Knob And Tube Wiring: A Potential Fire Hazard

Knob and tube wiring, often abbreviated K&T, was used from when homes were first electrified (starting in the late 1800s) up to the mid to late 1940s. This type of wiring has only 2 conductors (a hot and a neutral) which run independently, yet normally parallel within 6”-12”, of each other. Unlike modern Romex/Non-Metallic sheathed wiring which has all of its conductors assembled together, the hot and neutral conductors in knob and tube wiring were kept separated until the wiring met a switch, receptacle, light fixture, or appliance. Knob and tube wiring is very easy to identify since its outer covering is round rigid-cloth often light to dark grey and you’ll normally see 2 similar parallel conductors near each other. Where this type of wiring passes through joists or plates, a white ceramic tube is often installed. Where the wiring makes a splice or otherwise needs support, a white ceramic knob exists. K&T wiring was installed in this fashion to prevent a short circuit should the 2 conductors contact each other unexpectedly.

This photo shows knob and tube wiring leaving a modern metallic junction box and passes through a floor joist. No tubes were installed in this installation where the wiring penetrates the joist.

Knob and tube wiring, on its own, is not inherently a problem. It becomes a problem, however, when modified by unqualified people or when the quantity of modern conveniences in the home draw more power than the installed wiring can handle. Another disadvantage of K&T wiring is that it was not a grounded type of wiring by design. While the home’s main electrical service would be grounded, the actual branch circuits were not grounded. A branch circuit is wiring that leaves a breaker panel or fuse box to power the home’s lighting, receptacles, appliances, etc.

Branch circuit grounding is a safety requirement added to residential electrical standards back in the 1960s. Modern branch circuits should be grounded for safety reasons. Certain appliances, such as computers, power strips, refrigerators, etc. need to be powered via a grounded circuit to protect the appliance and the user from potential shock. Trying to power an appliance that came with a grounded plug with an ungrounded branch circuit presents a safety hazard. In older (say, pre-1965) homes that were originally wired with older ungrounded wiring types, sometimes oblivious home owners or unqualified professionals replace the original 2 wire (non-grounded) type of receptacle with a 3 wire (grounded-type) of receptacle. In many cases, based upon my experience, the actual wiring to the receptacle doesn’t get replaced. This is a safety hazard since the receptacle, which people will assume is grounded, is actually not grounded.

I also very commonly see power strips and surge suppressors in older homes. If the circuit powering the power strip or surge suppressor isn’t grounded, the appliances plugged into it have little to no protection.

The photo to the right shows knob and tube wiring along a wall and at the underside of a floor. Notice the ceramic knobs installed for support and ceramic tubes where the wiring passes through joists and plates.

When 2 wire receptacles are replaced with 3 wire receptacles, new grounded wiring should be installed to provide the extra grounding conductor that was missing with older 2 wire systems. This very often was not done in older homes.

QUIZ: You walk into a room of a pre-1965 home and see the above receptacle installed in the wall. Is it grounded? The answer is “you have no idea without testing the receptacle with a proper tester”. If it powers a lamp, for example, you will know if the receptacle is powered but still have no idea if the receptacle (and its branch circuit) is grounded. The cords on lamps, for example, are not grounded and don’t require a ground to operate. Most home owners and buyers sometimes wrongly assume that if the receptacle has 3 prongs, that means it is grounded. This is definitely not true and can present a safety issue. Two conductor wiring (such as older Romex which had only 2 conductors but no grounding conductor) powering a 3 prong receptacle presents the safety issue of an open ground. The receptacle’s ground pin is therefore not connected to anything so the ground is called ‘floating’.

Another consideration with K&T wiring is that it was designed to be air cooled, which means there should be no thermal insulation (or other materials) around the wiring. Sometimes while inspecting an older home, I will find that insulation was added in the attic or basement, yet live K&T wiring still exists in these areas. When wiring is powered, it warms up. The thermal insulation can hold this heat to the wiring instead of letting it dissipate to the air around it. Overheated wiring can lead to house fires.

Splices in K&T wiring were often not properly performed where it connects to more modern wiring. Simply wrapping and electrical taping the splice is not a proper splicing method. Wire splices should be wire nutted and enclosed in approved junction boxes. Sometimes, old splices in K&T wiring were soldered and then taped. While the soldering equates to wire nutting the splice, the splice itself is often exposed. Over time, the tape at the splice or the outer protective sheathing of the K&T wiring deteriorates, thus leaving the actual live metal conductors exposed. This, of course, is a serious electrocution risk.

Many insurance companies will no longer insure homes with active knob and tube wiring. The reason is due to the higher risk of house fires related to this old wiring. As wiring gets old, it can get brittle. Brittle wiring can lead to arcing which can lead to a fire.

As part of a home inspection, when/if I see knob and tube wiring, I use a voltage sniffer on both conductors to determine if they are still live (energized). If so, I take a photo to include in the home inspection report and make a note to my client about its presence and explain its issues. I know of instances that buyers didn’t find out that they were being denied insurance coverage until reaching the closing table. I recommend buyers check with the home insurance carrier they are planning to use to verify coverage, and if need be, get quotes from several carriers that will insure knob and tube wired homes. Doing this right after the home inspection can save you headaches down the road.

As part of a home inspection, the home inspector should report the type(s) of wiring found within the structure. In most modern homes, it will be 3 wire (grounded-type) Romex/NM wiring. In pre-1965 homes, it may be a combination of K&T, 2 wire (ungrounded Romex/NM wiring (used from the mid to late 1940s to the mid-1960s), and/or modern 3- or 4-wire grounded Romex/NM wiring.

In many cases, a home’s knob and tube wiring may no longer be in service and newer modern grounded wiring was added to power the various circuits. K&T wiring may be visible, but may have been removed from service (no longer used). A qualified home inspector or electrician can determine whether this wiring is still live or not. As long as it is no longer live, the remaining K&T wiring is no longer an issue. As part of a home inspection, we are doing a non-invasive visual inspection. Most of the home’s wiring is contained within the walls and ceilings; thus, an inspector can only inspect and identify what he can see.

As a real estate agent, it is critical that you are aware of issues common in older homes, such as knob and tube wiring. When showing an older home to a buyer, look for the signs of this type of wiring in places such as the basement or attic. Make sure when it comes time for the buyer’s home inspection, that the inspector properly identify this type of wiring and determine if the visible K&T wiring is live or not.

A related article that you may find of interest is Electrical Inspections Of Older Homes.

Matthew Steger is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), and an electrical engineer. He can be reached at matthew@thehomeinspectorsnotebook.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.