mapj1 said:

The only time voltage comes into it is in terms of the insulation, usually plastic, that separates the wire - high voltage cables need thicker plastic.

But thicker insulation acts as thermal lagging, so  the car version of the same cable, with a thinner plastic jacket, may be OK running at a higher current than its mains equivalent that is insulated and sheathed.

Going off at a bit of a tangent, how big is the difference in the thickness? If lagging is irrelevant, how much does the thickness matter? So if I make up some meter leads with a bit of flexible cable from Halfords, will I come to grief?

mapj1 said:

The only time voltage comes into it is in terms of the insulation, usually plastic, that separates the wire - high voltage cables need thicker plastic.

But thicker insulation acts as thermal lagging, so  the car version of the same cable, with a thinner plastic jacket, may be OK running at a higher current than its mains equivalent that is insulated and sheathed.

Going off at a bit of a tangent, how big is the difference in the thickness? If lagging is irrelevant, how much does the thickness matter? So if I make up some meter leads with a bit of flexible cable from Halfords, will I come to grief?

Depends what voltage you put on your meter leads! I suspect car hook up wire will be fine to a few kV

A few years ago I was involved in a job that 'mis' used a solid core polyethene core coax cable (URM10)  to carry an HT supply of about 15,000 to a transmitter valve. The insulation is about 5mm thick.

As the makers spec is only 1000V this raised eyebrows, and before acceptance, a destruction test was attempted in our test facility.

The stuff held off 80kV DC without mishap, and the only thing that stopped it going higher was the lash-up test rig breaking over...  Informal tests on 1mm T and E have been similar - you just need to spread the ends quite wide, or the air breaks down first.

It is common to put full mains between adjacent turns of enameled wire as part of transformers, and that is 0.1mm thick polyurethane varnish and that breaks at a few KV when twisted together, but only if you d not scratch the varnish while twisting.

Plastics are really good insulators, the problem that sets the regulatory requirements is mostly damage, and in EHV insulation, bubbles. Car wiring is normally loomed and taped, and perhaps the price of failure is considered lower though thinking what damage a car battery can deliver, that is probably a mistake.

Mike