300.3
(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (4).
Also:
250.184 Solidly Grounded Neutral Systems.
(A) Neutral Conductor. The minimum insulation level for neutral conductors of solidly grounded systems shall be 600 volts.
Exception No. 1: Bare copper conductors shall be permitted to be used for the neutral of service entrances and the neutral of direct-buried portions of feeders.
Exception No. 2: Bare conductors shall be permitted for the neutral of overhead portions installed outdoors.
FPN:See 225.4 for conductor covering where within 3.0 m (10 ft) of any building or other structure.
