Knob and tube wiring is one of the original forms of electrical wiring in homes, popular from the late 1880s until the 1930s when it fell out of favor. At the time, it was the standard for wiring new electrical installations due to its relatively low cost compared to the other options of the time like armored cable and conduits, which could cost two or three times as much as knob and tube wiring.
Today, there is a definite fear of old house wiring, especially when it comes to knob and tube wiring. Some of that fear is reasonable and some of it, like anything, is just blind fear from lack of knowledge. I wanted to lay out the facts about knob and tube wiring for you so that you can be informed and understand exactly what the issues are and how you can safely resolve them.
I’ll preface this by saying that I am not a licensed electrician. I am a licensed general contractor who works exclusively with historic buildings and have had plenty of experience with knob and tube, as well as learned a lot from the very skilled electricians on my job sites. Dealing safely with knob and tube is something that everyone who lives in an old house can and should learn, so keep reading below.
What is Knob and Tube Wiring?
Knob and tube wiring is a simple form of wiring that gets its name from the porcelain knobs and tubes used in the installation process. The wiring itself is simple copper wires wrapped in a rubber sheath (earlier versions were wrapped in asphalt soaked cotton cloth). These wires were installed by running them in the walls between porcelain knobs, which kept them properly tensioned, away from the wood framing, and facilitated easy direction changes in the wiring. So, that’s the “knob” in knob and tube, now lets look at the “tube”.
The tube part of the name came, once again, from porcelain tubes that were drilled and inserted into any framing that the wiring needed to pass through. These tubes allowed the wiring to pass through the framing without touching it. This helped prevent the wires from contacting the framing, being crushed, or the sheathing being rubbed off as the house settled or moved with the seasonal weather changes.
The whole system was cleverly built to accommodate for the construction methods and electrical needs of the time. The wires generated a decent amount of heat when a electrical current passed through them. That heat was why they were kept away from the framing elements by the porcelain knobs. This allowed the heat to dissipate in the surrounding air keeping them at a safe temperature. Most installations were intended to safely service 10-amp loads per circuit without generating too much heat which was more than adequate for the needs of the time.
Splices were typically installed by twisting the two wires together and then soldering the joint and wrapping it in asphalt soaked cloth, which made for mechanically and electrically very strong joints when done properly.
Advantages of Knob and Tube Wiring
Yes, believe it or not, knob and tube wiring had some distinct advantages over its historic counterparts!
Knob and tube was considerably cheaper to install than its competitors, which were two and three times as expensive. Early on in the electrical field, the materials were the expensive part of the equation rather than the electrician’s wages. As electrical systems became more advanced and electricians became more skilled, labor costs for electricians went up much faster than the materials costs for electrical components.
Knob and tube wiring is very labor intensive to install, so this change in labor vs. materials caused knob and tube installation prices to outstrip its competitors and was one of the leading causes of its decline in popularity.
Another advantage of knob and tube wiring was the use of porcelain insulators. Porcelain is extremely resilient and has an almost indefinite lifespan providing excellent protection with almost no expiration date, which is hard to find. Also, the porcelain knobs were excellent at preventing the wires from being tensioned too much during the installation.
Disadvantages of Knob and Tube Wiring
You knew I was going to get to it eventually, right? The doom and gloom part? Yes, knob and tube wiring has a very distinct set of disadvantages too.
That’s right, there is no ground wire included in a knob and tube setup. Just a hot and neutral wire were included. It wasn’t until the 1960s that a dedicated ground wire was consistently included in every circuit. The ground is important should there ever be power surges or overloaded circuits. If everything operates perfectly then you don’t need the ground wire, but since when does everything go perfectly?
Knob and tube works best when it has air to breathe. If you cover it with blown-in insulation it is not able to cool down and dissipate heat like originally intended, and this increases the potential for fire. It’s not just insulation that can cause this issue but really anything else that is stacked around the wires that blocks air flow. Even the current building code does not allow for insulation to be installed on top of active knob and tube.
Unlike the porcelain insulators, the natural rubber or asphalt soaked cotton sheathing naturally wears out and breaks down over time, causing the wires to be exposed and increasing the potential for electrical shocks and also fire. During renovations old brittle sheathing is very easy to tear or damage, creating even more chances for danger.
Increased Power Needs
This is, in my mind, the biggest issue with knob and tube wiring. The electrical needs of a house in 1918 are dwarfed by a house in 2018. Electricity was used mainly for lighting purposes in the first half of the 20th century with only minimal appliance usage. Now, with central AC, computers, TVs, internet, washer, dryers, refrigerators, etc. our homes have become huge energy users, and antiquated electrical systems have real trouble keeping pace without modern needs.
A typical house in the 1920s had a 60-amp main service panel (the total electrical service capability the home can handle). Today that main service panel is usually at least 150-amp with most larger houses needing 200-amp service. Knob and tube wiring was never intended to handle loads anywhere close to that.
Another big issue is from modifications done with shoddy workmanship along the way. Improper splices are just one part of it. Overloaded circuits like we just discussed often happen when the power needs of the home increased marginally. For example, an original 10-amp knob and tube circuit may have been slightly overloaded so the homeowner changes to a larger 15-amp fuse to prevent it blowing so often. Years later, that same circuit gets unknowingly overloaded even more as new appliances are installed and gets even more dangerous.
Should You Replace Knob and Tube Wiring?
Alright, you’ve got the advantages and disadvantages of knob and tube wiring, and now you need some answers about what to do? In my opinion, knob and tube wiring should be replaced in almost all circumstances. There is very little benefit other than saving money that knob and tube provides today and when compared to the potential dangers, it just doesn’t add up.
Does that mean you need to tear your walls down to replace it? Definitely not! Knob and tube does not actually need to be removed from your walls, it just needs to be disconnected so it is no longer active. A quality electrician can completely rewire an old house without taking down whole walls, but rather punching small tactical holes to fish their new wires into place. Don’t spend the money to fully remove every trace of knob and tube, just be sure that you have every circuit fully disconnected and rewired with modern wiring.
You’ll sleep better, and ultimately get better insurance rates when your house has modern wiring. And those insurance savings can pay for the cost of the rewire in less than 10 years in most cases. When you account for the decreased risk of fire or other electrical problems rewiring is a no brainer. What do you think?
Founder & Senior Editor
I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.