Hi Mike,
Will be needing a couple of harnesses myself shortly and have been looking at options.
With a 5amp alternator and 5 or 6 amp/hr battery, the electrical system is marginal as confirmed by CL-100. Totting up the electrical load with everything on gives a max wire loading in the order of 7-8 amps, hence the 10amp fuse. In theory you could wire it all in 20awg or 0.5mm2 !!
To keep the voltage drops down and beef up the system, I will use 16g for the main wires, IE, All black/red wires to the battery, earth wires and the blue switched live supply. Everything else can be wired in 17/18g to give a useful upgrade. Some of the minor circuits IE 3w bulbs, neutral switch etc could be 20g. This compares well with previous replies
I would have no qualms about using the current thinwall wires, they use a harder pvc insulation and should provide good abrasion resistance. The harness is well protected with sleeving, grommets etc. Just make sure none of the unprotected wires are rubbing against metal parts or protect with stick-on foam sheet.
Did find the more sites I looked at, the more confused I became. Everyone has different ideas or use various wire designations and max current or wattage ratings, so put this table together :-
AWG Area mm2 Strands/dia Amps
20 0.5 16/0.20 11
18 0.75 24/0.20 14
17 1.0 14/0.30 or 32/0.20 16
16 1.5 21/0.30 21
May not be an exact conversion between imperial/metric but near enough.
Hope this helps, Brian.