# What size wire?

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Registered Users Posts: 5
I have 6 - 255 watt panels each at 37.2 VOC to power 2 pairs in series together in parallel ,[ 4 ea - 6 volt 420 AH
Batteries ] to give me a 12 volt battery bank. Question 1; what to set up my solar panels? Question 2 ; what size wire between panel array and charge controler? [ about 80 ft apart.] Thanks Loren

• Banned Posts: 17,615 ✭✭✭
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Re: What size wire?

Well let's see if we can wriggle this into standard terms.

Six 255 Watt panels. Based on the size and Voc they are probably Vmp around 30 and Imp about 8.5.
Automatically we know this will require an MPPT type charge controller due to the 'non-standard' Vmp.

You have four 420 Amp hour 6 Volt batteries configured for 12 Volts, which means two parallel strings of two in series giving 840 Amp hours @ 12 Volts.

First suggestion: put the batteries all in series and go 24 Volts if you possibly can, because the current you are looking at is going to be very high (around 84 Amps).

Second suggestion: same as the first because your array is 1530 Watts. On 12 Volts that's 98 Amps which no single charge controller can handle.

If you must stay with 12 Volts you are going to need a MidNite Classic 150 to take advantage of as much of the array as possible: it will pass 96 Amps and just 'clip' whatever is over that.

Ideally you would use those panels all in parallel for a 12 Volt system as that would keep the controller efficiency at the maximum. But unfortunately that would mean having to handle 50 Amps from the array to the controller at nominal 24 Volts and that means 8 AWG minimum just to handle the current, and depending on the distance it may need to be much larger.

If you go with a 24 Volt system you would have three parallel strings of two panels in series and that would be 60-ish Volts and only 25 Amps. This reduces the minimum wire size to 10 AWG, but again the distance will factor in to reduce Voltage drop between array and controller. This would also work for a 12 Volt system, and over-all be more efficient despite the small reduction in controller efficiency.

The parallel panel connections should be individually fused/breakered for protection against shorted panels.

I really, really hope you can use a 24 Volt system here; it will make things so much easier for you.

• Registered Users Posts: 5
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Re: What size wire?

Cariboocoot, yes the 24 volt system sounds much better. thanks, may be running wire soon I hope. too many projects!!!
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: What size wire?

best pv arrangement would be 2 strings of 3 in series and use a good mppt cc like the midnite classic 150. the 2 strings allow for lower losses in the wiring to the cc while also foregoing the need to have fuses or combiners being you will only have 2 strings. any expansion on the number of strings will require fusing (most use dc circuit breakers) and combining.