History of Knob and Tube Wiring

History of Knob and Tube Wiring

Introduction to Knob and Tube Wiring

Knob and tube 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 cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported on nailed-down porcelain knob insulators.

As the first widespread and standardized method of electrical wiring in buildings, knob and tube wiring played an important role in the electrification of urban and suburban America in the early 20th century. It allowed for the wiring of older buildings and homes for lighting and electrical appliances without the cost and disruption of installing conduits. However, knob and tube wiring has some drawbacks compared to more modern wiring methods.

In this article, I will provide an in-depth history of knob and tube wiring, covering:

My goal is to give the reader a comprehensive understanding of this important early electrical wiring method that powered the electrification of America.

Origins and Early Development of Knob and Tube Wiring

The origins of knob and tube wiring can be traced back to the first experiments with interior electrical wiring in buildings in the 1870s and 1880s.

Some key early milestones in the development included:

So by the early 1890s, knob and tube wiring had emerged as an inexpensive and effective method for electrical wiring inside buildings. The separate knobs and tubes prevented wires from touching each other or combustible materials in walls.

How Knob and Tube Wiring Worked

The knob and tube wiring system consisted of:


This left an air gap around the insulated wires rather than filling space with conduit or solid insulation. Allowed heat dissipation and flexibility.


The Rise of Knob and Tube Wiring

In the early 20th century, knob and tube wiring rapidly rose to become the predominant standard for electrical wiring in American buildings.

Some key factors driving its popularity:

By the 1920s:

Knob and tube wiring enabled the mass electrification of urban, suburban, and rural America in the early 1900s, powering the adoption of electrical appliances and lighting across the country.

Advantages and Disadvantages of Knob and Tube

Compared to previous wiring methods like open cleat wiring, knob and tube offered some important improvements in safety and performance:


However, knob and tube also had some disadvantages compared to more modern wiring:


So while knob and tube was an improvement over previous methods, it lacked some of the performance and safety of contemporary conduit and modern cable wiring. The risks tended to increase as electrical loads grew over time.

The Decline of Knob and Tube Wiring

Starting in the 1930s, knob and tube wiring began a steady decline in use in favor of newer wiring methods:

Factors driving the decline:

So by the 1960s, knob and tube was essentially extinct in new construction. But millions of existing knob and tube systems remained in older homes and commercial buildings.

Legacy and Ongoing Issues

Despite being obsolete for over 50 years, knob and tube wiring still lingers in some older structures today. This creates some ongoing issues:

So while knob and tube played an important historical role, its challenging lingering legacy continues to impact older building infrastructure today.


In summary, knob and tube wiring powered the mass introduction of electricity and lighting into American homes and businesses in the early 20th century. As an inexpensive retrofit wiring method, it enabled the rapid electrification of urban and rural areas across the country during a time of major expansion and modernization. While knob and tube wiring has been obsolete for over half a century, its legacy persists as an ongoing infrastructure issue in some older buildings today. Hopefully this article provides a comprehensive overview of the technology and history behind this important early electrical wiring method.