Wire gauge depends on the length of the run, and the wire construction. 14 gauge varies between .025 and .035 volts of drop (with the low side being stranded and the high side being solid) for every amp of current and 10 feet of run. So with #14, at 10 amps and a 10 foot run, you lose, on average, 0.3 volts which is, at least for me, a little on the high side. For every wire size you go up, i.e. to #12, the loss is cut in half. So #12 would be about .15 volts, getting to be an acceptable number.
One caveat, you have to consider the total length of the run, both positive and negative sides, between the panel and the battery.
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