Wait…wrong knob and tube.
Knob according to the Cambridge Dictionary “a rich person whose family has been important for a long time” this would mean someone like King Charles
Wait…that’s Nob not Knob…lets try this again.
Now we have the right ones! So you know these were uncovered in a major renovation of a building from 1920, and at the time of this image they were still live. Many people (and insurance companies) would be shocked to know just how many buildings still have this type of wiring active in them.
The knob portion refers to the porcelain insulator that is standing out from the board. The tube is a porcelain tube that fully passes through the board. You’ll notice that both wires are black which makes for a lot of testing to work out which is the line and which is the neutral–knob and tube has no ground hence outlets are only two prong. Most modern wiring is classified as NM or non-metallic sheathed, popularly called Romex (a brand name of wire manufactured by Southwire) Also available and spotted in many locations is BX cable wich can have black, white, green, and sometimes red, wires in it, but the wires are wrapped in a spiral aluminum shell to protect them–handy in places where abrashion could be an issue like metal studs.
As mentioned before in knob and tube both leads are usually black compared to modern wiring having a color coded system that in the United States is:
Black = line or 110v
Red = line or 110v
White = common or neutral
Green = ground
Red and black usually have two uses: they run to the same appliance to provide 220v or they are used for a single pull of two circuits sharing a common and ground, though red is also used as a traveler in a three way switch set up–typically used at the top and bottom of a staircase.
Beyond the lack of a ground wire there were some disadvantages to knob and tube:
It was more labor intensive to install. Splices and drops–the lead going to an outlet or switch–required being twisted and soldered, which was time consuming and electricians have always been well paid.
The copper wires were covered with either asphalt soaked cloth–yep the same asphalt that they make roads out of–or natural rubber. Over time the insulation dries out, cracks, becomes very brittle, and sometimes has rodents chewing on it leading to electrical leaks and potential shorts.
I have two buildings where I work that have had several insurance companies refuse to insure them because of existing knob and tube.
Is knob and tube dangerous? Not really in my experience, but it is no longer legal to install inside buildings–most electrical poles still use it. Should it be replaced if you have it? This is a judgment call between yourself and a locally licensed electrician, but to me any two wire system should be upgraded to a modern system with a ground circuit.
Our modern lives use a significant amount more electricity than did our predecessors a century ago. As a result we are asking wiring from yesteryear to carry modern loads and needs it wasn’t meant to.
We spend a lot of money on buying our homes. We ask the building systems to do a lot for us. We really need to consider that our homes are a living structure, that they need to grow, to have cosmetic surgery at times, to acknowledge when their veins, arteries, lungs, and nervous systems need work or replacement and wiring designed in 1880 is one of those things.
Now which of those wires was line and which was common? The one closest to you was the common, but neither are there any more.
“Electricity is really just organized lightning”
―George Carlin
Maura out
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