Electrician - dedicated welder circuit wiring?

[Whoaru99]

Sometime in the coming weeks I expect to be helping a buddy install a dedicated welder circuit.

The welder is an old AC/DC buzz-box stick welder. The nameplate rating is 42A primary current @ 20% duty cycle. It has a 6-50 plug (two hots and equipment grounding wire).

According to what I gather from NEC 630 re. arc welders, the 42A can be derated by 0.45 because of the 20% duty cycle. That gives 19A rounded to nearest amp. That means I could use 12ga cable, but will use 10ga just because.

Again, per NEC 630, it looks like the dedicated welder circuit overcurrent protection can be up to 200% of the conductor rating. So, with 10ga conductors, could be up to 60A breaker. However, I don't see any adjustment factor for the 6-50 receptacle so that limits overcurrent size to 50A max.

So, based on how I see it, the install would be fully NEC compliant (assuming correct install techniques) using a 6-50 receptacle, connected to a 50A breaker, using 10-2 w/ground wire?

Sound right?

Edited October 11, 2015 by Whoaru99