“The Forgotten Science of Knob-and-Tube Wiring”

The Forgotten Science of Knob-and-Tube Wiring


Knob-and-tube wiring was the standard method of electrical wiring used in buildings in North America from about 1880 to the 1940s. Despite its antiquity, there is still a significant amount of old knob-and-tube wiring in use today. However, most people are unaware of how this early wiring system works or the science behind its design. In this article, I aim to shed light on the forgotten science behind knob-and-tube wiring and why it was an important milestone in the development of electrical systems.

A Brief History of Knob-and-Tube Wiring

The knob-and-tube wiring system was introduced in the 1880s as an early standardized method of electrical wiring. Thomas Edison's lightbulb and commercial power plants made interior electrical lighting popular, but there were no safety standards. Knob-and-tube consisted of insulated copper conductors run through non-combustible knobs and tubes, which were nailed or strapped to framing members and ceilings. This open wiring system allowed air to circulate freely and prevent overheating. By separating wires into tubes and open air, the risk of electrical fires was greatly reduced compared to earlier systems.

Knob-and-tube remained the standard wiring method in North American homes and buildings until the 1930s and 1940s. The introduction of plastic insulated cables like Romex eventually displaced knob-and-tube as the wiring method of choice due to lower cost and easier installation. However, existing knob-and-tube wiring continued to be used for decades in old buildings.

The Science of Knob-and-Tube Wiring

While it may seem crude by today's standards, knob-and-tube wiring exemplified careful scientific reasoning about electrical safety back in the late 1800s. Here are some of the key scientific principles behind this old wiring system:


Early electrical wires had rubber insulation that hardened and cracked over time. Knob-and-tube wires were insulated with woven cotton or asbestos wrapped around the conductor. This provided durable insulation that resisted heat and fire.

Air Circulation

The knobs and tubes physically separated wires from each other and building materials. This open-air design allowed heat dissipation and reduced fire risk compared to early insulated cables bundled together.

Isolation from Combustibles

Wires were nailed to non-combustible porcelain knobs and run through ceramic tubes. This isolated conductors from flammable building materials like wood framing and lath & plaster walls.

Simplified Wiring

Two-wire circuits with shared neutral wires were common. This differed from modern wiring with dedicated neutrals and grounds for each circuit. Less wiring reduced installation cost.

Accessible Wiring

Wires were run through unfinished spaces like attics and basements whenever possible. This left the wiring exposed for inspection and maintenance. Modern wiring is concealed behind walls, making it more difficult to access.

By incorporating these scientific principles, knob-and-tube wiring was a major safety improvement on previous electrical wiring methods. The open wiring prevented electrical fires, while the durable insulation minimized dangerous shorts and electrocution hazards.

Challenges of Knob-and-Tube Wiring Today

While knob-and-tube wiring was an engineering marvel in its era, the old technology does present some challenges for safety and modernization:

Despite these limitations, well-maintained knob-and-tube can often continue to function safely. But upgrades to modern wiring are recommended whenever feasible.

The Future of Knob-and-Tube Wiring

Knob-and-tube wiring will likely continue fading from use over time. However, this antique wiring method remains in an estimated 4-8 million homes in the US and Canada.

When discovered, homeowners have three options:

Rewiring older homes is highly labor-intensive, so knob-and-tube will remain in many buildings for the foreseeable future. Even as it disappears, we should appreciate knob-and-tube as an early electrical safety innovation. The forgotten science behind this antique wiring system improved public safety and made the modern electrical world possible.