Electronic – What gauge wire for 36V (DC), 5A peak

Over 18 inches you would drop about 0.675 volts with 18 gauge wire. 12 gauge would drop about 0.1 volts. You would need to use 6 or 7 gauge to handle 30 A over an extended period.

If you are not using continuous current, but pulses, you could get away with using a smaller gauge wire. However an undersized wire would heat up significantly in just a few seconds. Unless you are sending pulses of less then 1 sec, you will need a large diameter wire.

AS for the insulation, it depends on how the wire is being used and how it is routed. 30 A is a lot of current. If some one can come in contact with it, you need heavy insulation for safety. Especially if it flexes. If it is total enclosed in a non conductive housing, you could use thinner insulation.

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.