuration) and maximizing production accounting for any shade that exists.
            CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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            • #21
              Thanks, you're right of course, I think I was imagining the three parallel panels being on a PWM controller (even though I said MPPT) and the series panels on an MPPT controller.
              Steve

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              • #22
                So im ready go install in series but I think I bought the wrong size cabling. Can you please advise if it would be ok if used 12 awg cable on the whole system instead of 8 awg.

                also do I still need a 40amp fuse in etween the most controller and the battery bank?

                thanks for all your help and advice
                cheers
                sami

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                • #23
                  Originally posted by sami_h23 View Post
                  So im ready go install in series but I think I bought the wrong size cabling. Can you please advise if it would be ok if used 12 awg cable on the whole system instead of 8 awg.


                  also do I still need a 40amp fuse in etween the most controller and the battery bank?

                  thanks for all your help and advice
                  cheers
                  sami
                  Use this to help you determine your wire size. Circuit type is 3% voltage drop column. Current flow should be between 5-6a on a 100w 12v battery panel. That is for the wire size between the solar panels to charge controller.

                  Wire size between charge controller and battery is 300w / 12v = 25a. circuit type 3% voltage drop as well.

                  https://www.bluesea.com/resources/1437

                  If you click on part 2 it will help you determine fuse size. And YES you need a fuse between the battery and charge controller.
                  Last edited by Suprasoup; 02-03-2018, 11:05 AM. Reason: Clarifying a couple of points

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