The 35 A rated input power supply in question has a 50 A Nema plug and #8 SO cord. There is an existing 30 A circuit with a fused disconnect the customer wants to use to feed the power supply. The original question was posed to determine if we can install a 50 A receptacle on the load side of the disconnect to feed the power supply or is there a specific code requirement to install a new 50 A circuit for the power supply, even if the load will not exceed 80% of the rating of the 30 A existing circuit.
I don't see how 430.122 applies since the power supply is not part of an adjustable-speed drive system.
Someone brought up an ASD application that worked fine, made sense for what he was doing, but did not meet code. Was not related to your problem.
The bottom line is this. What you have can be made to work. It will be safe, and adequate. It maybe cannot be made to meet code, at least I cannot think of any way that is sure to meet code.
Do I personally have a big issue with just putting a 50A receptacle in place of the 30A one? Nope. It is perfectly safe, and will serve the purpose. It just probably cannot be made to meet code IF (note the IF) you decide that this outlet is dedicated to this particular power supply. If it is just a general purpose outlet, that is another story.
Personally, I would probably just add a 50A outlet in parallel with the existing 30A outlet, and put a sign on it that says "30A max".