Most homes built in the U.S. between about 1900 and 1950 were wired with a wiring system called knob and tube wiring.
If your home or a home that you are considering purchasing contains knob and tube wiring, then there are some things that you need to know.
In this post, I will discuss the things that you should know about this wiring and its potential hazards. I will answer the following questions:
Introduction
Knob and tube (sometimes abbreviated as K&T in this post) 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 as seen in the photos below.
Knob
Tube
Unlike most other wiring systems, when knob-and-tube wiring was installed, the hot wire and the neutral wire were run separately from each other.
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 K&T wiring, and it would be a very good idea for you to know if there is any of this wiring in your home.
By knowing if there is K&T wiring in your house, you can have the wiring inspected. Based on the inspection, you can then have the proper corrective action taken, if any is deemed necessary.
This post will help you to learn how to recognize this 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 about 1950.
There are several things that make it different from most other wiring systems.
1. Wires were run separately-
The hot (black) and neutral (white) wires are run separately. With most other wiring systems, the hot and neutral wires are contained together in an outer insulating jacket. (In more modern wires, there is also a bare (ground) wire along with the hot and neutral wires.)
(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.)
Knob-and-tube wiring
Modern wiring
2. No ground wire-
As in other older wiring systems, a K&T system contained no ground wire.
3. Cooled by the surrounding air-
Knob-and-tube wiring was designed to be installed and run through the open air of the attic, basement, and/or the crawlspace.
Wires heat up as they carry electrical current, and they can potentially get quite warm.
Because the hot and neutral wires are run separately (with space between them) 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 much of the heat from the wires that is generated as electrical current flows through them.
As a result, the air surrounding them helps to keep them at a safe temperature.
4. Supported by porcelain knobs and tubes-
To keep the wiring suspended in the air and not lying against wood or other building materials, porcelain knobs and tubes were developed.
Knobs- 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.
Tubes – 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.
5. Every connection was soldered-
When the K&T wiring was originally installed, the electrician would wrap one wire around the other, and then solder each splice in order to make a secure and permanent connection.
A properly-installed knob-and-tube system was a perfectly safe way to power a home.
Assuming the wiring was not damaged or modified, a K&T wiring system 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.
Is Knob-and-Tube Wiring Dangerous?
As a home inspector, I have seen a lot of knob and tube wiring. Some of it has been in really good condition.
Not all knob-and-tube wiring is dangerous or a fire hazard. As for myself, I, personally, would not be concerned about living in a home with knob-and-tube wiring – as long as the wiring was in good condition.
In the paragraphs that follow, I explain the most common dangers of knob-and-tube wiring.
It is important to note here that most homes with knob-and-tube wiring systems will not have all of these issues. Some homes will not have any of the problems listed below.
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.
Much higher electrical demands today –
As long as it was installed properly, knob-and-tube 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.
The increased demands have 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.
Improper splicing and other work –
New products and innovations have also required new circuits and new wires to be added to the home.
Often, 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.
Wires become damaged by heat and time –
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 become 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.)
This is an obvious fire hazard.
I have also seen insulation that has been damaged by rodents in the attic. This results in the same problem of exposed wires.
(I even saw a rat at one inspection that appeared to have been electrocuted years before as it chewed on the insulation on some wiring.)
Wires can stretch and sag –
Another thing that can happen over time is that the wires can stretch and sag until they are in contact with wood in the attic – another fire hazard.
As mentioned above, knob-and-tube wiring is designed to be cooled by the air around it.
Knob and tube wiring covered by attic insulation –
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 insulation prevents the air from cooling the wiring which can cause the wiring to overheat – possibly leading 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.
Wire damaged by people and stored items –
Another problem is that over the years, people go into the attic for various reasons. And yhey store belongings up in the attic.
People often aren’t really careful, and sometimes can’t see the wires – especially if it’s been covered by attic insulation. They can unknowingly step on wiring or kick it as they move around in the attic. They may also set objects on top of it
Over the years and decades, this takes a toll on knob-and-tube wiring.
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 Identify 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 K&T wiring 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 would be a good idea 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. It is possible that the accessible knob and tube wiring in the attic, crawlspace or basement was replaced, but there could still be some hidden in the walls.
On the other hand, if you see the knobs and tubes with wiring still attached, it is possible that the K&T wiring was replaced with more modern wiring, and that the abandoned wires were just never actually removed.
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.)
What Should You Do if There is Knob-and-Tube Wiring in Your Home?
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.
- What is the condition of the wiring?
- Has it been improperly modified over the years?
- 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 knob-and-tube wiring to be replaced, so your decision should be based upon what your electrician finds.
What Should You Do if You are Purchasing a Home with Knob and Tube Wiring?
Have the wiring inspected –
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.
Talk to your insurance company –
If you are considering purchasing a home with this wiring, then there is one more thing you should know.
While it may not be an issue to some insurance companies, there are some insurance companies that will not insure homes with knob and tube wiring. Other companies will insure the home but will charge a higher premium to do so.
How Much Does It Cost to Replace Knob and Tube Wiring?
It will definitely cost thousands of dollars to rewire a home.
There are a lot of variables that will affect the cost of a rewiring job.
- The size of the home
- The area of the country you are in
- How accessible the attic and crawlspace are
- How easy (or difficult) it is to pull wires through the walls. Sometimes, it is necessary to open up the walls in order to pull some wires. Opening up the walls and then the subsequent repairs will add a lot to the cost of this project.
- If the electric service to the home (the electric mast, meter base, breaker panel, etc.) are quite old, these will also need to be updated. This can typically add $1500 – $2500 to the overall cost of the project.
These are the main factors that can affect the price, but there are others.
You should probably expect it to cost between about $4000 and $8000 to rewire a 1500-square foot house. I realize that 4000 – 8000 is a wide range, but some homes will not even fall within that range.
Remember that every situation is different. Only the electrician who will be doing the work will be able to give you an exact price.
I would recommend that you obtain about three bids before you make your decision.
Make sure that you are comparing apples to apples when you look over the bids. It is important that you understand exactly what work each electrician is bidding to do, because different items/work may or may not be included in the job that different electricians bid.
Finally, it is critical that you use a licensed electrician for this type of work, and that all proper permits are taken out for the work to be performed. This will ensure that all work is up to code and that your home is safe for you and your family.
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.
© 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.
