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  • Wire Size after DC combiner box

    I am installing an off grid solar array with a battery bank. my house is in the woods, so I have to put the panels 200 feet from the house (and consequently from the charge controller and batteries) I have 6 300 watt renesola panels. They will be wired 3 in series and 2 in parallel. Planning to use Midnight Mnpv3 DC combiner box near the panels so I can use a slightly smaller wire to the house. How do I calculate how big of wire I need coming from the combiner box to the Charge Controller roughly 200 ft away? I calculated this online some time ago and got AWG-6, but forget how I got there. any help is appreciated!

    24 V battery bank, planning to use Midnight solar 150 lite for charge controller

  • #2
    You've to 1st know how the voltage of the wire. & then you take into account the voltage drop you are willing to accept. Since the typical voltage of the 300w solar panel is 24v, then the current is 25A. Probably. The voltage though, it operates higher than that. But for argument sake, let's take the 24v * 3 since the panes are in series & get 72v. The parallel string will bring the current to 50A. voltage drop = 2 * length of cable * current * resistance / cable size in mm2. Therefore, in our case - if you want a maximum of 3% volt drop - then it will. 03 * 72 = 2.16v. Then in order to find the cable, you just: 2 * 200/3 (length in metres) * 50A * cable resistance ie 0.017 (for copper I think) / 2.16 = 55.6mm2. You can convert that to AWG. Our metric system here is different. But that's some thick wire! The voltage of the panels could be higher. You know...

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    • #3
      I'll assume panels are 35V @ 9 amps. roughly.

      Nope, you got that logic wrong. 3 panels in series (If the Voc, on a cold morning, is BELOW the 150V limit of the controller) and then parallel both strings of 3.
      That gives you maybe 100V
      Parallel,amps adds, so your 2 parallel strings give you about 18amps @ 100Vdc (3 in series) [ 9A +9A & 35V *3 = 105V ]
      Run THAT (105V @ 18 A) the long distance, should only need 10 ga wire, maybe 8, depending on how much loss you want to tolerate.

      After that goes into the controller, you will get about 70A going into the batteries for charging. That 70A, you want fat wire, for a short distance, it's going to be expensive, much more than 18A rated wire.
      At 24V battery, using 105V Array voltage, the controller will be running pretty warm, and close to it's max amp limits.
      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


      • #4
        I used MC4 10 AWG cable. It is readily available in 50' lengths, with a male connector on one end and a female on the other end. So it is very easy to join them in any length you need.

        My array is using 18 photovoltaic panels. 6 breaker combiner box is mounted in the center of our array. I think I had to go 70' to get from their and into my house.

        The other option for me would have required building a heated structure at the center of the array to house the Charge-Controller, batteries and inverter.

        4400w, Midnite Classic 150 charge-controller.

        Comment


        • #5
          Thanks for the prompt responses. You are correct, Mike. I plugged the data of my panels into midnight solar's array sizing tool and got total VmP=109.8V. Array Current=16.4 amps. Though, with 3% loss the calculators are telling me 6 ga. wire or 8 ga. for 5% loss @ 200'. It sounds like I would be wiser to bump up to the midnight 200 lite, I am in southern Ohio, and on a rare occasion it would get cold enough to send VOC over 150v. Thanks for the help!

          Comment


          • #6
            No, don't bump the voltage up to the midnight 200. The higher the array voltage is above the battery voltage, the more loss you get in the controller (as heat)
            I'm running 160V into a 48V bank, and the poor controller (a midnight classic 200) is just cooking it's brains out. The losses for down-conversion are killing me.
            But I could not run fatter wire, so I was stuck.
            If anything, you should step your battery bank up to 48V, and that will help the downconversion losses in the controller. Figure out what 4-6% of 1800w is, That's what the controller will be dissipating as heat. Ideal is array voltage about 2x battery voltage, but when you have long runs, you have to have some trade off's
            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


            • #7
              I wasn't suggesting changing the voltage of the array, I was just thinking about going with the midnight 200 instead of the 150. At current array voltage=109x1.56 (extreme cold factor)=170V which would send the unit into Hyper VOC in cold weather.... any temps below 14 degrees would shut down the controller. So I figured swtiching to the Midnight 200 would leave more headroom before entering HyperVOC. Though the Midnight 200 can only handle 79 amps, which is much closer to the 62 amps Ill be putting into it, and maybe that factors in too. the midnight 150 can handle 96 amps. I don't think it would make economic sense for me to change to a 48v bank.

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              • #8
                I wouldn't suggest connecting a bunch of cables with connectors. Get yourself some decent burial rated wire (if you are burying it). That way the run is continuous, rated for being outside and you don't have to find the one bad connection out of 10 in the middle of the winter.

                I borrowed a spool from a local electrician then just paid for what I used when I returned it. You may not have that option if you don't live in an area where people trust each other. From what I see in your calculations, loss would be near 4% using 8 gauge so you seem to be running the calculator correctly, could be up to 8% depending on voltage. When you calculate based on open voltage, you are using the best case scenario, you should really check your panels to make sure the rating assumptions you are making are correct.
                Last edited by cabindave; 09-01-2016, 10:50 AM.

                Comment


                • #9
                  Originally posted by Mike90250 View Post
                  No, don't bump the voltage up to the midnight 200. The higher the array voltage is above the battery voltage, the more loss you get in the controller (as heat)
                  I'm running 160V into a 48V bank, and the poor controller (a midnight classic 200) is just cooking it's brains out. The losses for down-conversion are killing me.
                  But I could not run fatter wire, so I was stuck.
                  If anything, you should step your battery bank up to 48V, and that will help the downconversion losses in the controller. Figure out what 4-6% of 1800w is, That's what the controller will be dissipating as heat. Ideal is array voltage about 2x battery voltage, but when you have long runs, you have to have some trade off's
                  Mike, I don't see a technical reason why a higher input voltage MPPT charge controller would be less efficient. I am surprised that
                  a larger unit would operate with 6% loss. Grid tie inverters operate at much higher voltages and much lower losses. Perhaps
                  there just hasn't been the volume and competition in the battery area to justify the development effort? Bruce Roe

                  Comment


                  • #10
                    both MorningStar and Midnight state deep in the papers, and I can also attest to, that the further (higher) from battery voltage the array MPPT point is, the more loss in the controller. My 2Kw array @ 160v has my Classic running cooling fans and sitting at +50C. The TriStar with the 3Kw array @ 100v runs much much cooler.
                    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


                    • bcroe
                      bcroe commented
                      Editing a comment
                      I hear you; I seem to be accusing them of being behind the times. Bruce
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