Single conductors in a raceway are referenced in Chap. 9, Table 1. You will find it is very rarely legal, usually referencing a Grounding Electrode Conductor.
300-3(b) All conductors of a circuit, including the neutral SHALL be contained in the same raceway, trench, etc..
Without a very lengthy explanation, if the neutral is not in close proximity with the phase conductors, your fault levels become unmanageable. They must be very close to each other, unless DC, for the system to function through all it's parameters.
