Introduction to Knob and Tube Wiring
Knob and tube wiring was commonly used in homes and buildings in North America from about 1880 to the 1930s. As the name suggests, this early electrical system uses ceramic knobs, tubes, and cloth-covered wiring to deliver electricity throughout a building.
While knob and tube wiring was safe when originally installed, it can become hazardous over time as insulation breaks down. However, understanding how this antique wiring system worked can help homeowners and electricians evaluate when and how to safely update old electrical systems.
In this article, I'll give an in-depth look at:
- The history and rise of knob and tube wiring
- How knob and tube wiring systems were installed
- The components used in knob and tube wiring
- The pros and cons of knob and tube wiring
- Options for upgrading from knob and tube wiring
Let's start by looking at the origins of knob and tube wiring in the early 20th century.
The History of Knob and Tube Wiring
The late 1800s saw electricity become more widely adopted for lighting and powering devices in buildings. However, early electrical systems used basic splices and connections that left wiring exposed and vulnerable to shorts and fires.
Thomas Edison began developing a complete electrical wiring system for buildings in the 1870s. However, it was not until the 1880s that insulated copper wiring and wall-mounted outlets became widely available.
In 1898, the National Board of Fire Underwriters created the National Electrical Code (NEC), which established the first standards for safe electrical installations. The initial NEC guidelines described an early version of knob and tube wiring.
By the early 1900s, knob and tube wiring had become the standard for new construction. The American economy was booming, and more families could afford electrified homes that used ceiling lights, appliances, and wall outlets.
While knob and tube remained dominant into the 1930s, new wiring methods like BX (armored cable) and NM (non-metallic sheathed) cable eventually replaced it by the 1940s and 1950s as electrical needs continued to grow. However, you can still find knob and tube wiring in many older homes today.
Next, let's look at how electricians originally installed these antiquated wiring systems.
Installing Knob and Tube Wiring
Knob and tube wiring gets its name from the knobs, tubes, and wiring that make up the system:
- Ceramic knobs - These insulators separate and support the wiring.
- Tubes - Hollow spaces in wood framing or ceramic tubes protect wires running through open spaces.
- Wiring - Insulated copper conductors are covered in cloth or rubber insulation.
While the National Electrical Code provided overall guidelines, the specific methods used to install knob and tube varied. However, the general process involved these key steps:
- Running vertical wires through wooden stud cavities using porcelain knobs to separate wires from wood framing.
- Passing horizontal wiring through wooden joists or protective porcelain tubes.
- Using junction boxes made of wood or cardboard to join wiring.
- Having wires enter porcelain insulator tubes when exiting or entering metal junction boxes.
- Bonding neutral and ground wires to metal junction boxes for grounding.
- Installing ceramic sockets as connection points for light fixtures or devices.
Proper installations kept live wires separated from wood framing and grounded systems for safety. While not grounded by modern standards, these practices helped reduce fire risks when knob and tube was new.
Let's look closer at the knobs, tubes, and wiring that gave this system its name.
Knob and Tube Wiring Components
While knob and tube wiring may seem crude by modern standards, it represented major safety improvements for its time. Key components in these early electrical systems included:
- Made of glazed porcelain or ceramic to resist moisture and provide insulation.
- Featured grooved centers to separate wires and prevent contact.
- Installed at regular intervals to support and separate wiring.
- Hollow spaces through joists and studs allowed wires to pass safely.
- Porcelain tubes were also used to protect wires.
- Prevented contact between wiring and wood framing.
- Typically insulated with rubber and/or cloth coverings.
- Used 14 or 12 gauge solid copper conductors.
- Red insulation indicated live wires, black for neutral/ground.
- Insulation resisted moisture and abrasion when new.
- Originally made of wood or cardboard.
- Later had porcelain insulators for connections to metal boxes.
- Joined multiple circuits and wiring.
Along with ceramic sockets to connect devices, these components delivered electricity safely throughout homes and buildings when properly installed and maintained.
Next, let's look at some of the pros and cons of knob and tube wiring.
The Pros and Cons of Knob and Tube Wiring
Knob and tube wiring provided a number of advantages over previous electrical systems:
- Less prone to sparking and arcs than early splice-based wiring.
- Separates live wires from flammable surfaces.
- Ceramic insulation resists moisture and heat.
- Tubes and knobs protect wires from contact and abrasion.
- Listed as fireproof by insurance bureaus when properly installed.
However, knob and tube also had some disadvantages even when new:
- Not designed to handle high wattage loads.
- Lighting circuits were typically limited to 5 or 6 outlets.
- No grounding wires for shock protection.
- Complex installations prone to poor connections.
- Rubber/cloth insulation becomes brittle over time.
- Lacks capacity for major appliances and modern loads.
While considered high quality for its time, knob and tube wiring is now outdated and can be downright dangerous if left unmodified. Let's look at some upgrade options.
Upgrading from Knob and Tube Wiring
If your home or building still relies on antiquated knob and tube wiring, it's smart to consider upgrades for safety and capacity. Some options include:
- Running new NM cables and boxes while abandoning the old wiring in place.
- Fully replacing old wiring with modern systems and grounding.
- Installing AFCI and GFCI circuits to add protection.
- Re-insulating old wiring if it's in good condition.
- Upgrading electrical panels and service for added capacity.
Major upgrades often provide an opportunity to add more lighting circuits, grounded outlets, 240V circuits for large appliances, and increased overall capacity.
I always recommend consulting a qualified electrician to assess your existing knob and tube systems and provide repair or replacement solutions tailored to your needs and budget.
While rewiring can be a major project, it helps ensure your electrical system will meet today's needs and safety standards.
Key Takeaways on Knob and Tube Wiring
To summarize, here are some important facts about knob and tube electrical wiring:
- First appeared in the 1880s and was widely used until the 1930s.
- Allowed early electrical systems before modern insulation and grounding.
- Features ceramic knobs, tubes, and insulated copper wiring.
- Provided greater safety than previous splices and exposed wires.
- Was considered fireproof when properly installed.
- Now outdated and potentially hazardous due to deterioration.
- Can often be upgraded to modern wiring for safety and capacity.
While knob and tube served homes well for decades, current homeowners shouldn't rely on antiquated and aging electrical systems. Upgrading to modern wiring and safety systems remains the wisest choice.
I hope this article gave you a helpful overview of early knob and tube wiring and how it powered homes and buildings through the early 20th century! Let me know if you have any other questions on working with older electrical systems.