What gauge wire?

[Keith_Haw]

I'm going to be adding a second solar panel and need to know what gauge wire I need for 2/140w panels on a 35' run.

Thanks
Keith

[pianotuna]

Hi Salvo,

You know that pwm doesn't control voltage so the top 2.6 volts on a 17 volt panel don't matter a hoot, assuming a 14.4 charging voltage. #10 wire is good for 30 amps. 280 watts/12.8 volts =~22 amps max.

If it were an MPPT controller then wire size matters big time. My personal choice was and would be MPPT with a series connection for the panels.

[Salvo]

What's your rational for that conclusion? Where's the analysis? My hunch, it's not pretty.

Hell, let's do it right now.

10 awg wire resistance = 1.0 mohm/ft
We got 70 ft of wire.

Total wire resistance = 70 ft * 1.0 mohm/ft = 70 mohm

2 x 140W panels outputs about 2 * 8.7A = 17.4A.

Wire loss:
V = I * R = 17.4A * 70 mohm = 1.2V
P = I^2 * R = 17.4A^2 * 70 mohm = 21.2W

The cable causes a 1.2V drop and consumes 21.2W, or about 7.6% of 280W.

pianotuna wrote:
Hi,

#10 will be just fine with a pwm controller.

[westend]

Keith Haw wrote:
I've thought about welding cable and wou?ld like to use that but the controller is setup for 8ga max best I remember. So how do I get around that?

I bought gold plated connectors like these. They are cheap and effective.

[pianotuna]

Hi,

#10 will be just fine with a pwm controller. I would add a temperature sensor.

[Keith_Haw]

That's not a bad option either but don't know what the controller would cost, have to check that.

What's the difference in the way they charge? Or the charge rate?

[Salvo]

An alternative is to leave the 35 ft, 10 awg cable in place and go serial. You now need a mppt charge controller. You may find a controller cheaper than new cable. If you go serial then the two panels have to match. 140W each.

[Keith_Haw]

Thanks, not a bad idea. Will be back home in a couple weeks so can check my local welding supply house for cable. Thanks again everyone for the replies.

[mena661]

Clip some strands off the end of the cable or you can use reducers.

[Keith_Haw]

I've thought about welding cable and wou?ld like to use that but the controller is setup for 8ga max best I remember. So how do I get around that?

[CJW8]

Why put in high end Kyocera panels and cheep out on wire? 40 feet is a long way from the battery with a lot of voltage loss that you will never get back. Such a waste. If this is just going to be a battery maintainer, your little bitty #10 wire will be fine but if you are going to do some serious off grid camping, then go with at least # 4 cable as suggested. I have helped 3 others replace their factory installed #10 wire because their solar system didn't work as expected. In all cases there was big improvement.

[westend]

I wired all of my solar with #4 welding cable (about $1/ft.). I terminated everything with good lugs I bought at an electronics surplus outfit except for the connection terminals to the controller. For that, I sourced spade terminals from an online car audio supplier.
The cost of the wire is trivial compared to the installation labor and the anticipated loss of power with smaller wire throughout lifetime of the system. Using small wire for solar is like buying cheap paint, it's a false economy.

[CJW8]

I wouldn't use less than #4 if the panels are in parallel. Welding cable is very flexible, UV resistant and no more expensive than big box store wire.

[Gonzo42]

Wire size chart see 4th post in this thread.

[Keith_Haw]

Sorry, guess I should have gave a little more information.

I have a Blue Sky 30 PWM 30a controller and a single 140w Kyocerna panel and will be adding another Kyocerna 140w or 160w, haven't decided yet, panel.
The total run, panel to controller, is about 35'10ga then about 5'of 10ga from controller to battery. I will be updating the controller to battery to 8ga with the new panel but don't know if I should upgrade the 35' run to 8ga or run a second 10/2 wire along with the one that is already there.

[BFL13]

Besides the actual voltage drop comparison between the low amps at "24v" and the higher amps at "12v" is that the voltage drop with PWM doesn't matter very much for getting your "expected amps." Those amps are based on getting the Isc of the panel to the battery

It matters a lot for 24v MPPT which has the amps output very much affected by the voltage input to the controller.

They are affected the same on the part from controller to battery though, which are both at "12v"

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&pha...