Your wire size should be based off the amps, not the voltage, but your voltage under load should drop down to 13 to 14v, depending on your battery bank state of charge.
You can also get a Maximum Power Point Tracker Charge Controller (MPPT) that can handle a higher input voltage and adjust the output voltage to give you the highest wattage possible, often higher then you could otherwise get with a cheaper charge controller, also if you get a MPPT you can wire up your panels to a higher voltage so you can either run a longer run of wire and have your panels in an even more ideal location or you can use a smaller gauge wire.
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