Originally Posted by roger1
The breaker protects the wire.
The feed cord on your 20 Amp welder will be #12 wire. It will not be protected with a 30 Amp breaker and will create a hazard even if your extension cord is #10.
You are partly correct. Cord and plug connected equipment often has a cord of a smaller gauge than the circuit ampacity. Free air amperages are higher than enclosed amperages. You shouldn't run extension cords for a variety of reasons.
These include voltage drop and overheating of plugs and cord caps due to overload from high starting currents and loose fitting connections. Plugs are by nature loose fitting, otherwise you couldn't insert or remove them.
Hooking an air compressor to a 240 volt circuit with an extension cord is not a good idea.