I have been given a mig welder to supply power for. Rated input is 230volt 20 amp single phase, but plug supplied is 50 amp 250 volt - I guess that although oversized this is just the standard welding plug, but does one wire the receptacle for the demand of the welder or the 50 amp plug supplied?

I would wire for the demand of the welder. I have frequently run into the same thing with welders, seems there is no standard type of plug used on welders.

Demand of the welder. It's like a range that has a 50 amp receptacle but is wired for 40 amps.

Is there a demand calculation for a welder circuit, or is it just the 80% rule?

As a non Electrician, I would ask the customer if they plan to upgrade to a larger power supply in the NEAR future. If so, I would use a larger conduit to allow for an easier upgrade for the next electrician, which may be you.

Jay,
Welders are pretty hard on a power supply.
Is this in a residential or commercial workshop?
One thing I would note for a start is the nameplate on the welder itself, especially the Duty Cycle.
Welders with high Duty Cycles (of 60-70%) require heavier supply wiring, this is due to the weldor being able to use the welder for longer periods at a time and therefore increasing the amount of heating in the supply cabling.
If however, the welder only has a Duty Cycle of say 30-40% you can pretty much get away with a circuit that matches the current draw of the welder.
Another thing, welders and circuit breakers don't tend to go together that well, unless the CB has magnetic protection only, using a CB with thermal protection added in, will end in tears.
I've seen CB's bypassed by those that don't know any better in the past, because the wrong type of CB was installed in the first place.

26-700(2) say receptacle shall only be connected to voltage and ampere rating corresponding to the receptcle.

Is there some sort of exception for welders?

The welder is a pretty common Miller mig welder in a small commercial shop. Did'nt notice a duty cycle on the nameplate.

Section 42 does not seem to address this other than (42-004) letting the plug on the welder be less than the O/C rating but not less than conductor ampacity. With the price of copper these days I would want to justify the 50amp conductors.

42-004 says that the plug can't be less than the "conductors required". That's different than the ampacity of the conductors because you can always oversize the conductors without changing plugs. If you couldn't, engineers wouldn't be able to inflate the cost of the job by going "above code".

If the size of the cord end bothers you that much, change it. Why would you run #6 wires to a load that needs a maximum 24 amp circuit ampacity?

If you put the 50 amp receptacle on a 20 amp breaker what is to prevent the user from getting a full size welder that needs 40amps and plugging it into that receptacle.

I think you either need to change the plug on the welder or put in the full 50 amp circuit.

Just out of curiosity I had a look at a welder from same manufacturer with twice the rated input - and same 50 amp 250 volt plug on it. And I agree, why run the #6?

So it comes out of the factory as "listed and approved" equipment with one oversized plug that fits all. I could go and change out the plug on this approved and listed piece of equipment but then I am interfering with the approval - ironic.

The same thing that prevents a home owner from plugging in two 1500 W hairdryers into the same 15 A recepticle circuit.

Larry

What will stop the customer from plugging in a full size welder is that the breaker will trip. That's what breakers are for.

If you install #6 wire, you will use a 30 amp breaker to protect the welder. You can't fuse equipment rated for 20 amps at 50 amps. If someone plugs in a full size welder, the breaker will trip.

The comparison is a range plug. They are rated for 50 amps and are commonly wired with #8 (45 amps) and fused at 40 amps. If someone plugs in a larger range, the breaker trips.

There are several items I want to speak to besides the welder itself.
First is the reference to breaker size in section 8 or the match in section 26.
Sections 2m, 4, 6, 8, 10, 12, 14, 26 and 30 are basic code sections that generally apply to all installations. For example section 10 tells us the rules for bonding and grounding that apply across the board to installations under 750 V. Section 36 only applies when the voltage is above 750 volts and you better know the rules for grounding are very different there. Section 36 is amendatory.
welders are wired according to all sections outlined with specific rules that are applicable to welders. These rules ammend the requirements of section 14 and 4 with regard to wire size and protection. So if section 14 might require a 30 amp breaker and #10 but the welder was rated to only need a 20 amp breaker and #12 then the ammended nature of the section over rules the other two sections.
The range example is a good one as the wire can be bigger than #8 if the range is rated higher than 12 KW.
We might need #6 if the range is bigger than 12kw. You may be a chef with a 18 kw range from 8-300 that wire is based on 12 kw plus %40 of the rating in excess of 12 kw
8 KW plus 4kw * .4 = 9.6kw / 240 = 40 amps *1.25 = 50 amp breaker and 40 amp wire. If the range was 20 kw then the breaker, wire and receptacle would all have to be upgraded to a larger size. We can see the standard range wire and receptacle covers a broad range of stoves (pun intended)
In answer to this welder question the name plate and duty cycle are essential to matching the two.
Start with 42-006 and the nameplate. Wire and breaker according to 42-008 and you are on track.
The unit comes with a plug of certain configuration I assume was evaluated to the standard for transformer arc welders. There is no requirment to wire to the plug if it is for a welder as section 42 allows the match as an exception to other rules in earlier sections. Now if the plug was to be used for another load that is not a welder you could have another problem if 1 load uses a different set of rules than welders