Assume I have a portable piece of equipment which requires 480V,15A, 3 Phase power, this portable equipment has a 15A circuit breaker disconnect all properly size for it's load. I have a receptical rated 480V, 60A 3 Phase, and this is what I will plug into. Question, What is the (wire) size of the flex cable (10 feet) that I must put on the portable equipment to be allowed to plug into this receptical? Does the cable need to be sized for the 60A service and it's protection, or does code allow me an exception due to the 15A breaker?
This is equipment that is being developed, ...
Kerm -
Other than a few exceptions, the NEC neither specifies, nor cares what you plug into a receptacle. The device you are building is not premesis wiring - the NEC does not apply.
I'd guess the facility has a multitude of 480V, 3ph, 60A, "welding receptacles" scattered around. And I'm guessing you want to use which ever receptacle is in the area you need the device in. Pretty normal for industrial areas.
As for any listing concerns, that would depend on the intended use. If you are building hundreds for sale for other customers, maybe listing is a concern.
If you are building a one-off for your company's use, then any listing requirement is up to state law. Most do not require a listing for a one-off piece of cord and plug portable equipment.
As for the cord size, as Dave Luchini and some of the others said, 15A is fine. But it is a design issue.
cf