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  • Wire Size Voltage Calculator

    What Voltage would you use for a wire size calculator?

    There is the fact that it is a 12 Volt panel, Its open circuit voltage it 21.2, and the optimum operating voltage is 17.7 VDC. My gut tells me I need to use the more conservative leading to the thicker wire size, so 12 Volts. I'm trying to calculate what size wire I need for a 600 Watt system, and there's bit of difference in AWG for a 10' run based off voltage.

  • #2
    Originally posted by chrisski View Post
    What Voltage would you use for a wire size calculator?

    There is the fact that it is a 12 Volt panel, Its open circuit voltage it 21.2, and the optimum operating voltage is 17.7 VDC. My gut tells me I need to use the more conservative leading to the thicker wire size, so 12 Volts. I'm trying to calculate what size wire I need for a 600 Watt system, and there's bit of difference in AWG for a 10' run based off voltage.
    I have used both the 12V and Panel Vmp voltage in my calculators. To be more conservative I go with the 12V but it really comes down to making sure you do not exceed a 2% VD and that the final voltage is more than the minimum for your CC input rating.

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    • #3
      That actually gave me an idea. I think I'll wire these six panels to be 24 Volts instead of 12 Volts. That way the same watts would be pushed down to the charge controller, but because its at a higher voltage, I would not need as thick wires. The charge controller would still step the voltage down to 12 Volt battery voltage. The charge controller I plan on getting has a maximum rating of 100v, 50a MPPT charge controller, so I think that would work.

      It seems to easy a solution to work, and I will run the numbers on that when I come home from work.

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      • #4
        Voltage has nothing to do with the wire size (resistance) calculation.

        Resistance and Amps (current) determine the loss (voltage drop) This is a major reason for MPPT controllers, not because they can harvest 20% more in cold weather, but you can run a 100V string of panels at 8 amps and use skinny wire, instead of 20V panels at 40A (fat wires)

        Resistance is a known value. (say 0.001 ohms per 10 ft) Current is a projected (known) value 18A @ solar noon
        Measure the length of wire, throw in resistance and amps and you have the voltage drop at any given current. There are calculators on the web for wire loss at specific amps


        EIR_elect_triangle.png
        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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        • #5
          I am surprised at the difference in running the panels in series to increase the voltage makes in the wire size. The MPPT controller I am looking at is 100 volts, so I can run my panels at 24 volts and it decreases the wire size I need by quite a bit. Seems like I could even run the panels at 36 volts.

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          • #6
            And that's the magic of the MPPT controllers. Use thinner wires at higher PV voltages, use low cost/watt GT panels and expensive brains in the 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


            • #7
              As I'm proofreading my calculations, I'm wondering if:

              1) For running 15 feet of wires for panels 15', I need to calculate for 30' since I'm not using a frame ground. This 30 feet total would be 15' for the positive wire, and 15 feet for the negative wire.
              2) Since my goal is no more than 2% loss prior to the batteries, I need to get aim for 1% between the panels and the combiner, and 1% from the combiner to the MPPT power supply.

              Basically when I did these calculations at 36 volts, I can use 10 gauge wires on the roof for panels to the combiner and, and 6 gauge wires from the combiner to the MPPT charger. The MPPT charge controller is 100 volts and 50 amps, so I do not exceed the amps or volts of the charge controller with 600 watts of panels. That would leave me with three 12 volt panels in series, connecting two sets of these in parallel at the combiner before being fed to the converter.

              When I first did the calculations, I did 15' for 15' of wire, which meant I could set the 12 volt panels up for 24 volts as pairs, but adding both the positive and negative run for bumped them up to 36 volt panels. The limitations I seem to be working with are my panels have the MC4 connections, and I can't seem to be able to make my roof cables thicker than 10 AWG. I can't find crimp on MC-4 connectors that will take 8 AWG or thicker. There's a few 8 AWG for sale, but I don't want to plug in the pre-made MC4 connector with a 4 foot run of 8 AWG wire, and then have to splice my AWG wires in to complete the circuit. As for the 6 AWG wire, that is the thickest wire I can insert into my charge controller.

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              • #8
                Going to higher voltages, for batteries and PV, helps reduce the wire gauges needed for the same power. You have not given quite enough information about your configuration, if it's a 12, 24, or 36V system .Generally MPPT systems need about 1.5x max battery voltage (your EQ volts) to reliably start up. (check on this, each mfg is a bit different). PWM only needs about 4V over, but you have heave gauge wire all over.
                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


                • #9
                  Make it easy, use a chart.

                  DC_wire_selection_chartlg.jpg

                  MSEE, PE

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                  • #10
                    I see what part of my problem was. I used an online auto DC wire size calculator, and a footnote in that was along the lines of "this calculator is built for chasis grounding. If you don't chasis ground, multiply the wire size needed by two." Other solar calculators I found, such as Renogy's solar gauge calculator, is built assuming not using chassis grounding.

                    When I calculate loss, I'm going to do it in three stages. The first will be from the solar panels to the combiner, from the combiner to the Charge Controller, and then to the battery. I'm going to shoot for 1% loss at each of those places, which means I should have no more than 3% along that path from the panels to the battery.

                    Comment


                    • #11
                      Originally posted by chrisski View Post
                      I see what part of my problem was. I used an online auto DC wire size calculator, and a footnote in that was along the lines of "this calculator is built for chasis grounding. If you don't chasis ground, multiply the wire size needed by two." Other solar calculators I found, such as Renogy's solar gauge calculator, is built assuming not using chassis grounding.

                      When I calculate loss, I'm going to do it in three stages. The first will be from the solar panels to the combiner, from the combiner to the Charge Controller, and then to the battery. I'm going to shoot for 1% loss at each of those places, which means I should have no more than 3% along that path from the panels to the battery.
                      Since the distances for each run of wire may be very different, it does not necessarily make sense to size the wiring for a 1% maximum voltage drop on each run independently. For example, on the run from CC to battery may be very short and wiring which can safely carry the current may not produce a voltage drop of 1%. On the other hand, for proper operation of a CC without remote voltage sensing it is more important to keep voltage drop on this segment low. You really need to look at the most economical way of getting an overall 3% VD instead of deciding in advance to make the VD equal over all three segments.
                      SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                      • #12
                        I appreciate that.

                        I really think I'm going to need to do the math myself on that versus using an online calculator. The METRIC formula seems fairly straightforward:

                        ______Length x Current x 0.017
                        Volts=-----------------------------
                        __________Area

                        With the Area of a 10 AWG wire ad 6 mm2. And length in meters. Also, the formula is for 25° C and adds: voltage drop increases with wire temperature, at approx 0.4% per °C.. That voltage drop based on temperature seems fairly extreme.

                        Is there a different formula? I see some calculators use the rating changes for different insulation type.

                        The specifics of one run of wire is:

                        7 Strand Copper PV wire
                        90°C Insulation
                        ASTM B8
                        Listed as type USE-2 per UL 854
                        Listed as type RHH/RHW-2 per UL 44
                        Listed as type PV per UL 4703

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