I posted the example of 12 'switched hot' conductors as a thought experiment.
I agree with iwire: it is a violation of code not to derate these conductors, but isn't really a problem to protect them at 20A.
The worst case scenario for heating is if all of the current flows in a single one of these 12 wires. All other conditions result in lower I^2R heating. Clearly having multiple conductors sharing the current should be allowed whenever a single conductor is allowed (from a heating point of view....there are current sharing issues that would make it a bad idea to use small conductors as actual parallel conductors; but having them run in parallel to _different_ loads does not cause such problems).
IMHO a reasonable additional exception to the rules used in counting current carrying conductors would be: For the purpose of counting current carrying conductors, a set of two or more conductors supplied by the same branch circuit and carrying net current in the same direction shall be considered a single conductor.
I think that the wording would need some cleanup in order to avoid switch loops counting as a single conductor, but you get the drift.
I doubt that this is actually a frequent enough situation to actually warrant a code change, however.
-Jon
