Anyone who has lived in a building more than 75 years old has run the risk* of utilizing an electrical delivery system known as Knob and Tube. There are anecdotes a plenty about the dangers of this system, and plenty of theories and opinions about what to do if you encounter it. To form your own opinion, let’s talk about it’s history, use and common problems. Of course, a good idea on what household electrical delivery is all about is probably a good background to have first and foremost.
If you walk down any typical street in a city and look up, you are familiar with K+T’s ancestry. Electrical power lines overhead apply the exact same principles as early in-home wiring, and these were derived from their predecessor, telegraph lines. The idea was simple: metal wires conduct electricity over distances rather well, and as long as they are separate from each other, were generally safe. This generally comes from the principle that power/telcom lines are far over head and won’t run into many unplanned obstacles.
When brought into the home, it was quickly apparent that these wires could not be left uncovered. With where technology was at the end of the 19th century, fabric covering was the easiest and cheapest option for manufacturing. So we now have insulated (with fabric, called loom) copper wires that are run separately due to their heritage. This same heritage brought with it half of the namesake; the knobs. These white posts keep the wire up and away from the surrounding structure. The other half of the name, tubes, came at the same time the insulation did. These porcelain elements allowed the wire to safely go through a structural element without direct contact.
Electrification started in the 1880s, with widespread use in most metro areas by 1900. Indoor lighting was provided earlier by the use of various flame powered lamps (candles, oil, gas, etc). The reliability of gas light in most urban centers resulted in many homes from 1890-1920 being installed with both gas and electric supplies to dual fixtures. For the majority of installations, K+T provided a modern lighting option, but not much else.
Knob and Tube was installed commonly until the 40s and 50s, when post-war manufacturing was able to reliably introduce metal and rubber coated wiring products. Unfortunately, this same manufacturing boom is what started introducing dozens of new electrical products for the home that knob and tube was never designed to handle.
Installation was performed by well trained technicians and required great skill in planning due to the dual wire nature, and all the supporting porcelain knobs and tubes. Additionally, any splicing or joining was expertly soldered and re-wrapped in friction tape.
So far, it’s hard to see how there could be any problems with K+T. The wires are all separate, connections are soldered, and they are only providing power to light fixtures. How bad can it be? Within those constraints K+T is incredibly safe. The only self-imposed danger is the degradation of the textile loom around the wire itself. Without physical contact from something else hitting the wire, however, even this has a relatively low danger.
Something Else. These two words are the reason for the perceived dangers of K+T. As the world moved forward, these old houses with K+T did not. We now bought things that ran on electricity. And when faced with a house that had ample electrical wire within it, just not the places where we wanted to plug things in, what decision did we make? Simply add on!
Remember when K+T yielded to post-war wiring? Remember when newer was better? Remember when all those new things were introduced? This is the trivalent problem of K+T. Not the wiring itself, but the fact that we have all these new shiny things to plug in, the only way of getting them power is using a different delivery system than what was original, and the mantra that both these new products and wiring systems were inherently better than what was there before. All of this coupled with a mentality of DIY that exploded post WWII.
So we no longer have soldered connections. We no longer have two separate wires. Knob and Tube never used junction boxes, so why would I now when I join these systems together? Combine these things with some knowledge around how electrical fires start, and you immediately see the danger of Knob and Tube. Modification and over-taxing of a perfectly well designed lighting electrical system is what you should fear, not K+T itself.
People are quick to point to Knob and Tube as the issue when looking to purchase an older home, one that is daunting to overcome. But focus on the modifications. If you can verify the K+T is in good condition as far as the fabric loom and no broken knobs or tubes, and can show good evidence there are no additions/modifications to that system – keep using it as-is. The safest things you can do are remove any possible changes, and update the current limiting device in the house. Electricians or contractors who say you must rip out the whole thing and start from scratch are just using scare tactics. (There are plenty of instances where your insurance may want K+T removed. Typically, though, they just want the over current device, most commonly circuit breakers, to be as up to date as possible)
So there is my rant about Knob and Tube. Love it. Admire it. But most of all, understand it, and understand where the real dangers are. Oh, and never, ever, add on to a knob and tube circuit 😉