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:
- The origins and early development of the technology
- How knob and tube wiring worked and was installed
- The rise of knob and tube as the predominant wiring method in the early 20th century
- The advantages and disadvantages compared to previous methods
- When and why knob and tube wiring declined
- The legacy and ongoing issues surrounding knob and tube wiring today
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:
1878 - The first home in America wired for electricity was likely a New York City mansion wired with a primitive version of knob and tube. Wires were strung across the ceilings and walls, covered in wooden moldings.
1880s - As electricity use expanded, inventors began patenting improved insulators and techniques for running wires through walls and ceilings.
1886 - Thomas Edison filed a patent for a wooden knob designed to hold electrical wires. Porcelain insulators soon replaced wood.
1888 - Electrical conduit wiring systems for interior use were developed as an alternative to open wiring methods. However, conduit was more expensive.
1892 - Herbert G. Dyar patented a knob and tube wiring system using porcelain knobs and tubes to hold the wires in place. This became the basis of the standardized knob and tube method.
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:
Knobs - Porcelain spool-shaped insulators nailed to walls, ceilings, and studs. They held the wires in place and prevented contact.
Tubes - Porcelain tubes with an inner diameter greater than the wire. They protected wires passing through joist, stud, and rafter holes.
Wires - Single copper conductors with brittle rubber insulation. Typically run vertically and horizontally between knobs.
Holes drilled for routing wires through wood framing
Knobs nailed to surfaces every 4 to 6 feet
Tubes inserted into holes through framing
Wires pulled through tubes and attached to knobs
This left an air gap around the insulated wires rather than filling space with conduit or solid insulation. Allowed heat dissipation and flexibility.
- Simple, inexpensive, easy to install
- Did not require molding or conduit
- Wires accessible for maintenance
- Air gaps allowed cooling and expansion
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:
Code approval - Knob and tube wiring was formally included in the National Electrical Code in 1897.
Urbanization - As cities grew, existing buildings needed upgrading to electrical lighting. Knob and tube offered an affordable retrofit option.
New construction - Used extensively in new residential and commercial buildings, especially in urban areas.
Electric appliances - As more electrical devices became available, knob and tube wiring allowed homes to be upgraded for lights, irons, and other appliances.
By the 1920s:
Millions of American homes and businesses had been wired for electricity using knob and tube.
It was the standard wiring method endorsed by utilities and employed by electricians.
The only viable competitor was electrical conduit - too costly for most applications.
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:
- Less prone to insulation breakdown and fire risk
- Wires separated and supported, reducing hazards
- Air gaps aided cooling and heat dissipation
- Relatively easy and inexpensive to install
However, knob and tube also had some disadvantages compared to more modern wiring:
- Exposure to insulation breakdown at terminals
- Brittle rubber insulation prone to cracking
- Heat buildup in tubes inside confined spaces
- Only designed for low amperage circuits
- Difficult to service and replace wiring in walls
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:
Metal clad cables - Developed in the 1920s using flexible metal conduit. More durable and higher amperage capacity.
Non-metallic sheathed cables - Introduced in the 1930s using rubber and asbestos insulation. Forerunner of modern Romex wiring.
Conduit wiring - Fireproof metal conduit gained favor for some applications due to safety.
Code changes - The 1940 NEC prohibited new knob and tube wiring in some types of construction. By the 1950s, it was essentially obsolete.
Factors driving the decline:
Increasing electrical loads - Knob and tube capacity was insufficient for modern lighting, appliances, and motors.
Safety concerns - High resistance connections and cracked insulation raised fire risks.
New construction practices - Framing techniques changed from balloon to platform framing, making installation difficult.
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:
Fire and shock risks - Age degradation raises safety concerns. Difficult to fully replace inside walls.
Insurance problems - Many insurers refuse policies on homes with knob and tube. Or require full replacement.
Resale value - Homes with knob and tube sell for 10-25% less. Buyers are wary of safety and costs.
Electrical capacity - Often cannot support modern demands for electricity and appliances.
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.