I'd say worst case, your pushing the 250W power suplly at 10 Volts. but check out the input limiting fuse on the power supply. That's the ampacity you need to cover with the wire you choose. So if the fuse is 25Amps, then you can trust the engineering of the supply and say you need a wire that will handle 25 amps minimum. Then use an ampacity table to determine the sixe of the wire... like the kayjayco one I linked to in
this thread. 12AWG cuts it close, but 10AWG covers it nicely. I'd say you can implement your plan with no worries. And if the wire goes in the engine compartment, go XLP (CROSS LINKED POLY).