Article 398

Open Wiring on Insulators

Part I. General

  1. Scope. This article covers the use, installation, and construction specifications of open wiring on insulators.
  2. Definition.

Open Wiring on Insulators. 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.

Part II. Installation

398.10 Uses Permitted. Open wiring on insulators shall be permitted only for industrial or agricultural establishments on systems of 1000 volts, nominal, or less, as follows:

  1. Indoors or outdoors
  2. In wet or dry locations
  3. Where subject to corrosive vapors
  4. For services

398.12 Uses Not Permitted. Open wiring on insulators shall not be installed where concealed by the building structure.

398.15 Exposed Work.

(A) Dry Locations. In dry locations, where not exposed to physical damage, conductors shall be permitted to be sepa‐ rately enclosed in flexible nonmetallic tubing. The tubing shall be in continuous lengths not exceeding 4.5 m (15 ft) and secured to the surface by straps at intervals not exceeding 1.4 m (41∕2 ft).

Mineral-insulated, metal- sheathed cable

Multiconductor service- entrance cable

Multiconductor underground feeder and branch-circuit cable

Other factory-assembled, multiconductor control, signal, or power cables that are identified for the use




  1. Entering Spaces Subject to Dampness, Wetness, or Corro‐ sive 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 corro‐ sive location, through noncombustible, nonabsorbent insulat‐ ing tubes.Informational Note: See 230.52 for individual conductors enter‐ ing buildings or other structures.
  2. Exposed to Physical Damage. Conductors within 2.1 mPower and control tray cable 336TablePower-limited tray cable725.154,725.135(J),and 725.179(E)(7 ft) 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 protec‐ ted by one of the following methods:
    1. Guard strips not less than 25 mm (1 in.) nominal in thick‐ ness and at least as high as the insulating supports, placed on each side of and close to the wiring.
    2. A substantial running board at least 13 mm (1∕2 in.) thick in back of the conductors with side protections. Running boards shall extend at least 25 mm (1 in.) outside the conductors, but not more than 50 mm (2 in.), and the protecting sides shall be at least 50 mm (2 in.) high and at least 25 mm (1 in.), nominal, in thickness.
    3. Boxing made in accordance with 398.15(C)(1) or (C)(2) and furnished with a cover kept at least 25 mm (1 in.) away from the conductors within. Where protecting verti‐ cal conductors on side walls, the boxing shall be closed at the top and the holes through which the conductors pass shall be bushed.
    4. Rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing. When installed in metal piping, the conductors shall be encased in continuous lengths of approved flexible tubing.

398.17 Through or Parallel to Framing Members. 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 water‐ proof 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.

Informational Note: See 310.15(A)(3) for temperature limita‐ tion of conductors.

398.19 Clearances. Open conductors shall be separated at least 50 mm (2 in.) 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.

398.23 In Accessible Attics. Conductors in unfinished attics and roof spaces shall comply with 398.23(A) or (B).

  1. Accessible by Stairway or Permanent Ladder. Conductors shall be installed along the side of or through bored holes in floor joists, studs, or rafters. Where run through bored holes, conductors in the joists and in studs or rafters to a height of not less than 2.1 m (7 ft) above the floor or floor joists shall be protected by substantial running boards extending not less than 25 mm (1 in.) on each side of the conductors. Running boards shall be securely fastened in place. Running boards and guard strips shall not be required for conductors installed along the sides of joists, studs, or rafters.
  2. Not Accessible by Stairway or Permanent Ladder. Conductors shall be installed along the sides of or through bored holes in floor joists, studs, or rafters.

Exception: In buildings completed before the wiring is installed, in attic and roof spaces that are not accessible by stairway or permanent ladder and have headroom at all points less than 900 mm (3 ft), the wiring shall be permitted to be installed on the edges of rafters or joists facing the attic or roof space.

398.30 Securing and Supporting.

  1. Conductor Sizes Smaller Than 8 AWG. Conductors smaller than 8 AWG shall be rigidly supported on noncombusti‐ ble, nonabsorbent insulating materials and shall not contact any other objects. Supports shall be installed as follows:
    1. Within 150 mm (6 in.) from a tap or splice
    2. Within 300 mm (12 in.) of a dead-end connection to a lampholder or receptacle
    3. At intervals not exceeding 1.4 m (41∕2 ft) and at closer intervals sufficient to provide adequate support where likely to be disturbed
  2. Conductor Sizes 8 AWG and Larger. Supports for conduc‐ tors 8 AWG or larger installed across open spaces shall be permitted up to 4.5 m (15 ft) apart if noncombustible, nonab‐ sorbent insulating spacers are used at least every 1.4 m (41∕2 ft) to maintain at least 65 mm (21∕2 in.) between conductors.Where not likely to be disturbed in buildings of mill construction, 8 AWG and larger conductors shall be permitted to be run across open spaces if supported from each wood cross member on approved insulators maintaining 150 mm (6 in.) between conductors.
  3. Industrial Establishments. In industrial establishments only, where conditions of maintenance and supervision ensure that only qualified persons service the system, conductors of sizes 250 kcmil and larger shall be permitted to be run across open spaces where supported at intervals up to 9.0 m (30 ft) apart.
  4. Mounting of Conductor Supports. Where nails are used to mount knobs, they shall not be smaller than tenpenny. Where screws are used to mount knobs, or where nails or screws are used to mount cleats, they shall be of a length suffi‐ cient 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. Cush‐ ion washers shall be used with nails.
  5. Tie Wires. Conductors 8 AWG or larger and supported on solid knobs shall be securely tied thereto by tie wires having an insulation equivalent to that of the conductor.

398.42 Devices. Surface-type snap switches shall be mounted in accordance with 404.10(A), and boxes shall not be required. Other type switches shall be installed in accordance with 404.4.

Part III. Construction Specifications

398.104 Conductors. Conductors shall be of a type specified by Article 310.