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.)

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.

knob and tube electrical wiring
Knob and tube electrical wiring is no longer used to wire newly constructed homes. Due to its risk, many homeowners opt to replace knob and tube wiring in their homes. (Photo by Mike LaFollette)

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.

RELATED: Ask Angie: How Much Does It Cost to Rewire a House?

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.


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knob and tube wiring in attic
Knob and tube has separate hot and neutral wires that run parallel to each other and suspend in the air to help dissipate heat. (All photos by Mike LaFollette)

Knob-and-tube wiring still exists in some home built before 1940. Highly rated electricians describe the dangers of knob-and-tube and explain how they remove the outdated wiring.

Comments

I am a California Licensed Electrical Contractor. In paragraph #4 your article states INCORECTLY: ....As a result, outlets in a knob and tube home will have two prongs, not three, and won't support ground fault circuit interrupters (GFCIs).... A GFCI outlet measures the differential of the current on the black and white wires of the load. If the differential exceeds 3-4 milliamperes, the GFCI will trip. A GFCI outlet DOES NOT depend on current being present on the ground (3rd prong) to trip. Three conditions will trip a GFCI outlet: 1) The differential just cited; 2) Any current present on the third prong; 3) A neutral to third prong fault (that is, current flowing from the white wire to the third (normally grounded, but not in this case) prong. All modern GFCI outlets come packaged with a sticker to use in the above case with the words: "NO EQUIPMENT GROUND" for use with two wire (ungrounded) circuits. So YES, you may use a GFCI on a two wire circuit to protect personnel from ground faults (electric shock due to current leakage from a device). You still don't have a ground, but you do have shock protection. Note that ground-fault protection provided by a GFCI is NOT SURGE protection. Not all surge protectors (usually outlet strips with a surge protector built in) will not function correctly on a two wire circuit, with or without a GFCI. So if you want to surge protect your fancy TV and audio equipment, have a true three-wire grounding circuit installed.

Hi Jeff, We have updated the article. Thank you for bringing this to our attention!

My partner and I just bought (2014) a house built in 1889/1890. Going into this house we knew it was a fixer-upper, but that's what we wanted. The house we bought sat empty since 1980 when the owner passed away; He bought it in the mid 1930's. After buying it and cleaning it up a bit we found out that it previously had a fire, which most older home have had before. On the Tuesday prier of closing on the house we had a fire in the kitchen area when we both where on the third floor in one of the back bedrooms. Luckily we cot it in time and was able to put it out without any major damage. Later found out that it was caused by Knob and Tube wiring. Immediately we hired an electrician to have our entire house rewired (6,000 sq ft. cost us around 50,000 dollars.): That included Labor, Electrical, re-drywalling, moving a few things around, adding in outlets, light fixtures, etc.etc. We where lucky that we caught the fire in time that we did or we could have quickly watched our dream home go up in smoke. So i tell anyone if you have it change it, because spending a few grand now can save you time, money, and stress. We are blessed to have not had any kids at the home at the time, because it could have been a lot worse then it was.

I agree with Bryce that the reviewer used generalizations in his assessment of the safety of Knob and Tube methods. Knob and Tube wiring does not present a more significant safety concern than type NM installations. A homeowner that intends to modernize a residence will find that the overwhelming majority of electrical professionals will refuse to make modifications to knob and tube systems or even to integrate them into a service upgrade because of the difficulty of tracing and trouble shooting these systems specially if they have previously been modified. The NEC does not allow for soldering conductor junctions any longer and the true statement that the rubber insulation on the conductors will usually fall away at a touch means that continuing to add to or maintain these systems will eventually leave the homeowner high and dry. While knobs and tubes buried below 16" of blown or batted insulation are, I believe, safe, locating a broken wire or tap or crushed tube that occurred while insulating can be time consuming and more expensive than an upgrade before insulating. My experience includes thirty five years in commercial/ residential/ industrial electrical and HVAC.

I have to agree with Bryce in both his statements. Good info, sir. And valid.

This article is full of suspicious details like "basic installations only allow for 12 circuits in a home", which are simply not correct general assumptions. The bit about "exposing bare wires to air and moisture" is complete nonsense, since K&T wiring is copper and soldered at the junctions and conductors are widely spaced. K&T is more resistant to water than modern NM "Romex" which has a paper core and conductors right up next to each other. The ceramic tubes will outlast plastic Romex by thousands of years. K&T installations can have problems, but not the ones outlined in this article. In states including California an inspection is required prior to insulating. A home with K&T can be upgraded smartly with an AFCI breaker, new circuits for things like the dishwasher, a complete inspection for past poor work, and breaker derating to a conservative value for the wire size. The author cannot find a history of actual fires in inspected K&T, because there is no history of those fires. "If wiring insulation has cracked and caused any fire damage" is just fear uncertainty and doubt.

People lets just use good common sense. Today we encapsulate conductors in raceways such as EMT, GRC etc. Wires are spliced in metal boxes with covers. This raceway system is grounded adding further safety / unlike knob and tube wiring. Knob and Tube wiring is exposed to combustible materials. NOW what makes sense to you?

Modern NM cable has a paper core: that's combustible also. Knob and Tube wiring has an excellent safety record, when it's not pushed beyond capacity. Soldered knob & tube connections don't need a metal box with a cover, as there is nothing to work loose or spark. What makes sense to me? To inspect Knob & Tube wiring, conservatively reduce the ampacity especially if insulated, and follow the building code (which in my area allows insulating right over Knob & Tube).

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