1999 NEC
Art. 320 - Open Wiring on Insulators (read knob & tube)
320-3 - Uses Permitted
'Open wiring on insulators shall be permitted on systems of 600v or less only for industrial or agricultural establishments,indoors or outdoors, in wet or dry locations, where subject to corrosive vapors, and for services.
320-5(a)
Conductors shall be of a type specified by Art. 310
