Code Corner - May/June 2014


We bought a home with early 1920s knob and tube wiring as per the home inspection survey, and our plan is to remodel several areas. Can we keep this system in place and add to it?


The National Electrical Code (NEC) Article 394.1 Scope.

This section of the NEC clearly covers the use, installation, and construction specifications of concealed knob and tube wiring, which is designed for use in hollow spaces of walls, ceilings, and attics utilizing the free air in such spaces for heat dissipation.

Article 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, or in unfinished attics and roof spaces by 394.23, only as follows:

(1) For extensions of existing installations. Caution should be taken with this. The first approach should be to the local Authority Having Jurisdiction (AHJ) of enforcement of the code. The AHJ may be the federal, state, local, or other regional department or individual such as a building inspector, fire inspector, or electrical inspector. Municipalities can modify this requirement and not allow the extension of the knob and tube additions when the existing plastered walls are removed leaving open the stud/framing - requiring complete rewiring to the latest NEC accepted by the local municipality.

(2) Elsewhere by special permission. This permission is the written consent of the AHJ.

Article 394.12 (5) Uses Not Permitted.

Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating materials that envelopes the conductors.

Blown-in, foam-in or rolled insulation prevents the dissipation of heat into the free air space, resulting in higher conductor temperature, which can cause insulation breakdown and possible fire in the insulation.

Article 394.17 Through or Parallel to Framing. Conductors shall comply with 398.17 where passing through holes in structural members, where passing through wood cross members in plastered partitions, conductors shall be protected by noncombustible, nonabsorbent, insulating tubes extending not less than 75 mm (3 in.) beyond the wood members.

394.19 (A) 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.

394.19 (C) Clearance from Piping, Exposed Conductors shall comply with 398.19 for other exposed conductors piping, etc.

There are many more articles in the code to comply with. 394.23 (A) covers accessible attics or (B) not accessible by stairway or permanent ladder. Also relevant are 394.30 Securing and Supporting, 394.42 Covers Devices, 394.56 Splices and Taps, and 394.104 Conductors shall be of a type specified by Article 310.

Another item to consider is that this system is not a grounded system so you are limited to devices. In the old days, there was no concern about insulation and fuel cost, so this wiring was safe until insulation started to be blown into walls. Electrical contractors today would recommend that you rewire the home to conform with the latest electrical code and devices for protection of life and property.

Marc Ramirez has been a member of the NEC Code Making Panel 17 since 1993 and is a member of the IEC National Codes and Standards Committee. He is an instructor for the IEC Apprenticeship program for IECA of Arizona. He can be reached at (623) 256-9331.