Painless wiring problem?

[My68myway]

Ive got a painless wiring harness on my 68 and ive been having a few problems with the alternator not putting out good current back to the battery at idle once the car gets hot. I went back into the instructions and noticed that wire #915 which goes to the output on the alternator is a 12 guage wire instead of the 10 guage the instructions have it labeled as. My theory is that as the car and that wire gets hot the wire is too small and builds up to much internal resistance to carry the sufficient current. Does anyone have any advice or know what i may be able to do to fix this problem?