I'm trying to size wire for UL 508a panels. I have UL's wire gauge requirements, but those requirements are for continuous use. The device I'm designing will only run for two seconds, with minutes or hours between runs. Since currents of interest are 25, 50, 100, and 200 amps, there's a lot to be saved by not using wire rated for continuous use!
Is there a proper way to size wire for pulse applications like this? If the continuous ampacity of (for example) 75C copper stranded 4 AWG is 85 amps, how much can I run for two seconds? Is there some rule of thumb? Some equation? A table? Appropriate application of calculus?
Best Answer
If this question were in a physics exam, I would answer it as follows; whether this is a sensible idea in practice is entirely another matter. One would have to be pretty certain that no fault state could leave current flowing for more than two seconds.
We know from the specification of the wire the resistance per metre R and mass of copper per metre M. Given the current, I, we know that the power dissipated in the wire is I^2 R per metre. The total heat energy dissipated per metre of wire is therefore E=I^2 R t, where t=2 seconds is the time that the current is active for. We (conservatively) estimate that negligible heat leaves the copper wire during this 2 seconds, and therefore the rise in temperature T is given by
T=E/(MC) = I^2 R t / (MC)
where C is the specific heat capacity of copper. A wire needs to be chosen with R and M such that this temperature rise T is acceptable.