AC vs. DC wiring

adam1984
adam1984 Solar Expert Posts: 72 ✭✭✭✭
in General Solar Power Topics #1
On a 4KW system, grid tie with battery backup, i have to mount the panels about 100ft. from the charge controller/inverter/batteries. my question is:
Is it best to run DC or AC wire for the 100ft?
Should I have the charge controller, inverters, and batteries next to the panels and run the long distance on the AC side, from the inverter to the home. Or should i keep the charge controller, batteries, and inverter close and run the 100ft from the panels to the charge controller. I have heard both answers (run ac wire 100ft. to home and run dc wire from panels 100ft to charge controller).
Thanks,
Adam
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: AC vs. DC wiring

    Personally, I normally would prefer to setup a GT/Hybrid system that has the shortest AC Out wiring to main panel/pole transformer...

    Basically, with a GT inverter, they monitor the voltage of the utility grid... ~212-264 VAC for a "240 volt" split phase system.

    It is possible that you have low or high line voltage (summer A/C, winter electric heat, etc.) that, when coupled with the GT inverter can cause the inverter to go outside the UL limits.

    For example, my utility power runs around 240-255 VAC. If I had a long wire run from the GT inverter to the pole transformer, I may see a 3% voltage rise from the current pumped out by my inverter:
    • 255 VAC * 1.03 = 263 VAC
    For many inverters, they can set the cutout voltage floor to ~260 VAC minimum so they meet the 264 VAC setting required by UL (taking into account metering errors in manufacturing line).

    You could also have the problem with low line of, for example, an AC system or Well Pump starting surge taking utility voltage below ~212 VAC knocking the inverter out for ~5 minutes (UL timeout).

    If you run the long distance with the DC Array Wiring--you do not have the issue of long utility wire drops/increases in voltage adding to debugging later (apparently, limit voltages may be tweaked by the vendors--we read here where Enphase did that with one system).

    This is the one issue that can differentiate between between a distributed micro inverter vs central inverter system (the ability to run DC panel power hundreds of feet instead of AC branch circuit).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    #3
    Re: AC vs. DC wiring

    I am using a GVFX3648 Outback inverter, and it appears my best bet is to run DC wire from the panels. I am able to get them 40 ft. away. So the next issue is do i run 4 strings the 40 ft. or should i run the output of the combiner 40ft. and place the combiner next to the panels? The issue is i dont want to run 4 sets of wires, but they will be only 11A per string. If i run out of the combiner box 40 ft, the wire will be holding 44A instead of the 11A. Any suggestions?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: AC vs. DC wiring

    now to answer your question, it depends on which will show to be higher in voltage as the higher voltage will allow less losses. most battery backed gt systems will not show a very high voltage from the pvs in comparison to an ac voltage, but there's always exceptions. that means forget if it's dc or ac as you want the higher voltage to overcome losses. of course heavier wire will also come into play to attain lower losses if you need a lower voltage or if none of the voltages available to you suffice enough to lower those losses.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: AC vs. DC wiring

    And the difference in cost of running 4X 100 feet runs that will handle 11 Amps vs. 1X 100 feet run that will handle 44 Amps could come in to play. You might be able to re-arrange the panels to push the Voltage up higher if feeding an MPPT controller, which would make up for the long run. Just watch out for the possibility of Voc exceeding the controller's input max. Or buy one of the new Midnight 250 units! :D
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  • oil pan 4
    oil pan 4 Solar Expert Posts: 767 ✭✭✭✭
    #6
    Re: AC vs. DC wiring

    Run AC wireing for the distance. For a 4kw system try to make it 220VAC if you can.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

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  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    #7
    Re: AC vs. DC wiring

    As of right now, it will be 4 strings of 4 panels, so the voltage is around 88Voc or 72Vmp. From there, i think what i am hearing is to run the DC wire out of the combiner, that way i only need to run one wire set instead of 4 the 40 foot. The voltage will be 72 so its high enough i believe. Thanks neil for the voltage drop calculator
    Thanks,
    Adam Loucks
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