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  • Combiner box/DC disconnect

    Hi guys,

    I would be finalizing my PV design in the coming weeks. One thing that I have reconsidered from my original design was to use higher voltages on the PV side of thing.

    I am considering 24 solar panels, probably the Canadian Solar CS6K-280M (Vmp 31.5, 38.5 Voc, Imp 8.89) or the Heliene 72m-360 (39.71 Vmp, 48.19 Voc, 9.13 Imp).

    I would like to have 2 ground mounts of 12 panels each (IronRidge racking, 4 x 3 configuration) and connect all 12 panels per ground mount in series. That would be 378V @ 8.89A for the CS and 477V @ 9.13A for the Heliene. Each set of 12 panels would have a Schneider Conext MPPT 600-80 charge controller. The PV array operating voltage on the Conext MPPT 600-80 is 195V to 550V. The maximum PV array open circuit voltage is 600V on that thing. It should be more than ok with the CS (462 Voc) but will be close to its limit for the Heliene (578 Voc).

    The two ground mounts will be located 200 to 300 feet from the garage (where the inverter/CCs will be). I am considering using 10 AWG wires (at 2% acceptance loss), and a buried PVC pipe for each array to the garage.

    So, here are my questions:

    1. do I need a DC combiner box? I would think not because everything is connected in series
    2. each set of 12 panels will have a DC disconnect switch in the garage. With a 25% safety margin, I guess I should be looking at a 600V/15A DC disconnect switch - something like that: https://www.amazon.ca/Square-HU361RB.../dp/B00ZJIOA40 -- is that correct?

    Thanks!

  • #2
    Partially answering to myself.. the Heliene 72m-360 would be a no-go because of the temperature effects that can increase voltage by 8V at -30C and bring the Voc up to 685V, too much for that thing.

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    • #3
      Originally posted by extrafu View Post
      Partially answering to myself.. the Heliene 72m-360 would be a no-go because of the temperature effects that can increase voltage by 8V at -30C and bring the Voc up to 685V, too much for that thing.
      Right, you need to consider temperature when calculating Voc. Here is a link with appropriate minimum design temperatures for locations in the US. Since each string is going into its own charge controller, a DC combiner on the PV circuits is not needed (there is nothing being combined). You might still want a junction box to transition from PV Wire to THWN-2 for the long PVC run.

      The current rating used for the DC disconnect is 1.25 * the Isc of the string. The one you linked would be OK.
      CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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      • #4
        Originally posted by sensij View Post

        Right, you need to consider temperature when calculating Voc. Here is a link with appropriate minimum design temperatures for locations in the US. Since each string is going into its own charge controller, a DC combiner on the PV circuits is not needed (there is nothing being combined). You might still want a junction box to transition from PV Wire to THWN-2 for the long PVC run.

        The current rating used for the DC disconnect is 1.25 * the Isc of the string. The one you linked would be OK.
        As an example of part of what separates the science from the art of engineering, I'd suggest considering/checking and perhaps lowering the min. design temp. from what referenced sites suggest. Reason: Often, the min. ambient temps occur in winter around, or perhaps sometimes slightly after sunrise. In some locations, especially those with low humidity and or high(er) elevations, the ground, and by extension things that see a significant portion of the sky dome and its lower effective radiant sky temp. can be a temp. that may actually be lower than the ambient temp., sometimes by a fair amount, ranging up to 5 deg. C. or more (lower), depending on wind, humidity, atmospheric conditions and array orientation, among other things. That may or may not be a consideration, but it may be nice to know.

        Take it for what it's worth.

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        • #5
          This page http://www.schneider-electric.us/en/...nt&id=FA229310 shows exactly how to do the right math.

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          • #6
            Originally posted by extrafu View Post
            This page http://www.schneider-electric.us/en/...nt&id=FA229310 shows exactly how to do the right math.
            Except for not accounting for panel temps. that may possibly be lower than the design lowest ambient temps. due to unconsidered conditions.

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