The reason for the 12 gauge wire for an HEI is because the more voltage it gets the more it puts
out. They run without problems up to 16 volts.
To eliminate any voltage drop no matter how small, a 12 gauge wire is recommended for any HEI in a performance application. Over the years I have diagnosed many ignition problems with HEI's that were because of too small wire and the second big fault was poor grounding. They love voltage so we find its best to let them have it.
#12 wire is in fact rated for 20 amps in normal AC use. Interestingly enough at least one aftermarket fuse panel maker provides a 15 amp fused circuit for the ignition system and this circuit on the panel is in #12 wire. It may be overkill or just protection against someone wiring half the vehicle to that fuse.
