What knob-and-tube is
Knob-and-tube was the standard way to wire homes in the early part of the 20th century and into the 1940s. You will still find it in some of Edmonton's older neighbourhoods, in character homes and in houses that were partly updated over the years.
It uses single copper conductors, wrapped in rubber and cloth insulation, run separately through the house. White porcelain knobs hold the wires off the framing, and porcelain tubes protect them where they pass through joists and studs.
When it was installed properly, it was a reasonable system for its time. The problems come from age, from what has been done to it since, and from how much more we ask of our electrical systems today.
Why it causes concern today
- No ground conductor. There is no equipment ground, so there is no path for fault current from a metal appliance case and no proper grounding for three-prong devices.
- Aging insulation. After 80 or more years, rubber insulation can become brittle and crack, especially where it has been handled or heated.
- Buried in insulation. Knob-and-tube was designed to shed heat into open air. When attics and walls are packed with insulation, as many were to cope with Edmonton winters, the wires can run hotter than intended.
- Amateur changes. Decades of additions often mean splices, extensions and connections that were never done to any standard.
- Not enough circuits. Homes wired this way usually had very few circuits, which leads to overloading and long extension cord runs.
How to spot it
You can often see knob-and-tube without touching anything:
- In an unfinished basement or attic, look for pairs of single wires running several centimetres apart on white porcelain knobs.
- Porcelain tubes sticking out of joists or studs.
- Two-prong receptacles throughout the house, though these can also be found with other older wiring.
- An older fuse panel or a panel with very few circuits.
Even when a home has been "rewired", some knob-and-tube circuits are sometimes left in service, often to ceiling lights. An inspection will tell you what is still active.
Knob-and-tube conductors may be live and the insulation can crumble when handled. Do not move, pull or disturb it, and do not cover it with new insulation. Keep insulation contractors away from it until an electrician has assessed it.
Insurance and buying
Many insurers ask about knob-and-tube wiring, and some will not cover a home with active knob-and-tube or will only do so with conditions such as an inspection or a plan to replace it. Policies vary, so ask your insurer early.
If you are buying a character home, get a home electrical inspection before you firm up the deal. It tells you how much active knob-and-tube remains and what replacing it would involve. Our guide to buying an older home has a full checklist.
Living with it until it is replaced
Replacing knob-and-tube is rarely an overnight job, and many families live with it for a while as they plan and budget. In the meantime:
- Do not overload circuits. Spread heavy appliances across different rooms and avoid running a space heater on an old circuit.
- Avoid extension cords as permanent wiring. They are often a sign the house needs more circuits.
- Keep the attic and any spaces where the wiring runs free of stored boxes and new insulation.
- Never cut off a ground pin or use a cheater adapter to fit a three-prong plug into a two-prong outlet.
- Watch for flickering, warm devices and burning smells, and act on them quickly. See our list of electrical warning signs.
It is also worth having a licensed electrician check whether any circuits have been extended or tapped into by previous owners. Those spots are often where the real risk lies.
Replacing it
For most homeowners, the long-term answer is to replace active knob-and-tube with modern grounded cable. This is usually done as part of a whole-home rewiring, often with a service upgrade at the same time because older homes commonly have 60 A or 100 A services.
A licensed electrician will take out the required permit, plan new circuits based on how you live today, and fish new cable through walls and ceilings where possible to limit damage to plaster. Old wiring is disconnected so it cannot be confused with live circuits later.
What drives the cost:
- Size of the home and number of circuits.
- Plaster and lath walls versus drywall, and how much access there is from the attic and basement.
- Whether the panel or service also needs upgrading.
- Whether walls are already open for a renovation.
If you are opening walls for a kitchen, bathroom or basement project anyway, that is the ideal time to replace old wiring in those areas.
Common questions
Is knob-and-tube wiring illegal?
Existing knob-and-tube is not illegal on its own in Alberta. New work cannot use it, and extending or modifying it is generally not allowed. The bigger practical issues are condition, safety and insurance.
Can I add insulation to my attic if I have knob-and-tube?
Not over active knob-and-tube. It needs open air to shed heat. Have an electrician assess it and, ideally, replace those circuits before insulating.
Can I put three-prong outlets on knob-and-tube?
Not as a simple swap. Without a ground, a three-prong outlet gives a false sense of protection. In some cases GFCI protection is an accepted option, but an electrician needs to assess your wiring.
How long does a rewire take?
It depends on the size of the house, wall finishes and access. An electrician can give you a realistic schedule after seeing the home.
Codes and permit rules change and every building is different. For work on your system, talk to a licensed electrician and your local permit authority.