Originally Posted by bluesman2333
No, the problem is having two different loads on one dedicated circuit.
To my understanding, he has a 30 Amp receptacle that is dedicated running #10 wire with a 2 pole 30 Amp breaker.
He will either plug a 30 Amp dryer into this receptacle or unplug the dryer and plug in a welder (with an extension cord) which is supposed to be on a 20 Amp 220V circuit.
So, the problem isn't what you state. And, the problem isn't running the dryer, it is running the welder on a 30 Amp breaker when it is designed for a 20 Amp circuit.
If his extension cord is #10 then he's probably running Romex as an extension cord which is not designed for that.
If he is using a standard extension cord, the largest available for those is #12 and it wouldn't be good to run #12 wire on a 30 Amp breaker.