iwire: If, rather than being cord and plug connected loads, the welder and compressor were both _hard wired_ onto the same circuit, would your answer be different?
My opinion is that if it can be reasonably anticipated that 210.19(A)(1) will be violated, then the installation is not up to code. This may not be enforceable, because as you note you can't determine ahead of time what will be plugged into the receptacles, but having 50A of load on a 30A circuit _is_ a code violation.
This does require a judgement: Do you as the installer think that the customer will be serious about not operating the two loads at the same time.
I know many people for whom the standard of circuit overload is 'does the breaker trip'. In the case of a welder and compressor on the same circuit, both very intermittent loads, I would not be surprised if you could have both devices 'on' and work with both effectively at the same time (welding, then using a grinder, then going back to welding, etc) without tripping the breaker too frequently......
-Jon
