DIY Powersupply cables
So, generally, I'd advise to analyze the voltage drop, resulting power converted to heat, the resulting temperature raise etc., but honestly:

6 A is below what common household power cables are designed to carry. AWG 18 has 1mm in diameter – not really much. 

I'd simply oversize a bit and take a > 2.5 mm² cross-section cable. They come cheap, are used all over the place in e.g. automotive control and can easily be bought speced to have more than sufficient isolation.