"All conductors of the same circuit and, where used, the neutral and all equipment grounding conductors shall be contained within the same raceway, cable tray, trench, cable, or cord." [Article 300-3(b)]
and
"Single conductors ... shall only be permitted to be installed where part of a recognized wiring method of Chapter 3." [Art. 300-3(a)]
The only methods recognized by Chapter 3 which use single conductors are "open wiring on insulators" and "concealed knob and tube". The former is permitted only in agricultural and industrial installations, and the latter is permitted only as an extension of an existing system (not as a new installation).
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?