Wiring Question 200dx

[Rambozo]

Yes, if you wire a 30 amp circuit you can usually use 10 ga. Check your local code for the specifics on what is required in your area.
Yes, if you want a 40 amp breaker, you will need 8 ga. minimum.
The chart posted was the NEC section on derating wire for welders with limited duty cycles. It only applies to extension and input cords, not building wiring, as Sportbike mentioned. That chart is why the machine only has a 12 ga. input cord, even though it can draw 31 amps. Basically you size the wire to control heat. In a nutshell, because welders can't run at full power constantly, they give the wire time to cool off, so a smaller wire can be used. What it means for you is that you can use slightly smaller extension cords than the amp draw, and distance, would normally dictate.

[Rus T1]

Yes, if you wire a 30 amp circuit you can usually use 10 ga. Check your local code for the specifics on what is required in your area.
Yes, if you want a 40 amp breaker, you will need 8 ga. minimum.
The chart posted was the NEC section on derating wire for welders with limited duty cycles. It only applies to extension and input cords, not building wiring, as Sportbike mentioned. That chart is why the machine only has a 12 ga. input cord, even though it can draw 31 amps. Basically you size the wire to control heat. In a nutshell, because welders can't run at full power constantly, they give the wire time to cool off, so a smaller wire can be used. What it means for you is that you can use slightly smaller extension cords than the amp draw, and distance, would normally dictate.

Thanks Rambozo, I think I understand a little better now. I guess I will buy some 8ga. and use the 40 amp breaker
just for the added safety factor. Will the 10ga. work for an extension cord?

[Rambozo]

Thanks Rambozo, I think I understand a little better now. I guess I will buy some 8ga. and use the 40 amp breaker
just for the added safety factor. Will the 10ga. work for an extension cord?

That will work fine.

[sportbike]

Thanks Rambozo, I think I understand a little better now. I guess I will buy some 8ga. and use the 40 amp breaker
just for the added safety factor. Will the 10ga. work for an extension cord?

Plugging the welder into a 40A outlet doesn't really make things any "safer". If all you are going to use is the welder ont eh circuit, and as Rambozo noted, local requirements allow it, the 30A circuit should work just fine using the 10awg you have on hand.

Actually, one could argue that using the 10awg extension cord on a 40A circuit is less safe than on a 30A circuit. If the welder were to pull 40A for some reason, the hard wiring would be protected, but the extension would overheat.