Home & Garden Columns

About the House: Insurance: Knob and Tube Wiring

By Matt Cantor
Friday October 26, 2007

The other day a recent inspection client of mine called up and asked if I could help answer a few questions. She proceeded to ask if her new house had copper piping. 

I clicked, dragged and said, in sequence, “You have mainly copper piping with some galvanized steel, you have knob and tube wiring and fuses at a subpanel but breakers at the main, your roof is roughly 3 years old and your foundation is bolted and braced.”  

She laughed, asking how I knew what the rest of her questions had been. I told her that she was pursuing the answers to a series of questions that her insurance provider had asked her to declare. I could hear the smile over the phone. We talked a bit more and said our so-longs. 

Although these queries are all fairly silly and not very useful in determining the safety of the home (mostly because of the lack of specificity), insurance companies continue to ask these same questions. I tried to sit down with a S.F. based insurance provider many years ago and help them to develop a better form but it was, sadly, beyond them. Something about falling on dead ears comes to mind. 

Though there was much in my client’s list of questions worthy of further discussion, I’d like to explore just one today, that being the merits or failings of knob and tube wiring. 

Knob and tube was the first wiring type installed in our homes. The term comes from the insulators used to keep the wiring out of contact with the flammable wooden framing it bobs and weaves though on its way though walls, floors and ceilings. Knobs are two-part insulators that nail onto wooden framing, snugly sandwiching the insulated wire between two neatly cast porcelain cylinders as the nail gets its final smack. 

Tubes are also porcelain and are long hollow pipes with a flared end that can be slid partly through a hole drilled in a joist or stud. A wire can be run through the inside of the tube and be thus kept away from rough and flammable edges. These also sometimes get used when wires cross over one another. 

Knob and tube wiring is inferior to modern wiring in a few respects but these deficiencies are often not significant, especially in the face of the poor wiring sometimes done using modern methods. 

Knob and tube wiring is almost never grounded. That means that the devices in your home that need grounding, such as your fridge, computer or microwave oven are left without a requisite safety feature; so for these, please add a new grounded outlet. However, if you were to spread all your plug-in devices out on the lawn (yardsale!), you’d find that a very small number of them had three-prong (grounding) plugs and therefore do not require grounding.  

This means that, whatever upgrade you may require, it is not axiomatic that those old, two-prong, knob and tube fed outlets, need to be replaced. They need only be augmented with a few modern circuits bearing three-prong grounded outlets. 

This works out nicely since almost all houses with knob and tube wiring (which haven’t been the subject of upgrade) are lacking in an adequate distribution of outlets and possibly lighting. Therefore, the need for more outlets and the need for some to be grounded can be fulfilled in one fell swoop. 

Some of the other disadvantages of knob and tube wiring include its unsuitability to 240 volt devices such as electric stoves, oven or dryers. This can be done, but is rarely done properly as knob and tube was being phased out before most of these devices had arrived on the scene. Also knob and tube is generally coupled with fuses (our next article will go there) and so there is no way to make one of these big devices fully disconnect, when a fuse blows, as can be done with breakers. 

Knob and tube systems were also designed for far fewer electrical devices than most people have today. In 1925 there were no computers, microwave ovens or TVs and refrigerators were few and far between. The systems are generally too small for today’s watt-hungry world. 

Early knob and tube systems are also sometimes wired with the fusing on the neutral wire, allowing a blown fuse to leave current running and to shock the explorer who attempts to fix the seemingly dead device. 

All this said, I like knob and tube and have rarely found original, unaltered, wiring to be less than impressive in the neatness and care of its layout and assembly. One of the best things about this wiring is that it is soldered at the connections. The Western Union splice, developed during the era of telegraph wiring, is far superior to today’s methods of connecting wires because they virtually never pull apart. A sticky, thick, cloth-covered tape was well applied to the wire splices and is rarely faulty after as much as 85 years.  

One of the great things about these soldered connections is that they are much less prone to overheat because the connections have very low resistance. I have often opened a “modern” junction box to see wires, bound in a plastic “wire-nut” spring apart due to having been joined with almost no diligence or care. It’s actually much easier to perform this sort of screw-up today than it would have been in 1925 when knob and tube wiring was king (and quality control was rote). 

This bit about soldering means that fires are less likely to occur and circuits are less likely to behave badly. 

So with all these plusses and minuses, what should a person do if they’ve just moved into a 1925 house with two, two-prong outlets per room and no grounding outlets for the fridge or computer. My advice is to have the wiring checked, upgrade the panels that feed the knob and tube wiring and add some new circuits. That’s all. Now see, that didn’t hurt a bit!