mopar ja,
Quick preface, lets say you have a 14v alt @ 55a max output, your 'R' = .255 (rounded)
E=IxR, or in this case solve for R, = E/I. But then what is the cable length here ?????
Unknown !!!! just some circuit.

Rather than to get loooong winded as to the the formula for cable you asked about
The math & literally dozens of variables have been factored in already (luckily)

read - http://www.powerstream.com/Wire_Size.htm - plug in the #'s & 'upsize' cable
until You have < .4 volt voltage drop.