Electrics - wire thickness

[AWJDThumper]

He goes on to suggest, under " General recommendations", after talking about 12-volt systems, "For 6-volt motorcycles where voltage drops are more of a concern it is advisable to use 44/0.3mm (3 sq mm) wiring for the main battery and charging connections. The remaining connections can be made with 28/0.3mm (2 sq mm) wiring, but any thinner cables are best avoided because of the significant voltage losses they will introduce." ( By the way, where I say sq mm he uses mm with a small 2, but I think it's the same thing.) So no mention of heat there, the focus being on voltage drop. But the wiring he recommends is thicker than the stuff we seem to be using.   Phil

If I was building a new 6 V wiring harness for a classic BSA bike that I intended to ride regularly then I would probably over engineer it in the way that Dr James Smith recommends. A 0.3 V voltage drop experienced in a 2 m total length of wire back to the battery (or rectifier) is something that I would definitely avoid - this is what you would get from 1 sq mm, 14/0.3 cable. Upgrading to 2 sq mm 28/0.3 cable would reduce the voltage drop to < 0.1 V.

However, BSA took the view that the voltage drop indicated would be tolerable and at the same time 1 sq mm cable would save them 40% of the cost of 2 sq mm cable. If you want to avoid a voltage drop of 0.3 V then all you need to do is upgrade the wiring to the headlight from its sources of power (battery or rectifier). However, it is also important to address the earth return path as well especially if this is through the frame since this path can be the sources of even larger voltage drops. I personally, always upgrade wiring harnesses to avoid return paths through the frame or the rear mudguard.