So I've gotten myself into a healthy, educational Facebook discussion. Link is here if you're a member of that group.
When I thought about it, everyone defines wire capacity by current only. The voltage only comes in when talking about wire insulation.
However, I've always thought about a wire's capacity by total power transmitted, not just the current.
Assuming arcs and corona discharges aren't a problem, could I transmit say 10 kW via a #40 AWG wire at 10mA, 1MV DC? The ampacity isn't violated, but that doesn't look right for wire that's around the width of a hair.
I'm aware of the math behind it (5th year Electronics engineering student) and I can judge what voltage/current I need for X watts versus safety/other constraints, but wires being rated by current only hasn't bothered me up until this point.
Best Answer
The wire will heat up because of its resistance and current passing through it, and this also makes a voltage drop over wire. The potential of the wire itself is irrelevant, assuming the insulation does not bŕeak down. 10mA flowing in a wire still heats up the same amount whether it comes from 1.5V battery or 1MV generator.