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  • Quick question about panel wiring

    Currently I have 6 100 watt panels wired as two 3 panel series strings in parallel. Would it be better to connect them as three 2 panel parallel strings in series or does it really matter?

    Thanks,

    Justin B.

  • #2
    It all depends on the max DC input voltage rating of your CC and the Voc rating of those panels.

    A smaller MPPT CC may not have a DC voltage input rating that will accept 3 panels in series.

    What are the specs of your CC and panels?

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    • #3
      The way I figure it both methods should yield the same voltage. The panels have an Voc of 22.5V and CC is a 60 amp MPPT. Three panels in series is well within the operating limit of the controller which is currently 90 V supplying a 12V battery. This weekend I am changing to a 24V inverter and rewiring the batteries in series/parallel for 24V and will be able to feed up to 105V. I will also be adding my last two 100 watt panels to the array this weekend as well so plan on having 4 in series for 90 V.

      So, I am basically curious as to whether either method would have an advantage over the other (partial shade, etc) or not.

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      • #4
        Originally posted by Justin B. View Post
        The way I figure it both methods should yield the same voltage. The panels have an Voc of 22.5V and CC is a 60 amp MPPT. Three panels in series is well within the operating limit of the controller which is currently 90 V supplying a 12V battery. This weekend I am changing to a 24V inverter and rewiring the batteries in series/parallel for 24V and will be able to feed up to 105V. I will also be adding my last two 100 watt panels to the array this weekend as well so plan on having 4 in series for 90 V.

        So, I am basically curious as to whether either method would have an advantage over the other (partial shade, etc) or not.
        First off while 4 of those panels with a Voc = 22.5 v may equal 90volt you have to include a cold temp multiplier which can be as much as 20%. So you could see over 100V when the sun first comes up on a cold day. That may hurt your CC so do the math.

        As for shading. Any "string" of panels can be affected by shade on only one or even just partial shading. So it would seem the having more "strings" in parallel would help. The bad side is when you add more parallel strings you increase the amperage that runs through the wires to the CC as well as reduce the voltage. Both of those issues could require you to install larger wire between the panels and the CC, first to allow the higher current and second to reduce the voltage drop.

        If it was my choice I would make my "strings" as big as possible to reduce the number of parallel "rungs". So with 8 panel I would go 4S2P if my CC can handle the higher voltage from a string of 4 and make sure both the wire size and MPPT specs work with it.

        With an MPPT type CC it really doesn't matter how you wire the panels you should still get production from the total wattage.
        Last edited by SunEagle; 07-24-2018, 08:33 PM.

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        • #5
          Thanks. Is this "cold weather multiplier" in my panel specs somewhere?

          Would this be it and it so how do I interpret it?

          Temperature Coefficient of Voc -0.30%/

          that should read -0.30%/Degrees C (don't know why the cut and paste wouldn't take all of it...)
          Last edited by Justin B.; 07-24-2018, 10:22 PM.

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          • #6
            > With an MPPT type CC it really doesn't matter how you wire the panels you should still get production from the total wattage.

            Actually, it does, and the higher power you go, the more it matters.

            First, you need to wire panels so that Vmp is at least 1.5x battery charging voltage. Otherwise, the Buck circuit may not be able to function.

            Second. After about 2x battery voltage, you start to work the buck converter circuit harder and it generates a bit more heat. You have to balance your wire losses with low voltage against the controller losses at higher voltages.
            I had to use a 250V controller because I could not overfill my conduit with fatter wires, but my controller is really cooking and the fans scream most of the time.
            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

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            • #7
              Originally posted by Justin B. View Post
              Thanks. Is this "cold weather multiplier" in my panel specs somewhere?

              Would this be it and it so how do I interpret it?

              Temperature Coefficient of Voc -0.30%/

              that should read -0.30%/Degrees C (don't know why the cut and paste wouldn't take all of it...)
              Yes but using NEC Correction factor should be good enough. Add up your Voc and multiply by 1.25. So if you come up with 4 x 22.5 = 90 volts x 1.25 = 112.5 volts which is greater than 100 volts.

              MSEE, PE

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