Chusing wire size

Hi, I am new on the forum thing so forgive me if I mess up. Not sure of all the correct terms..

Subject: wiring up PV panels- wire size
I am trying to create my own PV system with PV panels in my price range and assembling other panels out of individual cell from E-Bay myself.
I have so far mounted on by one story roof : 2- 45W panels (1 more soon) and 1- 75W panel. All is for a 12V system. The distance for the wire is about 25 ft. I would like to have quite a few panels up there.

My concern is selecting the wire size and wiring method from the panels to the charger.
1. I heard some place that when selecting the wire size is to measure the actual distance and then DOUBLING this figure for the other wire. ( say- 25ft for POS and 25ft for NEG = 50ft) is this true???
2. If this is true, then from my DC wire sizing chart- 6A for 50ft (47.5’) = #6 wire. That big wire just for 6A DC –*+?????….!!!!!
3. According to “The Basic PV Voltage Drop Calculator” for #6 wire with 6.62A @ 12V the Array V-drop is 0.134V over 50ft. For #10 wire the Array V-drop = 0.33V ..
Question: Is #10 ga CU O.K. to use???? How is other homes with pro systems wired as far as wire size????

Thanks....

Comments

  • RoderickRoderick Solar Expert Posts: 253 ✭✭
    Re: Chusing wire size
    Silas683 wrote: »
    ...
    1. I heard some place that when selecting the wire size is to measure the actual distance and then DOUBLING this figure for the other wire. ( say- 25ft for POS and 25ft for NEG = 50ft) is this true???
    That sounds right; your total wire length is 50 ft.
    3. According to “The Basic PV Voltage Drop Calculator” for #6 wire with 6.62A @ 12V the Array V-drop is 0.134V over 50ft. For #10 wire the Array V-drop = 0.33V ..
    Question: Is #10 ga CU O.K. to use???? How is other homes with pro systems wired as far as wire size????
    It sounds like the #6 wire is a 1% voltage drop, and the #10 is a 3%, approximately. The power you lose to the wiring will be roughly proportional to these, so with the fat wire, you'd lose a little less than a watt, and the thin wire, a little more than 2 watts. For this small kind of system, when cost is critical, I'd say the fat wire is not worth it. Copper is expensive nowadays.

    Another thing to remember is that for a "12 volt" system, the voltage coming out of the panels should be more than 12 volts, maybe a lot more, depending on what controller you are using to charge your battery. So the loss of 0.33 volt might only be 2% or even 1%.
  • System2System2 Posts: 6,290 admin
    Re: Chusing wire size

    Thanks Roderick, This is the first time i have messed with DC to this extent, I have many yrs. with AC and electronice... with AC, a #12 wire is good for 20A @120V but is is strange that with just 12V you need #6 wire just for 6.6A...
    So then I guest that it would be good for me to run one run from each pannel and not to combine them on the roof. That would be a very large wire to supply all the pannels..

    Is there any thing that I mite need to do this wiring job??

    Thanks
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Chusing wire size

    AC and DC are identical on the wire sizes, NEC allows for upto 5% line losses and RE is expensive so losses are designed to be minimized.

    While 12 awg is correct for a typical sub-circuit, the electrican must do the line loss calculations to verfiy its under the 5% rule and thats were the lenght comes into play ... 300ft would be unusal for a typical home outlet

    For a 6 amp load 10 awg is a good choice, keep in mind, that 6 amps is peak ( well not considering edge of cloud or reflection effects ) and it will almost always be less
  • BB.BB. Super Moderators, Administrators Posts: 28,851 admin
    Re: Chusing wire size

    Also, the voltage drop itself is an issue...

    Run a 120 VAC line xx feet @ yy amps and get 110 VAC at the end under starting/load--things still work.

    Run a 12 VDC line xx feet @ yy amps and get 2 VDC at the end under starting/load--things will not work... (assuming, for sake of discussion, both wires have the same 10 volt drop at the same amperage)...

    With 120 VAC, you have +/- ~10 VAC of head room.

    With 12 VDC, you have +/- ~1 VDC of head room (over or under, and things may not work/get damaged)... and in some cases (like charging the batteries), your accuracy in voltage at the battery must be within +/- 0.05 VDC (I think Jim showed that being off by 0.1 VDC is an error in battery capacity of, very roughly, can be ~~10%)...

    Another reason, many times it turns out that it is better if you run 24 or 48 VDC battery systems--you can have more voltage drop to work with (as well as lower currents).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Chusing wire size

    elaborating just a bit further on this from bb:
    "Another reason, many times it turns out that it is better if you run 24 or 48 VDC battery systems--you can have more voltage drop to work with (as well as lower currents)."


    this is where the use of percentages come into play because the voltage drop itself being a fixed number will have a lesser impact on a higher voltage base. i do believe the nec would frown on the 10v drop because that exceeds their 5% rule(10/120=8.33%), but is good for example's sake in showing that not only is the impact larger for losses at smaller base voltages, but that as bb said, it may not work at all. that 10v drop at 12v represents 10/12=83.33% lost. nothing designed for 12v operation will work with 2v being outputted. at 24v with 10v lost=41.67%. at 48v=20.83%. see the pattern here!? if the wire is x gauge and has a resistance y ohms then going to a wire size that offers 1/2 of the resistance will have half of the voltage drop and thus half the losses. this will occur when you go 3 gauges smaller(can get confusing here because smaller gauge=larger wire). for example going from #10 to #7 will cut the resistance in half for the same number of feet of wire used.

    edit to add:

    i forgot to mention that the examples are good if the current flow is the same for all of the examples. cutting the current flow in half will also have the same effect on the voltage drop as cutting the wire resistance in half did. this usually isn't a factor or an option that can usually be changed easily in a manor to go with smaller wires(larger gauges) unless you produce less through that wire(pv,wind,etc.) or you would use less through that wire.
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