What gauge wire?

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.

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

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?

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.

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.

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.

Wire size chart see 4th post in this thread.

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.

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&phase=dc&noofconductor=1&distance=35&distanceunit=feet&amperes=18&x=49&y=10

#10 if fine, or kill it with #8.