Is it okay to use an outdoor 25 foot 12 gauge extension cord for an outlet to appliance (temporarily) to make sure there will be no overheating of any wires, which will be running on a 15 Amp Circuit (which probably has 14 gauge wire inside the outlet within the walls)?

In other words, would this be dangerous, being the circuit breaker might not detect overheating (if any) during an extended period of continual usage?

As I mentioned, that the wall outlet leads to a 15 amp circuit, all my 20 and 30 amp circuits are used to the max).

I have used several 6ft 14/3 gauge extensions listed for 15A with a max of 1875 watts, of course this is bad, at each end they heat up and is very dangerous, because of the resistance. I was think a heavier wire - directly from wall outlet to appliance might be better (for now). Thanks in advance