Voltage drop

brulaz
brulaz Solar Expert Posts: 31 ✭✭
in Solar Beginners Corner #1
When calculating the Voltage drop over the wire from your solar panels, what voltage and current should you use? I'm looking to hitch up 2 Kyocera 135W panels in series. But the specs for the Kyocera 135's give several possibile volt/amp combo's, and the graphs are all over the place.

Choose max sunlight, typical sunlight, or ... some other criterion?

Thanks ...
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: Voltage drop

    for 2 135w kcs you can consider those as 2 12v pvs in series for 24v. i know the vmp is higher on them, but it takes some extra voltage to charge the batteries. now they have an imp of 7.63a so that is what is being sent down the line at max. if using the v drop calculator here on the forum use the 90° c for the wire temperature as a maximum.
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  • brulaz
    brulaz Solar Expert Posts: 31 ✭✭
    #3
    Re: Voltage drop

    Thanks Neil,

    With those #'s it looks like I'll want 8 ga for a 50' run (100' total).
    But for 20' run I could get by (under 3% loss) with a 12 ga extension cord.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: Voltage drop

    that is correct. i have to point out that if you plan on expanding in the future it is best to design the wiring for whatever you will have later.
    an interesting tid bit for you is that going 2 gauges better in both cases will save around a full percentage point on the voltage drop. when you minimize voltage drop you are recouping wasted power.
    example for your 20ft run.
    the v drop is .618v using #12
    the v drop is .389v using #10
    the difference is .229v at a current of 7.63a. multiplying the 2 is 1.747w recovered for every hour this pv will output over its lifespan. if 3hrs average/day for 4 days per week, this is 12hrs/week. over 20yrs is 12480hrs at 1.747w and is 21802.56wh or over 21.8kwh just by going from #12 to #10. returns slowly diminish if you were to continue with larger wires, but increases are there non the less. would it be cost effective is another question as copper wire costs are high, but so is the cost of pv watts. that small jump from #12 to #10 imho is worth it.
    play with the calculator and do the math as you may be surprised even with your 50ft run.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
    Re: Voltage drop

    Interesting calc Niel. I probably up size my wire for the sake of ~22 kwh over 20 years. That said however, nearly every Pv system I have ever seen has grown with time, so building in "head room" makes great sense at a small up front cost.

    Tony
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