No, the provisions of 300.20 only come into play when the arrangement of the conductors can lead to inductive heating. If the conductors, as installed, in a common ferrous enclosure or raceway do not result in inductive heating then, be default, their grouping (what ever it actually is) is acceptable to the NEC. However, if inductive heating is present, then the conductor grouping must be modified (i.e. reduced spacing) so the heating is no longer a problem.
Don, can you show us a sample calculation you use to 'cover yourself'?
Jim,
I don't see the section being enforced that way. It is enforced any time there is a single conductor passing through ferrous metal. I don't see any inspector ever permitting a single AC conductor through ferrous metal, no matter what the current level is, even though the CEC clearly says there is no issue with currents less than 200 amps. I also don't see very many inspectors that would require grouping within a common enclosure.
I don't have any calculations as my installations comply with the way the rule is commonly enforced...that is no single AC conductor through ferrous metal.
As far as the original question, I really don't see any real world issue with inductive heating where all of the conductors are within the same ferrous metal conductors, but I see the first and second sentences of 300.20(A) as standing alone, and the second gives a prescriptive requirement, that the conductors be grouped without exception.
If that second part was not there, then I would agree that you would only need to take action where there would be inductive heating, and that is why I see no need for the second sentence.