Knob and tube wire is the original form of branch circuit wire (wire that runs through the home to switches and outlets). It was used in homes up until the1940s, and can be found in these vintage homes all around the US. It was most commonly used between around 1900-1920. This means that if you own a home or are considering buying a home dated pre-1940s, you might have knob and tube wire.
Knob and tube wire has several concerns - and usually, when you are a new owner of an old home that has knob and tube - you're going to need to replace it. Replacing this wire can be inexpensive (as little as a few hundred dollars if there is just a little wire present in accessible areas) or very costly (as much as $60,000.00 if the home is wired completely with this type of wire and requires rewiring.)
Here's why it is an issue and why you would need to replace it:
-The wire has no equipment ground (the third prong in a three prong outlet) - so you should not be using any appliances with three prongs on the plug on outlets fed by this type of wire.
-If there are walls or attic spaces with this wire present, those areas cannot be insulated until the wire is replaced.
-The wire is old (100-120 years!) so it can become brittle, corroded, and damaged.
-Insurance companies do not want to insure homes with this wire (or will charge you extra!)
-Fixtures and splices in the wire are not as safe as connections and splices used today.
Homes can appear to be fully updated, but knob and tube may still be present in concealed, inaccessible areas. Electricians can wire into and out of junction boxes, and splice new wire into the old knob and tube wire, and keep it live in the walls and ceilings. So if your home looks like it was fully updated, that is not always the case!
Rewiring circuits of knob and tube wire often involve putting holes in walls and ceilings, so the total cost of rewiring any circuits with knob and tube wire is increased by the added cost of patching and repainting.
Vintage electrical fixtures fed by knob and tube wire often have uninsulated and exposed live contacts - this can be a shock hazard - so be careful around these components!
Take a look around - especially in the attic and basement- and look for wire similar to what is in these photos. The ceramic "tubes" might be found in wood floor framing, running through the joists, and the ceramic "knobs" might be found attached to the underside of framing. In the interior living areas, look for two-button light switches and old two-prong outlets in the baseboards on your walls. Look out for anywhere you see single wires run on the insides of wood framing, and look out for anywhere light fixtures look like they are wired with exposed connections. Other things to look out for: ceramic "twist" type light switches, or wood conduit (looks like a "track" where old knob and tube wire would be run.
So, if you are buying a home, make sure your home inspector checks for this type of wire in the home! Even if they don't find any - and the home is of this vintage, be aware that you may find knob and tube wire when doing other repairs and/or renovations. Give us a call or send us an email if you have any questions!