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.