Introduction
The knob and tube (K&T) wiring system was commonly used in homes and buildings in North America from about 1880 to the 1930s. This early form of electrical wiring used insulated copper conductors run through porcelain knobs and tubes, which provided protection and structural support. K&T wiring was an important innovation that allowed electricity to be distributed safely and efficiently in buildings for the first time. In this article, I'll provide an in-depth look at how K&T wiring worked and the role it played in powering homes and buildings in the early 20th century.
What is Knob and Tube Wiring?
Knob and tube consists of two separate copper wires, one for the hot wire and one for the neutral, running through porcelain knobs and tubes. The tubes and knobs keep the wires separated and provide an insulating layer between the wires and surrounding surfaces.
Here are some key facts about knob and tube wiring:
-
Insulated copper conductors - The wires were insulated with rubber, cotton, or asbestos to prevent shorts and shocks. This was an improvement over earlier systems with bare wires.
-
Open air design - The wires were separated from each other and building materials, allowing air circulation to keep the wires cool.
-
Porcelain knobs - Circular porcelain knobs were nailed or screwed to surfaces to hold up and separate the wires.
-
Porcelain tubes - Straight porcelain tubes were used to guide wires through open spaces and protect them from damage.
-
No ground wire - K&T wiring did not include a ground wire, unlike modern electrical systems. This increased the risk of shocks.
The open air nature of K&T wiring was an advantage over early insulated cables. Keeping the wires separate and surrounded by air allowed heat to dissipate, reducing fire hazards.
When Was Knob and Tube Wiring Used?
Knob and tube wiring was prevalent from the 1880s to the late 1930s. It was the standard wiring method for homes and buildings during the electrification boom in the early 20th century.
Some key history about when K&T wiring was in use:
-
1880s - K&T wiring was first introduced as an early building wiring method. This open air, insulated wire system was a major improvement over previous dangerous methods.
-
1890s - As electricity became more widely available in cities, K&T wiring allowed homes and buildings to be wired for lighting and electric appliances. Its use expanded rapidly.
-
Early 1900s - During the electrification of rural America, K&T wiring enabled many remote farms and towns to get electrical service for the first time. It was economical and reliable.
-
1920s - Even as new wiring methods like NM cable were introduced, K&T continued to be the standard for wiring existing homes as electricity demand increased.
-
1930s - K&T wiring began to be phased out in favor of newer methods like BX and NM cable, which included grounding. However, a lot of K&T remained in use.
So in summary, K&T wiring powered electrical systems through the early days of electrification and continued to deliver electricity to homes and buildings through the 1920s and 30s.
How Did Knob and Tube Wiring Work?
The installation and components of knob and tube wiring systems followed specific techniques to maximize safety and reliability:
Installation
- Wires were run in straight lines parallel or at right angles along joists and beams. Following building framing provided support.
- Porcelain knobs were nailed or screwed to surfaces every 4 to 6 feet. Tubes were installed between knobs for wire runs through open spaces.
- Adequate separation was maintained between wires and any grounded metal surfaces. Air gap spacing prevented shorts.
- Splices were carefully wrapped in rubber tape and soldered for waterproof, insulated connections.
Components
- Knobs - Glazed porcelain, nailed or screwed to surfaces. Provided anchor points for wires.
- Tubes - Glazed porcelain or wood fiber tubes protected wires through open framing.
- Wires - Rubber or fabric wrapped copper conductors. Insulated and sized for electrical load.
- Insulators - Porcelain stands separated wires from wall surfaces and each other at ends and splices.
- Junction boxes - Enclosed splices and connections. Early ones were wood or cardboard.
Proper installation was crucial for safety. The components worked together to allow wires to be run safely throughout buildings.
How Reliable and Safe Was Knob and Tube Wiring?
When properly installed and maintained, knob and tube wiring provided reliable and safe electrical service. However, it did have some drawbacks compared to modern wiring:
-
No grounding - The lack of a ground wire meant risk of shocks from faults. Modern wiring grounds electrical systems.
-
Overloading - Heavy use could overheat wires run through combustible knobs and insulation. Fire hazard.
-
Brittle insulation - The rubber or cloth insulation dried out over time, becoming brittle and flammable. Danger of shorts developing.
-
Exposed splices - Early unsoldered connections could loosen, arc, and overheat. Potential fire starter points.
-
Difficult upgrades - Adding circuits required running new wires through limited space in walls and ceilings.
On the positive side, K&T's open air nature allowed heat dissipation, reducing fire risks compared to insulated cable. The porcelain components didn't deteriorate like early insulation materials. Overall, knob and tube systems had lower fire rates than even modern wiring when properly maintained. However, problems developed when overloaded or neglected over decades.
How Were Knob and Tube Systems Upgraded Over Time?
As electricity demand grew in homes over decades, knob and tube wiring required upgrades:
-
Pigtailing - New NM cables spliced into K&T circuits added capacity and grounded outlets. Mixed systems remained common for decades.
-
Splicing in junction boxes - Early unsoldered connections were redone in covered metal boxes for safety.
-
Circuit breakers - Fuses were replaced by circuit breakers that could handle increasing loads.
-
GFCI outlets - Ground fault circuit interrupters helped prevent shocks in ungrounded K&T systems.
-
Full rewiring - For heavy updating like kitchens or appliances, homes often had K&T replaced fully with modern NM cable and grounds.
-
Capping off - In accessible places like basements, K&T was sometimes left capped off for the future when doing partial upgrades.
While labor intensive, homes with knob and tube wiring continued to be upgraded bit by bit over the decades to take advantage of improving electrical technology and safety. Even today, some remnants of old K&T systems remain in buildings wired over 80 years ago.
Summary
From powering light bulbs in the 1880s to running refrigerators half a century later, knob and tube wiring served as the vital electrical circulatory system for homes and businesses through the early 20th century. This innovative open wiring method enabled the widespread adoption of electricity and improving standards of living during the electrification of America. While it has been outdated for generations, there's a good chance that those porcelain knobs and tubes are still hiding in walls, reminding us how yesterday's technology laid the groundwork for the electrical capabilities we rely on today.