Electrical system change - cables sizes?

[nuke]

Hi guys.
I'm about to change the entire electrical installation of my XS400SE ('81). I was trying to find some info on the size of the cables, but wasn't able to find such anywhere. In Hynes is not written, too.
Using some calculators online it appears that for the headlight (50Watts on 12 Volts, consumption ~4Amps) I need cable with 4mm2 size. Looking on the original electric system of the bike, I don't have such a big cable.

Any ideas where I can find the correct cable sizes or how to calculate them?

 

[BBS360]

This should give you a good idea:
http://www.xs400.com/threads/rewire-how-to.5934/

 

[nuke]

Thanks a lot. A great topic!