Are you talking about the main charging lead from alternator to battery? You want something larger than 12. Look up what circuit protection should be for #8 wire 8 might not be ALL that large for an 80A alternator
If you are running a "one wire" alternator the charging wire needs to be MUCH larger than "normal" because the voltage sense is AT the alternator, and whatever voltage drop is in the charge wire will allow the system voltage "at the other end" to sag
According to this website, no6 wire should be fused for 80A
Wire Sizes and Fuses
