New solar install wire size?

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  • Island mon
    Junior Member
    • Mar 2020
    • 11
    #1

    New solar install wire size?

    I have 18 255 watt panels Im going to connect 3 panels in series, 3 strings x2.
    they are 30.4 volt, 37.4Voc 8.39amps
    so, 30.4v@8.39ax3=91.2v@25.2a x 2=91.2@ 50.4 amps.
    what wire size do I need between the panels and the controler?
    I also have 16 T105 6v golf cart batteries set up for 48vdc.
    what interconnecting cable size do I need?
    thanks, stay safe!
    Tags: None
    • Mike90250
      Moderator
      • May 2009
      • 16020
      #2
      > 3 panels in series, 3 strings x2.

      Not sure if you have 6 parallel strings, or what ??

      Each series string will only need to handle 8.39A, and their 12ga cable should be fine to wire all the way to your combiner box.

      If you array is wired 3S6P, then the output of the combiner would be the sum of 6, 8.39A strings = 50.34A

      But, it's likely you need 2 controllers to manage all that power, so it would possibly be better to only combine 3 strings per controller, at about 25A per controller, which, if the distance was short, #8 wire would handle it, or go to #6 if you have a long run from the combiner to the controllers

      clear as mud ??

      Also, your Voc of 3 panels in series, is 112V at 70F, depending on how cold your area gets in winter, you could be close to the fry voltage of your controller
      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

        Comment

        • Island mon
          Junior Member
          • Mar 2020
          • 11
          #3
          I'm a electrician, but not familiar with solar.not sure what the s & p means.
          But, I believe you understood me and are correct!
          91.24v, @ 50.34 amps total

          I'm located in the carribean, so it never gets colder than 75 degrees . If anything in the heat of the day, there will likelybe a voltage drop.

          I have 2 60a mppt controlers.
          I'm going with #6.

          What about the wire interconnecting the batteries?
          16 T105 6 volt golf carts?
          thanks for your answer!

            Comment

            • Island mon
              Junior Member
              • Mar 2020
              • 11
              #4
              Oh, I'm putting the controlers in parallel!

                Comment

                • SunEagle
                  Super Moderator
                  • Oct 2012
                  • 15126
                  #5
                  Originally posted by Island mon
                  I'm a electrician, but not familiar with solar.not sure what the s & p means.
                  But, I believe you understood me and are correct!
                  91.24v, @ 50.34 amps total

                  I'm located in the carribean, so it never gets colder than 75 degrees . If anything in the heat of the day, there will likelybe a voltage drop.

                  I have 2 60a mppt controlers.
                  I'm going with #6.

                  What about the wire interconnecting the batteries?
                  16 T105 6 volt golf carts?
                  thanks for your answer!
                  Actually you will need to add up the Voc of 3 panels so your total will be 112.2v not 91.24v.

                  How are you wiring those 16 T105 batteries? The wire between the CC and them needs to handle the total amps that can come from the CC or in your case 2 CC's.
                  Last edited by SunEagle; 04-02-2020, 11:43 AM.

                    Comment

                    • Mike90250
                      Moderator
                      • May 2009
                      • 16020
                      #6
                      To clarify, each controller must only be connected to it's own solar array.

                      The battery connections should be independent all the way to the batteries, where then, the controllers would be parallel.

                      So you have 2 independent arrays, 3S3P & 3S3P ( 3 series, 3 parallel )


                      Interconnecting the batteries, you should use the same size cable you use to wire to the Inverter. All pieces of cable length count toward your length to inverter. So you may have 7 one foot interconnects and 2 two foot inverter cables, a total length of 11 feet inverter cable. All that resistance counts toward you voltage drop when a large load surges on the AC output of the inverter
                      (say your fridge starts up and needs 2300w starting for 1 second, with a 48V bank that will pull about 50 amps from the batteries, through the 11 feet of cable.)
                      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

                        Comment

                        • Ampster
                          Solar Fanatic
                          • Jun 2017
                          • 3650
                          #7
                          Originally posted by Island mon
                          I'm a electrician, but not familiar with solar.not sure what the s & p means.
                          .....
                          Ohms law applies to electricity whether it is DC (solar) or AC
                          S = series (Voltage adds)
                          P = parallel (current adds)
                          Last edited by Ampster; 04-02-2020, 02:19 PM.
                          9 kW solar, 42kWh LFP storage. EV owner since 2012

                            Comment

                            • Island mon
                              Junior Member
                              • Mar 2020
                              • 11
                              #8
                              Thanks guys!

                                Comment

                                • RShackleford
                                  Solar Fanatic
                                  • Sep 2019
                                  • 311
                                  #9
                                  By controller you mean inverter ?

                                  I think most string inverters want to work at higher than 100v or so; you sure strings of only 3 is the best configuration ?

                                    Comment

                                    • SunEagle
                                      Super Moderator
                                      • Oct 2012
                                      • 15126
                                      #10
                                      Originally posted by RShackleford
                                      By controller you mean inverter ?

                                      I think most string inverters want to work at higher than 100v or so; you sure strings of only 3 is the best configuration ?
                                      I believe the OP is talking about the Charge Controller for an off grid system. So 100V is pretty much typical for most battery CC's.

                                        Comment

                                        • Island mon
                                          Junior Member
                                          • Mar 2020
                                          • 11
                                          #11
                                          Mav Voc is,150v.
                                          I have 20 panels max each controler is 2.8kw.

                                            Comment

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