Copper wire and overcurrent.

Lefty Wright

How much current can a 12 guage copper wire pass without melting?

The ampacity is based on the type of insulation. When the insulation fails the circuit will ground or short out.

I was once on a job when an overloaded life support circuit blew a 100A fuse and there were none on hand with which to replace it.

I tecscrewed a length of 12 AWG copper wire to the blown fuse and replaced it while someone ran to the supply house to get some new fuses.

The insulation burned off in a few minutes and the copper turned blue. But it held long enough to run the system until the new fuses arrived.

Now I don't recommend doing this and I was aware of the dangers involved and would not have done it if it had not been an emergency. It did show me that in rating conductors it's the insulation that is really being rated.

mike95490

Re: Copper wire and overcurrent.

Not always. We use teflon coated wire in our labs, and sometimes, we get power supply settings "wrong" and have actually vaporized the copper, inside the teflon. The teflon gets soft, and a big bubble blows out the side, stinks like heck. Other insulation would have flamed and started a fire.
Also the heat from your copper bypass, could have damaged the insulation on the fuse contacts, or ruined the temper of the contact clips, and they may not make good contact anymore.
I've found 300 W halagon lights, that the bulb clips are just brittle trash when the bulb burns out, and I have to scrap the fixture. Cheap parts. Hopefully, your 100 A fuseholder survived the "copper heating element".

Lefty Wright

Re: Copper wire and overcurrent.

You are right, of course, Mike.

Like I said I wouldn't have done it if there had been no emergency. There is quite a large thermal mass in a 100A 600V fused disconnect. The fuse clips should be periodically read with a non-contact pyrometer as a part of preventive maintenance anyway.

On a job in a steel mill we stripped the insulation from 4/0 locomotive cable and pulled it through radiator hose to make water cooled conductors. We ran 1200A at 12V through them to power the rocking resistor furnaces.

Other than that I follow the NEC on wire sizing.