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  • Location of roof mount combiner box and advice

    Greetings, I'm about to start on the installation of 9 pv panels. These will be in landscape orientation. After reviewing posts here I know it's not what most here would recommend but I already have the panels and have figured out the racking installation. I'm planing on configuring as 3 strings of 3 each wired in series. The panels are Canadian solar 72 cel 320w, 9 amp (max)

    Wiring: I was planing on using a combiner box as a junction box mounted on the roof connected to 1 1/4 in metal conduit going to the solar control area in the basement.
    I was planning on connecting the negative of the three strings together and returning using a #6 wire. I was planning on using 3) #8 wire drops to bring the output of the three strings back to the solar control area. Once in the solar control area the strings will be connected to 15 amp breakers in a big baby box, these combine into a single #6 wire going to a master DC breaker and then to the charge controller.

    Question: The roof if roughly a # 4 pitch is it typical to mount the combiner box under a corner of the panels -or- is it typically mounted just close to the pv array?

    Conduit: I'll be running two #6 wires ( one is the common negative for the three strings, the other is the ground going to the racking system) and three #8 wires for the three pv strings.

    Does all this sound reasonable?

    Thanks in advance
    Jan Siegrist

  • #2
    Before getting to your question, are you sure your charge controller can handle the voltage of three of those panels in a string? What is the maximum Voc you calculate, after adjusting for temperature?
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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    • #3
      Approx what distance will your wiring runs cover?


      Here is a simple junction box on the roof, mounted to the rail:

      j-box.JPG



      Here is an example of an Wiley ACE passthrough:

      j-box closed.JPG


      Sola-deck makes commercial rooftop combiners / passthroughs as well.
      CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

      Comment


      • #4
        sensij - thanks so much for the reply, You have answered some of my questions from the pic's given.

        Regarding my string setup: Each string will consist of three panels in series for an output voltage of approx 108 vdc total. My charge controller can handle 150 vdc. It's rated to charge up to 60 amps at 48v, which is my battery bank voltage.
        The three strings will combine (in parallel) to give the charge controller 108 vdc at 27 amps max.
        For the panels I have (Canadian Solar) I've been told to expect less than the rated maximums - so I'm figuring a realistic 7 amps per string for a total of 21 amps to the charge controller.
        The wire runs from the roof to the charge controller will be approx 40 ft. According to the charts I've seen I should have a minimal loss using # 8 wire for the 3 pv strings going back to the solar control area in the basement.

        Is my choice of #6 wire for the common negative going back to the charge controller Ok? - I'm figuring the amps to be between 21 ~ 30. #6 wire is good for 50~60.

        Grounding: I was going to go with a #6 wire there as well but do you think an #8 is adequate?

        An alternative would be to mount the breakers for the three strings in the combiner box and bring back a single 'combined' wire to the charge controller. Frankly I was not fond of having to possibly service breakers on the roof if needed hence my decision to bring back the three string circuits separately. Also, if my panels prove to provide more power than expected, I could easily add another charge controller in my solar control area in the basement if needed since I have the three separate string circuits.

        Comments are most welcome.


        Thanks in advance
        Jan Siegrist





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        • #5
          Originally posted by JanS48 View Post
          sensij - thanks so much for the reply, You have answered some of my questions from the pic's given.

          Regarding my string setup: Each string will consist of three panels in series for an output voltage of approx 108 vdc total. My charge controller can handle 150 vdc. It's rated to charge up to 60 amps at 48v, which is my battery bank voltage.
          The three strings will combine (in parallel) to give the charge controller 108 vdc at 27 amps max.
          For the panels I have (Canadian Solar) I've been told to expect less than the rated maximums - so I'm figuring a realistic 7 amps per string for a total of 21 amps to the charge controller.

          Most charge controllers with 150 V limit will require that the Voc be within that limit at all times. Voc for a 320 W Canadian Solar panel (not sure if this is the model you are using) is 45.3 V. Three in series is 45.3 * 3 = 135.9 V. So far, so good, except we need to account for the fact that Voc rises as it gets colder. The datasheet value is based on 25 deg C, and give -0.31% / deg C as the temperature coefficient. .31% * 45.3 = .1404 V / deg C * 3 panels in series = 0.4213 V / deg C.

          (150 - 135.9) / 0.4213 = 33.5 deg. By this math (which is used by NEC), if your ambient temperature can drop below (25 - 33.5) deg = -8.5 deg C (=16.7 deg F), you would exceed the voltage rating of the controller. In reality, there is a small irradiance effect and overnight radiant cooling to consider, so if you are close to the limit, you might find yourself with over-voltage faults in the morning.

          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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