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Knob and tube wiring (K&T wiring) was the most commonly used wiring system from the late 1800’s until the 1940’s. It gets its name from the porcelain knobs and tubes that are used to hold the wiring in place. In this blog, I’ll discuss what it is, the dangers of knob-and-tube wiring, how to identify it, and what to do if your home has knob-and-tube wiring. 

knob and tube wiring
knob and tube wiring

Unlike most other wiring systems, when K&T wiring was installed, the hot wire and the neutral wire were run separately. Also, as in other older wiring systems, there is no ground wire in a knob and tube wiring system.

When this wiring was initially installed in homes, there was nothing inherently dangerous about it. What causes it to potentially be dangerous today are mainly age, abuse, and the greater electrical demands of the modern home.

If you own a home built around 1950 or earlier, then it is possible that it has knob-and-tube wiring. It would be a very good idea for you to know if there is any of this wiring in your home so that you can have it inspected, and have the proper corrective action taken, if necessary. This article will help you to learn how to recognize knob and tube wiring and will tell you what to do from there.

What is Knob-and-Tube Wiring?

Knob-and-tube wiring was a very popular wiring system used in millions of homes from the late 1800’s until close to 1950. There are several things that make it different from most other wiring systems. First, the hot (black) and neutral (white) wires are run separately, and there is no ground wire. With most other wiring systems, the hot, neutral, and ground wires are all contained together in an outer insulating jacket.

knob and tube wiring

Knob-and-tube wiring

romex cable- knob and tube wiring

Modern wiring

(The photo on the left is knob-and-tube wiring, and the photo on the right shows some modern wiring with the hot, neutral, and ground wires all wrapped together in the yellow plastic insulating jacket.)

knobs and tubes

Knob-and-tube wiring was designed to be installed and run through the open air of the attic, basement, and/or the crawlspace. This is because wires heat up as they carry electric current. Because the hot and neutral wires are run separately instead of being in contact with each other (as they are in other wiring systems), there is air surrounding the wires which is able to remove the heat generated, thus helping to keep them at a safe temperature.

To keep the wiring suspended in the air and not lying against wood or other building materials, porcelain knobs and tubes were developed to essentially hold the wires suspended in the air. The knobs were designed to be attached to a joist with a nail and the wiring would be wrapped around it to hold it in place above the rafters as it runs through the attic. The tube, as the name implies, is a round porcelain tube that is inserted into a hole drilled through a joist. The wiring is then run through the tube.

When the knob-and-tube wiring was being installed, the electrician would wrap one wire around the other, and solder every splice in order to make a secure and permanent connection. 

A properly-installed knob-and-tubing system was a perfectly safe way to power a home, and, assuming the wiring was not damaged or modified, it could go on safely powering a home for decades – perhaps for a hundred years in ideal conditions. Unfortunately, for most homes, ideal conditions do not exist. Also, the electrical demands on homes has increased significantly in the last 80 to 100 years. That brings us to the next question.

Why is Knob-and-Tube Wiring Dangerous?

I will preface this section by telling you that not all wiring of this kind is dangerous or a fire hazard. In the paragraphs that follow, I explain the most common dangers of knob-and-tube wiring. Please remember as you read, that not all homes with K&T wiring systems will have any or all of these dangers. As I list the dangers, I am not trying to scare you, but to inform you of the possible hazards to your home if it has this type of wiring in it.

As long as it was installed properly, K&T wiring was perfectly safe to provide for the small electrical demands of homes back in the day. However, due to all of the wonderful inventions and innovations that have come along, electrical demands have increased. Things like air conditioning, electric heat, refrigerators, electric ovens, clothes dryers, and water heaters have significantly increased the demands on a home’s electrical system. This increased demand has caused knob-and-tube wiring systems to be strained beyond what they were designed to handle. Higher demands mean higher current which leads to higher temperatures and accelerated breakdown of the rubberized insulation on the wiring. New products and innovations have also required new wires to be run and additional circuits to be added to the home. Sometimes, these new circuits were spliced into the existing knob-and-tube wiring incorrectly. This improper splicing creates additional problems besides just the increased demands on the wiring.

Over time, the heat in the attic, the heat generated by the electrical current flowing through the wires, and time itself will make the rubber insulation on the wiring brittle. This insulation eventually cracks and can fall off of the wire, leaving a bare live wire exposed. I have seen several homes that have had bare live wire exposed in the attic due to brittle insulation crumbling off of it. (See the photos below.)

damaged knob and tube
knob and tube wire | Morgan Inspection Services
knob and tube wiring
damaged knob and tube wire

This is an obvious fire hazard. I have also seen insulation that has been damaged by rodents in the attic which leads to the same issue. I have even seen a rat that appeared to have been electrocuted years before as it chewed on the insulation on some wiring. Another thing that can happen over time to this wiring, is that it can stretch and sag until it is in contact with wood in the attic – another fire hazard.

As mentioned above, K&T wiring is designed to be cooled by the air around it. Unfortunately, in an attempt to make their homes more energy efficient, many people have unknowingly created a fire hazard by covering up their knob-and-tube wiring with attic insulation. This prevents the air from cooling the wiring which can cause the wiring to overheat which can lead to a fire. Knob and tube wiring should NEVER be covered with attic insulation. It must be allowed to have its heat dissipated into the surrounding air.

Another problem that can be caused by insulating over knob-and-tube wiring is that as people go up into the attic and move around over the years, they will not be able to see the wiring, and can, therefore, damage the wiring as they step on it or bump or rub against it.

The issues listed above are the main hazards associated with knob-and-tube wiring. In order to keep your family and your home safe, it is important to know if you have any of this wiring still in use in your home today.

How to Know if Your Home has Knob-and-Tube Wiring

The simplest way to know if your home has knob-and-tube wiring is to look for the ceramic knobs and tubes wherever wiring is visible such as in the attic, basement or crawlspace. If you find any then it would be a good idea to have it inspected by a licensed electrician to determine its condition. If you have an older home but are not able to determine if you have any of this wiring, then it is advisable to have a home inspector or licensed electrician inspect your electrical system to determine if you have any of this wiring still in use and what its condition is.

If you look for this wiring yourself, there are a couple of things to keep in mind. If you don’t see any signs of knob and tube wiring, that does not mean that it is not there because it is possible that the accessible knob and tube wiring in the attic, crawlspace or basement was replaced, but there could still be some in the walls. On the other hand, if you see the knobs and tubes with wiring still attached, it is possible that it was replaced with more modern wiring, but was just never actually removed from the home, even though it is no longer in use. One indication that this is the case is if you see the knobs and tubes, but also see a lot of newer, more modern wiring crisscrossing your attic or basement. The only way to be certain of your situation is to have your home inspected by a professional. (The photos below show some typical knob-and-tube wiring.)

knob and tube wiring
knob and tube wire | Morgan Inspection Services
knob and tube wiring
knob and tube wire

What Should You do if Your Home has K&T Wiring?

If it has been discovered that your home has live knob-and-tube wiring, then there are several other things that you need to know in order to make a fully-informed decision, including the following:

  1. What is the condition of the wiring?
  2. Has it been improperly modified over the years?
  3. Is it covered by attic insulation?

The only way that you can know the answers to all of these questions is to have it fully inspected by a licensed electrician. After the electrician has inspected your home’s wiring, he or she will be able to give you a recommendation as to the best course of action that you should take. To my knowledge, there are no state or national codes that require K&T wiring to be replaced, so your decision should be based upon what your electrician finds.

Whether you already live in a house with knob-and-tube wiring or are purchasing a home with this wiring, your course of action should be essentially the same – have it inspected and get the recommendation of an expert. If you are considering purchasing a home with this wiring, then there is one more thing you should be aware of. While it may not be an issue to some insurance companies, there are some companies that will not insure homes with knob-and-tube wiring. Others will insure the home but will charge a higher premium to do so.

In this article, I have tried to help you to learn what knob-and-tube wiring is, how you can determine if you have it in your home, and what course of action to take if you do. I hope that this information is helpful to you and helps you in your decision-making about knob-and-tube wiring if it is present in your home or in a home that you are considering purchasing. If you have any questions about this or about home inspections in general, please feel free to contact us.

See NACHI’s article on K&T wiring for more information. 

Do one or more of your outlets have reversed polarity? See our article on how to fix reversed polarity.

© 2020 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the Central Texas area since 2002. He can be reached at 325-998-4663 or at mike@morganinspectionservices.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.


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