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I am totally a house geek. In fact, I own several historic code books, and I enjoy internet sleuthing all things houses. Naturally, I enjoy writing my own articles for others. Click here to go down past the cool stuff and learn what is knob and tube wiring anyway.
First, knob and tube wiring actually only referred to the two wire system concealed in the walls. On the other hand, the NEC (National Electrical code) referred to exposed knob and tube wiring as OPEN WIRING ON INSULATORS.
Chapter 2 in the 1947 NEC defines Open Wiring on insulators as
So, “Open wiring on insulators” is the exposed type and knob and tube is the concealed type.
Second, some knob and tube contains asbestos. The potential for fire associated with open wiring limited the wiring types to those with heat, or fire resistant qualities. Specifically, this included rubber insulated, slow-burning, or asbestos varnished cambric. Cambic is essentially the black asbestos embedded cotton cloth around the wire.
Third, the NEC did not require boxes for knob and tube splices. Specifically, the NEC suggested that “it is better practice to make no splices” in K&t wiring. However, they allowed unboxed soldered and taped splices such as the “Western Union Splice."
Fourth, the NEC did not approve knob and tube wiring in the following locations:
Commercial garages and hazardous locations makes sense. However, it took a little more digging to solve the knob and tube prohibition in theatres and motion picture studios.
Apparently, before 1950 most film contained nitrate. Nitrate is highly flammable and theatre fires became a very real and common problem. In fact, the film booth itself had specific requirements. Only the electrical equipment and devices which was absolutely necessary to be in the booth was allowed in the booth. Accordingly, inspectors prohibited switches, overcurrent protection devices and other equipment from the booth.
This would presumably be to prevent sparks in the booth. Similarly, modern code prohibits ignition devices within 18 inches of a residential garage floor.
Lastly, conduit wiring existed Since at least 1912.
I could not find an exact date for the invention of conduit. However, I found at least one reference to conduit wiring in 1912. That means for most of the life of k&t systems, there were safer alternatives. However, conduit wiring was twice as expensive in materials and labor, and k&t was often chosen for its price.
If you want to check out this full estimate from 1912 here is the link.
Knob and tube wiring is a wiring system that is found in older homes (built in the late 1800’s to about 1940). It consisted of two wires (copper wire)- one hot wire, and neutral wire. Manufacturers used varying types of insulation, such as rubber insulation, or asbestos treated cotton insulation.
Electricity is inherently a fire hazard, especially in timber homes. Accordingly porcelain knobs and tubes insulated the wires from the combustible wood framing.
These glass or porcelain knobs, or ceramic knobs suspended the wires at least 1/2 inch from the surface of the wood. The knobs were also referred to as cleats.
As stated, knob and tube consisted of two wires, the live wires, and the neutral wires. These systems, just like most older electrical systems, did not have ground wires. Ground wiring code requirements did not begin until 1956. Furthermore, the NEC did not require ground wires on all circuits until 1962. This lack of a ground wire is the most significant con of k&t systems.
The ground wire bonds all metal non current carrying components together and back to the main panel. In the event of ground faults, this wire carries the flow of electricity back to main panel allowing the circuit breaker to trip.
Without a ground wire present, non current carrying metal components, such as the metal frame of an appliance, will hold current during a fault, until it becomes grounded. If a grounded homeowner touches the frame, the electricity will flow through them. Obviously, this is also known as an electrical shock.
Insurance companies and home inspectors tend to insist that knob and tube wiring is unsafe. However, there is nothing inherently more unsafe about knob and tube wiring than any other electricity in the home. The mere fact that it is an old wiring system does not make it unsafe.
We already talked about the the lack of a ground wire. It’s important to realize that this makes it less safe than if it had a ground wire, but not UNSAFE.
There are two common issues regarding the safety of knob and tube wiring. First, is electrical work done by homeowners and amateurs. Second is home insulation added around the wires.
For example, an older house, built in the early 1900s did not have as many electrical outlets or light fixtures. Over the last 100 years it is likely that the homes electrical went system went through many repairs and upgrades.
Unfortunately, many homeowners improperly splice, and otherwise modify the system creating safety hazards. In other words it is not he original installation that is a safety hazard, it is the improper new wiring that is the safety hazard.
The other problem is homeowners and contractors blowing insulation over the wiring. Unsurprisingly, many professionals argue that installing insulation over maintained knob and tube poses no hazard. Regardless, it has been a violation of the national electrical code since 1987 to do so.
The theory is that the wires relied on the free air, or open air around the wires to dissipate heat. The addition of insulation around the wires causes excessive heat and fires. True, or not, it is unlikely the code will ever change, and that is what we must work with.
Maybe.
Unfortunately, most home inspectors are already under the false notion that knob and tube is just unsafe, and they will just recommend replacement without inspecting it.
Even still, a home inspection is only going to inspected a limited amount of the k&t wiring. They are only going be able to inspect what is visible at the time of inspection such as wiring in the attic (if not covered with insulation), on exposed basement floor joists, or in the electrical panel.
Of course, home inspectors can not inspect any wiring or mechanicals concealed in the walls.
Even if the wiring is in good shape, and it hasn’t been touched, you'll probably want to replace it. The fact that you can not insulate over it is reason enough. Of course insulation and air conditioners were not in use during the knob and tube ear.
The only way to insulate the attic and wall cavities of an older home is to replace the wiring.
If you want insurance coverage on your home, which is generally required anyway, you are going to need to upgrade to modern wiring.
Keep in mind you can repair any problems and new wiring can be added for modern appliances, but the inability to insulate your house, or insure your house are pretty sound reasons to replace knob and tube.
