Concealed Knob-and-Tube Wiring
Part I. General
- Scope. This article covers the use, installation, and construction specifications of concealed knob-and-tube wiring.
Concealed Knob-and-Tube Wiring. A wiring method using knobs, tubes, and flexible nonmetallic tubing for the protec‐ tion and support of single insulated conductors.
Part II. Installation
394.10 Uses Permitted. Concealed knob-and-tube wiring shall be permitted to be installed in the hollow spaces of walls and ceilings, or in unfinished attics and roof spaces as provided by 394.23, only as follows:
- For extensions of existing installations
- Elsewhere by special permission
394.12 Uses Not Permitted. Concealed knob-and-tube wiring shall not be used in the following:
- Commercial garagesARTICLE 396 — MESSENGER-SUPPORTED WIRING 396.2
- Theaters and similar locations
- Motion picture studios
- Hazardous (classified) locations
- Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors
394.17 Through or Parallel to Framing Members. Conductors shall comply with 398.17 where passing through holes in struc‐ tural members. Where passing through wood cross members in plastered partitions, conductors shall be protected by noncom‐ bustible, nonabsorbent, insulating tubes extending not less than 75 mm (3 in.) beyond the wood member.
- General. A clearance of not less than 75 mm (3 in.) shall be maintained between conductors and a clearance of not less than 25 mm (1 in.) between the conductor and the surface over which it passes.
- Limited Conductor Space. Where space is too limited to provide these minimum clearances, such as at meters, panel‐ boards, outlets, and switch points, the individual conductors shall be enclosed in flexible nonmetallic tubing, which shall be continuous in length between the last support and the enclo‐ sure or terminal point.
- Clearance from Piping, Exposed Conductors, and So Forth. Conductors shall comply with 398.19 for clearances from other exposed conductors, piping, and so forth.
394.23 In Accessible Attics. Conductors in unfinished attics and roof spaces shall comply with 394.23(A) or (B).
Informational Note: See 310.15(A)(3) for temperature limita‐ tion of conductors.
- 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 where conductors are instal‐ led along the sides of joists, studs, or rafters.
- 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, 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.
394.30 Securing and Supporting.
- Supporting. Conductors shall be rigidly supported on noncombustible, nonabsorbent insulating materials and shall not contact any other objects. Supports shall be installed as follows:
- Within 150 mm (6 in.) of each side of each tap or splice, and
- At intervals not exceeding 1.4 m (41∕2 ft).
Where it is impracticable to provide supports, conductors shall be permitted to be fished through hollow spaces in dry locations, provided each conductor is individually enclosed in flexible nonmetallic tubing that is in continuous lengths between supports, between boxes, or between a support and a box.
- Securing. Where solid knobs are used, conductors shall be securely tied thereto by tie wires having insulation equiva‐ lent to that of the conductor.
394.42 Devices. Switches shall comply with 404.4 and 404.10(B).
394.56 Splices and Taps. Splices shall be soldered unless approved splicing devices are used. In-line or strain splices shall not be used.
Part III. Construction Specifications
394.104 Conductors. Conductors shall be of a type specified by Article 310.