OPEN WIRING ON INSULATORS

320-1. Definition. Open wiring on insulators is an exposed wiring method using cleats, knobs, tubes, and flexible tubing for the protection and support of single insulated conductors run in or on buildings, and not concealed by the building structure. 320-2. Other Articles. Open wiring on insulators shall comply with this article and also with the applicable provisions of other articles in this Code, especially Articles 225 and 300. 320-3. Uses Permitted. Open wiring on insulators shall be permitted on systems of 600 volts, nominal, 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. Conductors. (a) Type. Conductors shall be of a type specified by Article 310. (b) Ampacity. The ampacity shall comply with Section 310-15. 320-6. Conductor Supports. Conductors shall be rigidly supported on noncombustible, nonabsorbent insulating materials and shall not contact any other objects. Supports shall be installed as follows: (1) within 6 inches (152 mm) from a tap or splice; (2) within 12 inches (305 mm) of a dead-end connection to a lampholder or receptacle; (3) at intervals not exceeding 4½ I feet (1.37 m) and at closer intervals sufficient to provide adequate support where likely to be disturbed. Exception No. 1: Supports for conductors No. 8 or larger installed across open spaces shall be permitted up to 15 feet (4.57 m) apart if noncombustible, nonabsorbent insulating spacers are used at least every 4½ feet (1.37 m) to maintain at least 2½ inches (64 mm) between conductors. Exception No. 2: Where not likely to be disturbed in buildings of mill construction, No. 8 and larger conductors shall be permitted to be run across open spaces if supported from each wood cross member on approved insulators maintaining 6 inches (152 mm) between conductors. Exception No. 3: In industrial establishments only, where conditions of maintenance and supervision assure that only qualified persons will service the system, conductors of sizes 250 kcmil and larger shall be permitted to be run across open spaces where supported on intervals up to 30 feet (9.1 m) apart. 70-208 NATIONAL ELECTRICAL CODE 320-7. Mounting of Conductor Supports. Where nails are used to mount knobs, they shall not be smaller than 10 penny. Where screws are used to mount knobs, or where nails or screws are used to mount cleats, they shall be of a length sufficient to penetrate the wood to a depth equal to at least one-half the height of the knob and the full thickness of the cleat. Cushion washers shall be used with nails. 320-8. Tie Wires. No. 8 or larger conductors supported on solid knobs shall be securely tied thereto by tie wires having an insulation equivalent to that of the conductor. 320-10. Flexible Nonmetallic Tubing. In dry locations where not exposed to severe physical damage, conductors shall be permitted to be separately enclosed in flexible nonmetallic tubing. The tubing shall be in continuous lengths not exceeding 15 feet (4.57 m) and secured to the surface by straps at intervals not exceeding 4½ feet (1.37 m). 320-11. Through Walls, Floors, Wood Cross Members, etc. Open conductors shall be separated from contact with walls, floors, wood cross members, or partitions through which they pass by tubes or bushings of noncombustible, nonabsorbent insulating material. Where the bushing is shorter than the hole, a waterproof sleeve of noninductive material shall be inserted in the hole and an insulating bushing slipped into the sleeve at each end in such a manner as to keep the conductors absolutely out of contact with the sleeve. Each conductor shall be carried through a separate tube or sleeve. (FPN): See Section 310-10 for temperature limitation of conductors. 320-12. Clearance from Piping, Exposed Conductors, etc. Open conductors shall be separated at least 2 inches (50.8 mm) from metal raceways, piping, or other conducting material, and from any exposed lighting, power, or signaling conductor, or shall be separated therefrom by a continuous and firmly fixed nonconductor in addition to the insulation of the conductor. Where any insulating tube is used, it shall be secured at the ends. Where practicable, conductors shall pass over rather than under any piping subject to leakage or accumulations of moisture. 320-13. Entering Spaces Subject to Dampness, Wetness, or Corrosive Vapors. Conductors entering or leaving locations subject to dampness, wetness, or corrosive vapors shall have drip loops formed on them and shall then pass upward and inward from the outside of the buildings, or from the damp, wet, or corrosive location, through noncombustible, nonabsorbent insulating tubes. (FPN): See Section 230-52 for individual conductors entering buildings or other structures. 320-14. Protection from Physical Damage. Conductors within 7 feet (2.13 m) from the floor shall be considered exposed to physical damage. Where open conductors cross ceiling joists and wall studs and are exposed to physical damage, they shall be protected by one of the following methods: (1) by guard strips not less than 1 inch (25.4 mm) nominal in thickness