We have knob and tube wiring in our home. But uncle Bob says there’s nothing wrong with leaving it alone. What should I be doing?
Have your home’s electrical systems evaluated by a licensed electrical contractor. See further details below:
Knob & Tube details
Knob and tube (a.k.a. K&T or KT) wiring was discontinued in favor of cables some time during the 1950’s. When installed correctly knob and tube wiring was, in some ways, superior to current wiring practices. In fact, when the switch was made there was some debate. Unfortunately, this system is rarely intact after 80 or so years of use. Things that happen well after the original installation can cause major problems. For these reasons, we would suggest any time an active Knob & Tube system is found it be evaluated or upgraded.Let us discuss this system a little further. The system uses porcelain insulators (knobs) for running wires through unobstructed spaces. Porcelain tubes protect wires that run through studs and joists. See the pictorial view below:
Some of the safety features of the system are as follows:
• The porcelain knobs suspend the wire in open air to dissipate heat.
• The wiring was usually installed along the of joists and studs away from potential nail punctures.
• Additional protection is provided by porcelain tubes where it passes through wood.
• The hot and neutral wires are always separated by at least 3 inches except near a connection to a box or fixture. At these places, an additional protective woven sleeve, or was used from the last knob. When we say “in some ways the K&T systems had something over current standards”, this is what we mean.
• Splices were joined by wrapping one wire around another and then soldering the joint. Knobs were then placed within 6 inches of the splices to prevent stress on the connection.
• Wires were usually never loose or placed on top of joists where they could be easily damaged.
If your basement is unfinished then looking up from the basement at the wiring may look like this if you have Knob and Tube wiring.
The two main weaknesses of this vintage wiring are:
• The lack of a ground conductor, and
• Switches were often placed on the neutral wire, turning off the circuit, but not the current.
Other Weaknesses that emerged with time
• As time passed the system ages. Since it has not been installed since the 1950’s it is old by any measure. A lot has happened since then.
• Somewhat related to age, as time passes people lifestyles have changed: houses got larger requiring more branch circuits and more appliances resulting in great load on each circuit. This creates the desire to extend the wiring or add more branch circuits.
The picture shown left is how modern wiring would look if your are standing in an unfinished basement and looking up. Unfortunately though, many homes that originally had Knob and Tube wiring may have had some extension of the Knob & Tube wiring, but using modern wiring techniques. This therefore means, because you do not actually see Knob and Tube, does not mean there is no active (electrified) Knob and Tube wiring.
Improper Alterations And Improper Additions
Unfortunately from a safety standpoint, the electrical system is one of the few things in a home that can be installed completely wrong and still work… for a time. So, improper installations and the more common improper alterations are the most consistent problems we find with knob and tube wiring, and they both pose a significant safety hazard.
Because this system was discontinued such a long time ago and people’s lifestyles have changed so much, there is always a need to extend the wiring. That is, by adding more branch circuits or adding more loads to existing branch circuits. Branch circuits improperly added to the original wiring is, one of the common problems with a Knob and Tube in a home. When additional branches or fixtures are added, the fuses protecting the old circuits are likely to blow frequently. Installing larger fuses is an easy, but unsafe, solution. Oversized fuses allow much more current to flow than originally intended, resulting in additional heat in the conductors. This heat causes the insulation protecting the wire to become brittle, and eventually to disintegrate.
Heat and critters
Heat directly above ceiling lights and in un-vented attics can also degrade the wire insulation over time. Critters that find their way into attics of these old homes have been known to eat some types of insulation used on the knob and tube wiring. Without the insulation on the conductor, there is an increased safety risk.
This picture left shows Knob and Tube wiring that has damaged or worn electrical insulation. You can see the insulation is missing in some part leaving just the pure copper conductor. This is a safety issue.
Thermal Insulation Problems
Back then, many homes was built with very little thermal insulation in the attic. Faced with old drafty houses and high heating bills, one quick solution is to add thermal insulation to the attic. Insulation on top of knob and tube wiring is another risk factor. This would defeat the one of the good characteristic on Knob & Tube Wiring: The wires are suspended in open air allowing heat to dissipate. Loose and rolled insulation counteracts the original open air installation of knob and tube wiring.
What We Do When We Find Knob Tube
The inspectors at Qualitex Home Inspection are trained to verify and report on what type of wiring is in a home during an inspection. In fact, our Lead Inspector and founder is a Master Electrician with over 30 years experience. First and foremost we inspect the panel. If there is or was Knob & Tube, we may see it here. Unfortunately, sometimes due to alterations it may not be readily identifiable that there is active Knob & Tube in the home as the panel most likely have seen an upgrade at some time prior. In such cases, we will examine unfinished areas in the home, and or open at least one wall receptacle at random to make a conclusive determination.
So to recap, Knob & Tube Wiring was a sound method of wiring when it was developed and remained so for several decades. However, much time has passed. Because of the missing ground, age and changes in people’s lifestyle, this wiring method has become obsolete. So, if your home has active Knob & Tube wiring, we recommend you have it evaluated by a licensed electrical contractor. As always, we recommend any deficiencies in a home that is electrical related to be treated as a priority because of the severe risks (fire) that it may expose occupants to.
Qualitex Home Inspection services the Greater Toronto Area: Toronto, Mississauga, Brampton, Milton, Oakville, Burlington, Vaughan.