Is Knob and Tube Electrical Wiring Safe?
This contractor replaced knob and tube wiring, installed a new panel box and separated some high-volume rooms that were on the same circuit. (Photo courtesy of Angie’s List member Kari B. of Riverdale, Md.)
If you live a home built prior to 1950, chances are you have what's known as knob and tube electrical wiring. Electricians no longer use this method to wire new houses, and it's widely believed to pose a hidden risk to homeowners. So, what's the verdict? Is knob and tube wiring safe or do you need a professional to rip out and replace your existing system?
Knob and tube's powerful history
Knob and tube (K&T) wiring was the go-to method for electricians in the United States from the 1880s to the late 1930s. Many pros continued to use this method through the 1950s, '60s and even '70s for new home construction. Now, many homeowners looking to upgrade their electrical system or complete a renovation are discovering "hidden" K&T wiring in their homes and wondering how to proceed.
Knob and tube wiring gets its name from the ceramic knobs used to hold wires in place and ceramic tubes that act as protective casings for wires running through wall studs or floor joists. Instead of the three wires found in modern electrical installations, K&T wiring has only two — a black (hot) wire and a white (neutral) wire. This means there is no ground wire in the system for excess charge or in the event of a short.
As a result, outlets in a knob and tube home will have two prongs, not three. However, many electricians opt to install ground fault circuit interrupters (GFCIs) so there will be some three-pronged outlets. GFCIs can trip when they sense an imbalance between the hot and neutral wires. So even though they are not grounded, they are presumed safe because they can cut the electrical current when needed.
The hot and neutral wires in a K&T setup are run and sheathed separately and approximately an inch apart, rather than being bundled together as with new wiring. Most knob and tube installations are restricted to a 60-amp service.
Problems with knob and tube wiring
If a K&T system is intact and working, it poses no immediate risk to you and your family. Problems often arise, however, because of the age of the installation or modifications made to the electric system by a previous owner or unscrupulous electrician.
One of the most common problems with this kind of wiring is its insulation, which is made of rubber instead of plastic. Over time, the rubber degrades, exposing bare wires to air and moisture, in turn increasing the chance of a short or a fire.
Extra circuits are also a problem because basic K&T installations only allowed for 12 circuits in a home. Often, homeowners who needed extra circuits would pay contractors to add new circuits at the panel or simply splice into an existing wire. Both of these modifications run the risk of overloading the system.
You also need to watch for older homes with renovations. A popular electrical scam making the rounds involves attaching new wiring to a switch or socket, which is then checked by a home inspector who declares the house free of K&T. In fact, the new wiring only runs the height of the wall and connects to K&T in the ceiling.
Building codes and insurance issues
The 2008 National Electric Code addressed some issues with K&T wiring, most notably its high heat dissapation that poses a fire hazard when combined with fiberglass insulation. As a result, the NEC now requires that K&T wiring not be in "hollow spaces of walls, ceilings and attics where such spaces are insulated by loose, rolled or foamed-in-place insulating material that envelops the conductors." It's important to note, however, that this code is not mandatory. States can choose to follow it at their own discretion.
Knob and tube wiring also poses a problem for insurance companies. Some demand higher premiums from customers with this kind of wiring in their homes, while other companies refuse to insure homeowners at all until the wiring is changed.
The to cost replace knob and tube wiring
If you want to replace knob and tube wiring in your home, you need to hire a professional electrician for the job. Expect to pay around $8,000 to $15,000 to rewire a 1,500- to 3,000-square-foot home.
You'll also need to upgrade the service to your home to at least 100 amps, though many contractors will recommend 200-amp service to account for any future electrical needs. If wiring insulation has cracked and caused any fire damage, your costs will increase.
So, is K&T wiring safe? Not exactly. The risk of faults and fires, coupled with difficulty finding home insurance makes replacement your best option.
Editor's note: This is an updated version of an article originally posted on Oct. 9, 2013.