You need to look at the data plate on the welder for the effective ampacity required (or some statement about the current requirements). I suspect that the presence of a 50A plug from the factory indicates that you probably were expected to give it #8. You can probably go to #10 on the ground then.

"I suspect that the presence of a 50A plug from the factory indicates that you probably were expected to give it #8."

Just as with induction motors, the conductor size and breaker nice are not 'matched' like in general purpose circuits (15 and 20 amp).

I do not have a code book handy, but there is a separate article for welding equipment, just like induction motors.

I know that. I have the code. That's why I said you need to know the amps of the welder from it's welding plate. You look it up in the table in art. 680.

The overcurrent protection and the wiring ampacity can be less than the normal branch circuit based on the duty cycle of the welder.

However that doesn't change what I said, the fact they put a 50A plug is telling, it probably means that the wiring ampacity probably also wants to be 50A. The welder may indeed draw more.

You can go higher (up to 200% of the conductor ampacity or current max of the welder) on the braeker.

I'll add my usual diatribe here.

I feel VERY strongly that a receptacle for a welder in a residence should NOT be wired using the exceptions in Art630. I think it should be wired using the "typical" conductor/receptacle/breaker combinations for general purpose circuits.

This is a house, not a welding shop. Homeowners are transient. What if they more and Mrs. Jones find a nice convenient 50A receptacle in the garage to power her 40A kiln? Now you have a 40A load that may be on for 15-20+ hours on a circuit wires with #10. NOT a good thing.

Agree with Petey. This is a residence.

Yes, and getting back to what I said, I suspect highly that the 50A plug on the welder implies the manufacturer wanted 50A ampacity on the wiring. The welder may indeed draw more. Then the only question is what size breaker to use and I'm with bb/petey, I'd put in a 50 and only consider a larger one if there are excessive trips in normal operation.

And the cost of the now over sized conductors for some future event is obviously not a cost object for you.

This is not "using the exceptions in Art630" it is following the code as it is written.

The same type of rules are used for large motors and hermetic compressors (and the hermetic compressors get even more room since they are cooled by the returning refrigerant.

You are not responsible for someone misusing an otherwise correctly installed circuit.

I am actually surprised that ANY plug was on the welder unless it is a small hobby unit, and those usually have very small duty cycles.

Every decent size welder I have seen or installed has come with either an unterminated cord or no cord at all.

Nope, plenty of larger professional welders come with cord and plug. The big-assed Mig welders from LE have them.

The big Lincoln and Miller welders in the welding shop of the institution where I teach are all corded from the manufacturer.

OK, sorry Brick. It is not an exception in Art 630, I know.
How about this:
I personally prefer not to use the relaxed code as written in NEC Art630 for a specific purpose when wiring a receptacle in a residential setting. I prefer to wire it as a general use receptacle considering I have no control over what it will be used for in it's life span.

I strongly disagree with you that the conductors would now be "oversized" if wired as I suggest. They would be larger than needed if the circuit is wired to the absolute code minimum for a welder, and safer for any other use the receptacle might see.

How would you do it? Wire it to the code minimum and put a little sticker saying "Welder use only"???

"I prefer to wire it as a general use receptacle considering I have no control over what it will be used for in it's life span."

It is a pretty hard push to consider anything about a 240 V 30+ amp circuit "general use."

You have no control and no liability, not worth worrying about.

Anyone stupid enough to plug in a large load like that without checking the circuit deserves what they may get.

The wires are oversize if they exceed what is required by the code.

It is your money if you want to waste it.

"Anyone stupid enough to plug in a large load like that without checking the circuit deserves what they may get. "

And you expect a new homeowner to have any clue what they are looking at, let alone open a receptacle or panel to check the wire size??????
All they care about is does the plug fit into the receptacle.

Yes, it is my money and I will waste it. My conscience will be clearer because of it.

There are certainly a few things I wire to code minimum. This is NOT one of them.