Hi all,

I recently accidentally installed 300V rated wire in a 480VAC circuit.
I am going to replace it (and the device has not been connected to the
480V source yet), but it got me wondering: what goes into wire voltage
ratings?

Current rating is easier to understand: the cross-sectional area tells
you how much heat will be generated per unit length. You then take the
maximum temperature which the insulation can withstand continuously
without softening or being compromised, take the maximum ambient
temperature, and get the maximum deltaT between wire and air. You
apply some safety factor to this, and then from the thermal properties
of the insulation (both the thermal conductivity and the ability to
transfer heat to still air by convection) determine the maximum heat
power per unit area, then work backwards to current.

>From my experience, the typical current rating for wire from tables is
a bit conservative, but not hugely so. For example, 10 AWG wire is
typically rated to carry 55 amps if it is not in a bundle and is in
free air. When I have put 55 amps through such wire, it gets fairly
warm, and if I were to try to put 80 amps through it, the insulation
would begin to melt.

However, if you take typical 300V rated wire with PVC insulation, the
dielectric strength of PVC is about 12kV/mm. The wire might have 1/2
mm of insulation, which SHOULD be good for 6kV. Now, I realize that
there are several other factors, such as a safety factor for
insulation which is partially cut-through, safety factor for E field
concentration if the wire is very close to a grounded sharp point,
etc. However, it seems to have a very high safety factor (20x)
compared with the safety factor for current rating (80/55= about 50%
over rated, and even in terms of amps^2, i.e. power, it is about a 2x
safety factor).

Can anyone point me to some web page or even book which discusses the
details of what goes in to the voltage rating of wire?

Thanks,

Sean
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