I had a bit of a look over lunch and found this table in one of my "how to build a kit car" books.
Cable sizes:
For side, tail, fog, flashers, radio and panel lamps. rating 8.75 amps. 14/0.3 metric or 14/0.012 imperial
Headlamps, horns, heated rear windows. rating 17.5 amps 28/0.3 metric or...