The idea is to limit the temperature of the conductor, the greater the current the greater the heat loss of the conductor.

Or to put it another way..... the heat created due to the resistance of the wire is I^2 * R. The voltage has no role to play in this equation so changing the working voltage doesn't have any effect on the heated generated - only the current does.

Voltages do not affect the copper thickness of the cable - the current warming it up is the failure mode.  Actually although folk like simple things like  '5 amp cable' there is no such thing - the same cable will take many times the rated amps for a few seconds,  and on the other hand needs to be de-rated if it is used in a place where it is already hot, or the heat cannot get out because it is in the middle of a bundle of other cables, or under the carpet, or something.

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.
Mike.

JD said:

Current carrying capacity of a cable. My understanding is that if I have a cable rated to 6amps this cable will always only ever be able to take 6amps.

I'd put it slightly differently. The rating depends upon the installation method (including the ambient temperature), but your "6 A cable" rated for that method will carry 6A (or less) indefinitely without overheating. It may safely take more than 6A briefly so perhaps 8A for 20 min. It does not suddenly go "phut" at 6.001 A.

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

The carrying rate of the wire is related to the cross section of the wire, and how much voltage is applied depends on the insulation

I have been suggested that this rating is only true at 230v and that if I was to drop the voltage the current capacity of the cable may increase. My understanding is this is not related, and even at 12v the cable would still only be rated to 6amps?

You often see "12V automotive" cables with an apparently much higher rating than the same c.s.a. building installation cables. But that's down to a number of factors - slightly different formulation of plastic than can withstand higher temperatures is one, higher allowable termination temperature is another, but probably the biggest is that plastic insulation degrades with heat and time - and vehicle wiring is only expected to last for the commercial lifespan of the vehicle (7-15 years say) whereas building wiring is expected to last generations (70 years is often quoted) so all else being equal it has to be treated a lot more gently heat-wise. The actual system voltage isn't really a direct factor as such.

  - Andy.

Of course if the load is fixed power, then dropping the voltage will increase the required current, and thus require a larger CSA.

Frequency also has an influence due to the skin effect. Large cross sections can cary less current as the frequency increases.