what guage wire?

I,m doing almost the same thing but my wire between the ign terminal of the switch and the regulator looks like it has been hot. Its brittle and melted. When I run a 4 0r 6 to the starter relay since i,m using a one wire alternator can I run a new wire from the switch ign terminal direct to the ballast feeding the ignition coil? A simple diagram would really be apreciated.