We have a main breaker in an electrical enclosure (300 A). We come off of the top of the breaker to a 30 A breaker in the same enclosure. This is within ten feet. Can we go to field devices outside the enclosure off of the load side of the 30 A breaker?

You can go anywhere you want off the load side of the 30 amp breaker as long as you use 30 amp wire. If by "come off the top of the breaker", you mean that you have landed the tap wires in the same terminal as the feeder conductors, you have a violation unless the terminals are listed for those two conductors.
Don(resqcapt19)