Transformer sizing for 240V 50A NEMA 6-50R Welding Receptacle

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stzu0125

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Wisconsin
I'm trying to size a 480V-120/240V single phase dry type transformer that will feed a panelboard that will power (4) 240V 50A NEMA 6-50R welding receptacles. Does NEC require that I assume that all (4) welding receptacles will be used at the same time? I believe this would require a 50KVA (208.3A) dry-type transformer to get the required 200 amps. Or is it safe to assume that not all welding receptacles will be used at the same time? If so, how many welding receptacles should I assume will be used? Thanks.
 
I don't know about the transformer part, but I do know for welders that the load is determined by the duty cycle. For example a welder with a 20% duty cycle can only be used 2 minutes out of every 10.
 
I would assume all 4 operating at the same time. You only allow for load diversity in residential not commercial situations. That's why you are limited on the number of receptacles on a 15 or 20 amp branch circuit in a commercial setting. IMHO.
 
This will be a hard one to calculate. I think that you would need the ratings of the actual welders.

As Tom notes, welders often have a duty cycle; they probably cannot run on a continuous basis, so you will get some diversity there. Article 630 specifically lists diversity factors for welders.

At the same time welders are known to draw high current for short periods of time, so breakers in excess of the wire ampacity and receptacle rating are permitted. Your transformer would need to be able to supply these high currents without excessive voltage drop. (Where excessive is defined by what the welder requires.)

Good luck!

-Jon
 
I would also think that the quantity of welders on the premises would come into play. If there is only one, then one circuit would be adequate, with a second circuit installed if a second welder was added in the future.
 
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