New looms

Modern wire uses thinner insulation. A lot of vehicles now use this type of cable as it saves weight and space. Go onto the Vehicle wiring products website, cable sub heading, thin wall. gives the current carrying capacity of the wire. choose accordingly.

Thirty years ago I bought a new loom for my 1960  99, I was so unimpressed by the thin wire that I re-fitted the old loom with a few alterations for 12v . It has worked fine up to now. 

AES also supply vehicle wiring products. I prefer their modern look website and easy to use drop down menus plus extra tech info like wire gauge conversion charts. I'm using their thinwall wire and snakeskin sleeving on custom looms for my Atlas now with the Mk3 later this year. The thinwall insulation also has a higher temperature rating.

https://www.autoelectricsupplies.co.uk/product/70/category/114

Staying with standard size and color wires using online sources including previous NOC forum posts as a guide. Some good (and bad!) advice there on rewiring bikes…  

'or that modern copper cable is far more effective'; NO this is a nonsense statement. The current carrying capacity of copper is exactly the same. 'Thin wall ' is not an answer to a maidens prayer.

The answer can be found on the Autoelectric supplies or VWP web sites. The current carrying capacity is linked to the cross sectional area of the copper ie 1mm or 2mm. Along with the PVC insulation. so our traditional British vehicle wiring has been rated at 8.75 A for many years. This is the ALLOWABLE temp rise at this current. By making the PVC insulation of higher temp PVC this allows the insulation to be thinner. So you end up with a warmer/thinner loom. BUT if you go for the higher current capacity, you will actual tend to increase the voltage drop. So to try to answer the first question. The manufacturers (Lucas-Rists) would use a 'standard' cable  to cover most situations, bearing in mind the intended years of use of the vehicle, and 'abuse' by the owner etc.

I do a standard colours wiring kit with standard cables or thin wall if you prefer aoservices.co.uk

I think my objection to the thin nature of  replacement looms was the flimsy feel that  i thought would flap about in the breeze and jolting that a classic bike provides  and  work harden connections  to break off.  I have had some expensive wiring failures on 20 year old  Fiat  ,Ford  and even the best era of Mercedes cars .Thin insulation can easily  wear through if any friction.

If using the thin wall variety and order it with the same overall OD as the standard type (so the actual copper cross sectional area is greater) the current capacity will be higher than the std type. So less voltage drop, cooler running loom, as you are actually going up a size in the copper content. 

Wiring failures that i have dealt with in cars almost always involve high flex areas. Door and tailgate wiring that get flexed every time. 

Fitting original looms is a recipe for failure. Corrosion, blackwire, age hardened insulation and connectors etc etc make for a built in breakdown.     

The thinwall cable seems to me to be less flexible than standard cable, and I would therefore not be inclined to use it to link the main part of the harness to the handlebars, headlamp etc..