Besides the electrical characteristics others have mentioned (inductance,
etc.), it all comes down to temperature rise. For steady state current,
you can use I^2*R to get the power dissipation for a certain length of the
wire. Then calculate the temperature rise based on conduction, convection,
and radiation. The current limit will be based on the temperature limit of
the insulation (if any) or the conductor itself.

For non-steady state, you can use the RMS current in the above formula.

For short pulses, you have to consider the heat capacity (joules per
Kelvin) of the conductor. If, for the moment, you don't consider power
dissipated by conduction, convection, and radiation, the number of joules
(watts * seconds) times the heat capacity of the conductor will raise the
temperature that number of Kelvins.

I think heat capacity of the determining factor for short pulses.

Harold




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