Yeah the wire is already rated at higher than 15amps. It is safe if it is modern wiring. I wouldn’t do it with knob-and-tube, but any solid core 14awg thhn should work at 12 amps continuously. Just use the right extension cord. Hell worst case you could probable just swap the head on a 20amp extension cord to a 15amp head, and never have to worry about the cord at all.