Classic Knob-and-Tube Wiring Is Still in Use: Take care working with this antiquated system

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Published On
Jul 1, 2021

I received an email recently from a good friend requesting I write an article on knob-and-tube wiring. This request brought back memories of when I was an electrical contractor in the Phoenix area, licensed for 600V and lower electrical installations and electrical work in the state of Arizona.

I provided a total electrical conversion of knob-and-tube wiring in the old Doctor Pleasant mansion, renovating it into a modern commercial office for the Girl Scouts of America regional headquarters. The Pleasant family mansion had one of the first servant-call systems powered by Eveready (patent pending) batteries and had an in-wall built-in vacuum system from the early 1900s. The utility company’s 120V electrical meter had been discontinued but was still in place on the wall in the basement when I started working on the project. At some point, the electrical system was converted to a 120/240V single-phase, three-wire system. Some of the knob-and-tube wiring was still being used in the mansion when I started the rewire project.

Knob-and-tube wiring was commonly used between 1880 and the 1930s, especially in dwelling units. It normally consisted of single insulated copper conductors installed in walls, ceilings and attics. A porcelain tube was used as a sleeve to insulate the wire where it passed through walls and ceiling joists, and the single insulated conductors were supported along their length using a porcelain knob insulator with a nail through the insulator. Where the conductor entered a box for connection to a wiring device—such as a switch, receptacle or lighting fixture—a flexible cloth loom provided an insulation sleeve to help protect the insulated conductors. The first insulation used was asphalt-saturated cotton cloth, until rubber became more readily available.

Where splices in the conductors were necessary, a running splice could be made by stripping the insulation back on the horizontal or vertical supported conductor and twisting the spliced conductor onto the supported conductor to make a mechanical connection, then soldering the two conductors together and wrapping the connection with a rubber-insulated tape. Splices were also made in metal boxes using the mechanical twisting of the connections together, soldering the connections by dipping them into a bucket of hot solder, and then taping the soldered connections together. This very labor-intensive type of wiring could be somewhat unreliable due to some of the older types of insulation on the conductors in the walls and attic areas. Additionally, it would not be able to withstand the higher loads required as we installed more appliances, lighting fixtures, and, sometime later, electronics, in the homes.

In my National Electrical Code collection, I located the knob-and-tube wiring requirements in my first edition, second impression of “Wiring for Light and Power” by Terrell Croft, which was printed in 1917 and based on the 1915 NEC. It was also on page 21 of my reprinted copy of the 1897 NEC. Knob-and-tube wiring was considered to be a Class C wiring method and only permitted for use in dry locations inside buildings. The text in the 1915 NEC was relatively unchanged throughout the newer editions, with assignment of Article 502 in the 1925 NEC and eventually becoming Article 324 in the newer editions up through the 1999 NEC. In the reorganization of Chapter 3 dealing with wiring methods in the 2002 NEC, concealed knob-and-tube wiring became Article 394. 

Generally, between the 1915 and 1993 versions of the NEC, there was very little change in the installation requirements for knob-and-tube wiring, with the exception of better insulation types being used, as they became available. Remember, however, that knob-and-tube wiring was designed as a free-air installation and was never designed to be installed in attic or wall insulation. Knob-and-tube wiring has never been permitted within a wall, ceiling, or attic containing loose, rolled or foamed-in-place insulation where the wiring is enclosed in that insulation. Enclosing knob-and-tube wiring within wall or attic insulation can cause heat damage to the conductor insulation and dielectric leakage, with the possibility of fire in the wall or attic.

There was a major change for knob-and-tube wiring installation requirements in the 1996 NEC with the addition of texts in 324.3, stating that concealed knob-and-tube wiring shall be permitted to be installed in hollow spaces of walls and ceilings or in unfinished attic and roof spaces for extensions of existing installations (such as connecting with NM cable) or elsewhere by special permission. Be especially careful about installing wall or ceiling insulation that may encapsulate the knob-and-tube wiring.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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