I first learned about knob and tube wiring when I bought my 1910s craftsman house. Upon inspection, the electrician told me the wiring was original knob and tube and would need to be replaced to meet modern electrical codes. This started my journey into understanding this important but often overlooked part of electrical history.

What is Knob and Tube Wiring?

Knob and tube (K&T) is an early method of electrical wiring used in buildings from about 1880 to the 1940s. It consists of insulated copper conductors passing through ceramic knobs to provide air space and prevent contact with framing members. The wires are supported along their length by porcelain tubes.

This primitive system eventually sparked the widespread adoption of electricity in buildings. While revolutionary for its time, knob and tube has been banned in most areas due to safety concerns. However, some of this original wiring still exists in older homes today.

The Rise of Knob and Tube Wiring

In the late 1800s, electricity was just beginning to be adopted for lighting in buildings. The arc lighting systems used at the time were impractical and even dangerous for home use.

Around 1880, Thomas Edison began marketing his incandescent lightbulb for indoor use. This created huge demand for a safe and effective way of running wiring through buildings to power these newfangled electric lights.

Knob and tube wiring was the solution. It was developed to prevent fire and shock hazards associated with early electrical systems. The knobs and tubes provided open air space between conductors and wood framing, reducing risk of short circuits and overheating.

By separating wires into individual ceramic knobs and tubes, knob and tube allowed electrical wiring to be run safely through wooden structures. This simple but ingenious system sparked the widespread adoption of electricity in homes and businesses.

How Knob and Tube Wiring Works

The knob and tube system consists of individual copper wires coated with rubber or cloth insulation. The insulated wires are supported by porcelain knobs attached to framing members and run through porcelain tubes where they pass through joist and stud spaces:

This clever spacing and insulation of conductors allowed electrical wiring to be installed safely in wood-frame buildings for the first time.

The Rise of Electrical Adoption

Knob and tube wiring sparked the rapid adoption of electricity across America:

Overall, knob and tube wiring acted as a catalyst that pushed America into the electrical age. Within just 40 years, electricity went from a novelty to an essential necessity of modern life.

The Demise of Knob and Tube Wiring

By the 1930s, knob and tube wiring was rendered obsolete by improvements in electrical systems:

By the 1950s knob and tube was obsolete and prohibited for new construction. However, the simple wiring system had already acted as a catalyst to usher in the Electrical Age.

Dangers of Old Knob and Tube Wiring Today

Although knob and tube sparked the adoption of electricity, it is now considered outdated and dangerous by modern standards:

For these important safety reasons, knob and tube wiring is prohibited by electrical codes in most areas today. Any existing old K&T systems should be properly updated or replaced.

My Experience Replacing Knob and Tube Wiring

When I discovered the knob and tube wiring in my century home, I knew it needed to be addressed. As an engineer, I found the system fascinating but realized it posed safety risks. Here is how I went about replacing it:

While costly, I highly recommend fully updating old wiring. The safety benefits are invaluable for both protecting your family and preventing catastrophic electrical fires.

The Knob and Tube Wiring Legacy

While prohibited today, knob and tube wiring holds an important place in the history of electrification. This simple wiring method enabled the practical and safe use of electricity in buildings for the first time.

By allowing electrical lighting, knob and tube wiring acted as a catalyst for the adoption of electricity across America in the late 1800s and early 1900s. This primitive system powered the lighting revolution and helped usher in the age of electrical appliances.

So while the knob and tube wiring itself was short-lived, its impact was immense. This overlooked wiring system paved the way for the Electrical Age and modern society as we know it today.