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“The Forgotten Knob and Tube Electrical Wiring of the Early 20th Century”

The Forgotten Knob and Tube Electrical Wiring of the Early 20th Century

As an electrical historian, I have always been fascinated by the early electrical systems that helped bring electricity into homes across America in the early 20th century. One particular system that many people today have forgotten about or are unaware of is knob and tube wiring.

What is Knob and Tube Wiring?

Knob and tube (K&T) wiring was an early standardized method of electrical wiring used in buildings in the United States from about 1880 to the 1940s. It consisted of single insulated copper conductors run within wall or ceiling cavities, passing through ceramic knobs attached to structural framing members and supported on porcelain tubes between the knobs.

This old two-wire electrical system was considered very safe when it was invented. The wires were separated from each other and surrounded by lots of air space rather than being bundled together. The porcelain knobs and tubes provided protection and insulation between the wires and the framing members. K&T wiring was the standard for new construction until the 1930s.

The Rise of Knob and Tube Wiring

The knob and tube system was invented in the 1880s as a safer and more practical alternative to open wiring methods. Before knob and tube wiring, electrical wires were often run along the surfaces of walls and ceilings, exposed and unprotected. This posed obvious fire and electrocution hazards.

As electricity became more commonplace in the early 20th century, the knob and tube method was mandated by insurance companies and electrical codes. It was faster and cheaper than conduit wiring, which encased wires in metal tubes. By embedding wires within walls, knob and tube wiring helped speed the adoption of electricity while minimizing the risk of electrical fires.

Thomas Edison’s electrical company was one of the main proponents and installers of knob and tube wiring in America. Millions of homes were wired using the knob and tube method during the 1920s and 30s electrification boom.

How Knob and Tube Wiring Worked

The ceramic knobs were nailed or screwed to framing timbers, straddling the two wires and keeping them separated. The wires were laid out with specific separation distances between them required by electrical codes. The porcelain tubes were used to bridge gaps and support the wires through cavities.

While very safe initially, knob and tube wiring did not include a ground wire, which provides an extra protection against electrocution not found in modern wiring. The hot and neutral wires were run through separate knob and tube assemblies to prevent sparking and ignition of the rubber or cloth insulation.

A typical knob and tube wiring installation

The porcelain knobs provided excellent insulation and protection for the wiring, which was rated for 600 volts. The hot and neutral wires "bundled" together by a knob could even touch without risk of touching each other through the porcelain.

The Decline and Issues with K&T Wiring Today

By the 1930s, metallic conduit wiring and non-metallic sheathed wiring like NM cables had displaced knob and tube as the wiring methods of choice. However, knob and tube wiring can still be found in many older homes across America today.

While fundamentally safe when properly installed, knob and tube wiring has become outdated and potentially hazardous in older houses due to:

Many insurance companies today will not insure a house with old knob and tube wiring due to the fire and shock hazards. Replacing knob and tube wiring has become a necessity in many old homes.

As an electrical historian, I find knob and tube to be a fascinating look into the early days of residential electricity. While no longer safe for modern use, it served an important role in America's electrification. We owe a lot to those shining porcelain knobs that first brought power to the people.