Q: What is Knob & Tube Wiring?
Knob and Tube refers to a type of wiring commonly used from the late 1800’s until the 1940’s. At the time it was cost effective and appropriate for the electrical systems of most homes. It is still found in many older homes today.
Knob and Tube Components:
Wire: K&T uses copper wire but only consists of a hot and a neutral strand. Commonly it is 14 gauge and supported a maximum of 15amps. Homes were often wired with a single circuit for the entire house that did lights and few or no receptacles. As electrical needs grew and plug-in appliances became more commonplace, knob and tube wiring became obsolete by the mid 1940’s.
Knob: Circular Porcelain disks with a center nail hole are attached to framing to hold the wire in place or allow it to wrap around and change direction. These “knobs” hold the wire away from any wooden structures and allow it to dissipate heat and insulate it from touching flammable surfaces.
Tube: Porcelain tubes are placed where wires need to go through joists or framing structures. These insulate the wire from the flammable wood.
Loom: Wires are wrapped in a flexible fabric or rubber casing to insulate and protect them. This was often cotton coated in asphalt or even asbestos. This served to protect the wire especially where it went behind wall spaces or attached to fixtures.
Is K&T dangerous?
Knob and Tube wasn’t inherently dangerous when it was installed but is obsolete and no longer allowed for modern construction. It should be replaced for several reasons:
The biggest concern with K&T systems is the lack of a ground wire. This presents a shock hazard and needs to be addressed from a safety standpoint.
This lack of ground is especially problematic for bathrooms and kitchen areas that require GFCI/AFCI protection or appliances requiring a three prong plug. These areas are often modified to try to accommodate modern standards.
Modern homes have higher electrical needs so face limitations in amperage for newer appliances and additional outlets and receptacles.
Modern layouts use more intricate circuits, often with multiple branches to route electricity to more outlets and appliances. Many knob and tube systems wired the entire home through a single looped circuit.
K&T systems can pose fire hazards when trying to insulate around them. Modern blown-in insulation is especially an issue as materials can touch uninsulated wires or allow heat to build up.
The loom materials often become brittle and crack with age, exposing live wires. Additionally, the cloth can be attractive to rodents and other pests and invite chewing. Loom materials are often not rated for exposure to moisture, so damp areas like crawl spaces can be an extra hazard.
By nature K&T is more rigid; wires are bent by a “knob” so offer less flexibility than modern Romex. Modern wiring has better flexibility to be fed through walls and throughout the home safely.
Standards for splicing wires together have changed, K&T required hard splicing but allowed for these to be wrapped or hidden, modern code requires a junction box that is accessible & visible. These hidden splices are often wrapped in plastic tape and are a fire hazard.
Knob and Tube systems are very likely to have had modifications made over the years to help them adapt to new requirements, these updates often create additional shock hazards or unsuspected dangers.
Insurance companies may refuse to insure homes with Knob and Tube Wiring or may have higher premiums for coverage.
What should I do if I have knob and tube wiring in my home?
A first step is to have an electrician check if it is still in use and how widespread it is throughout the home. It is common that knob and tube may be disconnected but left in place or that it may have been partially replaced as renovations and upgrades have been done. It is important to assess both conditions:
Disconnected but Left Behind: Most often this is a time saving measure to not remove the old K&T wiring. Unless it is being left in place to avoid extensive damage to the home, it is always best to remove old wiring. It prevents any accidental reconnection to a circuit or eliminates any missed connection that might still allow for a live circuit.
Mixed Systems: These are common, especially if part of the home has been renovated or an addition has been added over the years. Larger appliances and HVAC systems may have been upgraded because they require a thicker gauge wire but lighting and regular outlets may remain Knob & Tube. Almost any modification or mixing or new and old systems can create problems.
Takeaway:
If you have Knob and Tube wiring ideally it should be replaced, this addresses any safety issues and provides a modern upgrade to the entire system. Rewiring the entire house can be an expensive project but actual costs will depend on the layout of the home itself. Costs often depend on how easily wiring can be accessed or how much drywall and interior structures need to be removed versus if new wires can be fished through existing walls.
More commonly, we find homes that have partial knob and tube systems still in place so these may not require a full rewire. If you suspect you have knob and tube wiring in your home, give us a call and we can start with an estimate to assess options and determine next steps.
Ask the Electrician is a column we publish with our advertising partner WCHL/Chapelboro. In “Ask The Electrician,” Scott answers common questions about electrical work, from switches and outlets to safety issues and preventative maintenance. If you have a suggested question for us to answer – or need an electrician – please reach out to our office.
See the article on Chapelboro.com: Older Wiring and Modern Updates