MC4 Cable Size Help

Elmer TElmer T Registered Users Posts: 14
I have installed 3 Kyocera 135 watt solar panels (with the MC4 Cables) on my sailboat. I plan to run the MC4 Cables to a Midnite Solar MNPV3 3-Circuit Solar Array Combiner, which has 3 seperate 10-Amp circuit breakers. Then on to a Blue Sky Solar Boost 3024iL controller.

My Question is; Can I combine the 3 (-) negative MC4 Cables at the panels using 2 of the Multibranch MC4 connectors, thereby only having to run one (1) negative MC4 Cable to the combiner box? The MC4 Cable is #10 AWG stranded. Will this one cable, #10AWG be large enough? I still plan on running all 3 (+) positive cables to the combiner box.

Thank You for your help.

Jeff

Comments

  • XRingerXRinger Solar Expert Posts: 529 ✭✭✭
    Re: MC4 Cable Size Help

    People will be able to give you a better answer if you post the lengths of the cable runs.

    I've been using number 10 wire on my various hook-ups (4 200w panels)
    and the only problem I've had is with the connectors.

    MC4.jpg

    I'm soldering in the wires, and my cables are coming out fine.
    But the last batch of connectors had some pins that were drilled out
    a little smaller than normal. Number ten wire was too big and would not fit anymore..
  • Elmer TElmer T Registered Users Posts: 14
    Re: MC4 Cable Size Help

    Sorry about that, the length of the MC4 cable will be somewhere between 20 and 25 feet. But if there is a minimun lenght where this would work, I could possibly move the combiner box closer to the panels.

    Thanks,
    Jeff
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MC4 Cable Size Help

    Jeff;

    You can check the Voltage drop with the calculator here: www.solar-guppy.com/forum/download/voltage_drop_calculator.zip

    If there's not too much drop (greater than 3%) it should be okay, as the panels only put out about 8 Amps max each - so that's 24 Amps when paralleled. On average your panels will be running under 24; closer to 21. In general, 10 AWG is good for 30 Amps - but there are circumstances which could adversely affect it, such as bundling or placing in conduit - situations where heat can't be dissipated easily, mostly. The AWG table here http://www.powerstream.com/Wire_Size.htm shows 15 Amps conservatively for transmission wiring.

    Someone else might have a different opinion.
  • LuminastLuminast Registered Users Posts: 6
    Re: MC4 Cable Size Help

    The other replies didn't touch specifically on your point about combining the three negative terminals into a single MC4 cable: To the purist, this isn't the best practice, but it can be done, and for your short runs the difference between that and the preferred practice might be acceptable. The preferred practice is to keep the combined wire size, and resistance, and load, balanced on both sides of the circuit.

    When you connect the three negative terminals to a single wire, and connect the three positive terminals to three separate wires of the same gauge the total circuit resistance, and the total voltage drop, across your wire is 1/3 higher than it would be if you connected the circuit through a balanced two wires on the positive side, and two wires on the negative side.

    If your panels are each putting out the full 8 amps, and if your wire runs are 25 feet, the voltage drop across the combined wire runs (with a single 10 ga wire carrying the current to the negative terminals, and three parallel 10 ga wires carrying the positive terminals) will be about 0.97 volts.

    Using the same assumptions, but with each side connected to a pair of 10 ga in parallel, the total voltage drop across the wiring will be about .73 volts.

    The practicality of connecting three panels to two wires, with balanced load between wires, is cumbersome if you want to use MC4 cable combiners. Personally, I would just buy one more 50' MC4 cable, for a total of three, with a dedicated MC4 cable to each of the six terminals and a total voltage drop of .48 volts. This would cost a little more money, but you would also save some by not buying the pair of MC4 combiners, and it isn't many more dollars compared to the cost of the rest of the system.

    But then I am probably overly fussy, and if your budget is tight, the scheme you describe will work if the 0.97 volt drop doesn’t exceed your allowance for voltage drop.
  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: MC4 Cable Size Help

    Having balanced wire gauges/resistance/etc. is very important for current sharing on Voltage Sources (lead acid batteries, paralleling inverters/AC generators/etc. --that support paralleled outputs).

    For solar panels that are current sources--balanced resistance on wiring is much less important.

    With batteries, you can have a few tenths of a volt of difference and have double digit percentage difference in current sharing.

    With solar panels, a 10% difference in Vmp+wire-drop (2 to 7+ volt differences, depending on Vmp)--may only give you a 10% difference in current flow.

    Obviously, we still recommend keeping wiring drop/voltage drop low to avoid wasted energy (I^2 * R = Watts of "heating effect/losses).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LuminastLuminast Registered Users Posts: 6
    Re: MC4 Cable Size Help
    BB. wrote: »
    Obviously, we still recommend keeping wiring drop/voltage drop low to avoid wasted energy (I^2 * R = Watts of "heating effect/losses).

    -Bill

    Yes, as I tried to explain, it is only for the purpose of minimizing the system votage drop that balanced wiring is any "better".
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: MC4 Cable Size Help
    Elmer T wrote: »
    I have installed 3 Kyocera 135 watt solar panels (with the MC4 Cables) on my sailboat. I plan to run the MC4 Cables to a Midnite Solar MNPV3 3-Circuit Solar Array Combiner, which has 3 seperate 10-Amp circuit breakers. Then on to a Blue Sky Solar Boost 3024iL controller.

    My Question is; Can I combine the 3 (-) negative MC4 Cables at the panels using 2 of the Multibranch MC4 connectors, thereby only having to run one (1) negative MC4 Cable to the combiner box? The MC4 Cable is #10 AWG stranded. Will this one cable, #10AWG be large enough? I still plan on running all 3 (+) positive cables to the combiner box.

    Thank You for your help.

    Jeff
    Isc for that module is 8.37A. I don't know the conditions, but assuming 105-133 degrees F ambient and no conduit:
    For calculating ampacity, John Wiles says to calculate using upper bound current for excess insolation and continuous current and then calculate using upper bound current for excess insolation and conditions of use and use the higher of the two.

    1: (8.37)(3)(1.25)(1.25) = 39.23A

    2: (8.37)(3)(1.25)(1/.87) = 36.08A

    The ampacity of 10AWG 90 degree C wire is 40A. So, yes, assuming the voltage drop is acceptable.
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