What gauge wire?

The "run" is the one-way distance? Panels in series 9a, 44V; in parallel 18a, 22v

This calculator uses the one-way distance so it is easy to use

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

Assuming you're connecting the panels in parallel, the distance is 35' one way and PWM controller then 10ga would give you a 1.5V drop plus the additional drop to the battery.

You might want to consider a combiner box and then 8 ga to the controller. You should get more information if you post all of the details - controller type, panel specs and all involved wiring lengths and existing wire size.

Assuming you're connecting the panels in parallel, the distance is 35' one way and PWM controller then 10ga would give you a 1.5V drop plus the additional drop to the battery.

You might want to consider a combiner box and then 8 ga to the controller. You should get more information if you post all of the details like controller type, all existing wiring lengths and gauge and panel specs.

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Will your controller support a second panel?

deleted - sorry about that

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

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

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.

Wire size chart see 4th post in this thread.