Knob & Tube Wiring

Isn’t it good that this is the shortest month of the year?

Just as the days start getting longer, the sun starts feeling a bit warmer. You think you can make it through another Maine winter. Then, 10 inches of snowdrops on us.

The cold weather has stuck around just a bit longer than usual. Most of us are getting tired of it. If you had a chance to escape to warmer weather this year, you are one of the lucky ones!

Despite this difficult weather, we have been out there doing home inspections, radon testing, water testing, thermal imaging, and more. Thanks to you, our realtor, and homebuyer friends, we have stayed busy during the toughest, slowest part of the year. As always, we certainly appreciate your continued business and referrals.

This month’s topic is knob and tube wiring, which can often cause a lot of trouble and concern. I hope you can gain a little bit from this information. Be better prepared for dealing with this troublesome stuff if you suspect it is in a home that you or your client are considering for purchase or sale.

What is Knob & Tube Wiring?

Knob & Tube (K&T) wiring gets its name from the ceramic knobs and tubes used to carry the live wires throughout the house. See the picture at right. These wires have very little protective covering on them compared to today’s wires.

They need to be kept away from wood and other flammable materials, where a spark or heat could cause a fire. That is accomplished by running them along and thru ceramic pieces. Think of the electrical wiring on phone poles on a smaller scale.

K&T wiring is the oldest type of residential wiring. It’s common in homes built before 1930. If you are considering the purchase of a home built then, don’t be surprised to find it has some live K&T wiring in it.

Because this wiring is now considered unsafe and impractical due to today’s electrical demands and insulation requirements, it has become difficult to obtain insurance for a home where live K&T wiring exists. Check with your insurance company if you are unsure.

Why is it unsafe?

If left alone, K&T wiring is generally safe and has been for many years. However, today’s electrical demands are much higher than those of 80 or more years ago. Today, we use electronics everywhere in our homes, so the load that the wiring needs to carry is often much greater than previously wired for.

In a home with K&T wiring, you will often see many extension cords and surge protectors, all wired into one ungrounded outlet. This is not safe. Also, the electrical code does not allow K&T wiring to have insulation around it.

The wiring tends to get hot and needs the air around it to dissipate the heat. Before 1930, we didn’t care much about insulation, but today we want lots of it. It is not uncommon to see a house wired with K&T but with newer insulation smothering the wire. This can cause overheating and fires.

Lastly, older homes often have been renovated over the years. Old K&T wiring may have newer wiring attached to it. It is difficult to connect new wiring to older K&T safely, so we often see open-wire splices where old and new are combined. This is also unsafe.

How easy is it to fix?

First, realize that you don’t need to “get rid of” all the K&T. But you do need to get it all disconnected (non-energized). Once this is done, you will now have lots of lights and outlets without power.

Many electricians don’t like fixing K&T wiring because doing it right means fishing new wiring through old walls and ceilings. This is difficult, time-consuming work, often requiring putting holes in the walls and ceilings that are hard to repair. On top of that, the walls built before 1930 are mostly lathe and plaster, which is more difficult to repair than standard sheetrock walls.

The job’s difficulty is related to the layout of the house and how much renovation work will need to be done. We have seen K&T replacement jobs estimated anywhere from $3,000 – $25,000. Make sure that when you contact an electrician about this, you find one that is willing to work on replacing K&T wiring and doing it with as little damage to the home as possible. Also, be sure to ask who will be responsible for the wall and ceiling repairs.


With today’s electrical demands and insulation needs, knob and tube wiring should be upgraded to today’s electrical standards. If you or your client are considering purchasing or selling a home built before 1930, and are unsure if live K&T wiring is present, contact us to take a look. We can usually tell pretty quickly if it is and how extensive it is. Then, if needed, you can contact an electrician for a price to replace it with wiring up to today’s code.

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