Thanks for the reply. The wires are only running from point a to point b. They are not completing a circuit. So 15ft red and 15ft black. Am I looking at this wrong?

That's still 30' of wire length that you have to plug into the calculator, if the calculator is expecting round trip distance. Some calculators assume the number you're inputting is a one-way length and automatically doubles it.

40 amp is the

*output *of the charger. It will likely draw

__more amps on the input side__. To be on the safe side, I'm assuming 50 amps max on the input side, with a round trip distance of 30'. Using my favorite

wire gauge calculator (used by a lot of forum members), I come up with a recommendation to use 2 awg, with 4 awg being marginal. Note that the calculator is assuming 105° C wire. If you use cheaper 90° C wire you may need a heavier gauge.

We could fudge the voltage number up to 13v, with the expectation that the source is capable of producing a solid 13v+. In that case, 4 awg is acceptably under the 3% voltage drop. Would I rely on that? Probably not, unless I had a bunch of 4 awg sitting around waiting to be used and my DC-DC charger was going to be used to top off a LiFePO4 battery, not charge it from a very low state of charge. But we're quibbling here and my preference is to design/implement for the worst case scenario, so I would go 2 awg.