Knob and Tube Wiring

What is Knob and Tube Wiring?

So here’s the big question: What is this knob and tube wiring thing you keep hearing about? It’s antique wiring that was installed in homes between the 1880s and early 1930s. It was also used in commercial situations. But for the purposes of this webpage, we’ll only be referring to the knob and tube wiring installed in homes.

Knob and Tube Wiring

Dangers and Disadvantages of Knob and Tube Wiring

  • Insulation companies cannot insulate your home with knob and tube wiring. This wiring is run between joists with air space, in order to dissipate heat. Whether insulation is blown in or standard pink that surrounds the old wiring, it can overheat and potentially cause a fire hazard. The insulation company will require an electrician to inspect all wiring, in order to confirm there isn’t any concealed knob and tube. If knob and tube wiring is found, it must be rewired to the standard set forth in the 2020 National Electrical Code (NEC)
  • Knob and tube was never meant to last 100+ years. Homeowners sometimes take out the proper 15- or 20-amp fuse and install a 30-amp fuse because they don’t have enough power. Yes, this tactic will prevent your fuse from blowing, but it creates a fire hazard. Since the knob and tube wiring that is now over-fused wasn’t meant to carry such a high current, the wire insulation can start breaking down.
  • Squirrels, mice, and other critters seem to love the insulation on knob and tube wiring. These pests can leave bare copper wire exposed.
  • A knob and tube wiring system doesn’t incorporate a ground. Since the ground wire on any circuit is there for safety, it’s the most important one. Also, some electronic equipment requires a ground to operate correctly.
  • In the early 1900s, electricians would switch the neutral feeding of a ceiling light, instead of the hot wire (which is today’s standard). Switching the neutral would turn off the light, but live voltage potential was still present at the fixture. So this situation can be dangerous for a homeowner trying to change a lighting fixture.
  • When overheated, insulation becomes very brittle and breaks down. Older lighting fixtures had no insulation in them that would prevent overheating. These older fixtures usually weren’t marked with a maximum wattage for light bulbs. So if a homeowner installs a 100-watt bulb in a fixture that was never meant to have such a high wattage, the wiring above the ceiling fixture could break down, and the insulation could disintegrate.
  • Some insurance companies are not insuring homes with existing knob and tube wiring. Starting in 1903, electricians had the choice to use either knob and tube wiring or BX wiring (armored cable). The interesting fact is that electricians kept using knob and tube wiring instead of BX cable because the materials were cheaper. While the BX wiring was undoubtedly easier to install, electrical labor was cheap, so electricians opted for the cheaper knob and tube wiring.

I have a lot of respect for the electricians who installed knob and tube wiring. Compared to what we use today, the electricians at that time had to really plan out their wiring runs, drill holes by hand, and put screws in by hand. I can’t imagine being an electrician back then!

If I had to break it down, I’d say 60% of our customers are having their knob and tube replaced due to having insulation installed. Another 30% have an insurance issue, and the remainder want an up-to-date, safe, grounded electrical system.

Does My Home Have Knob and Tube?

Here are a few things to look for:

  • Old porcelain knobs and joist sleeves in an unfinished attic or basement
  • Old two-prong outlets in the baseboard
  • An open ground

Use a three-prong tester or voltage meter to test all outlets around the house. If you find an open ground (i.e., there is no ground), there’s a good chance that wiring is tapped into the older wiring.

The Majority of Homes Have Knob and Tube Wiring in the Same Locations

Homes that still have active knob and tube wiring seem to follow similar formats:

  • Front porch light and switch
  • Front entry light and switch
  • Dining room light and switch
  • Any three-way switches going up or down stairways and corresponding lights tied into these switches
  • Additional first floor wiring, including the older outlets still mounted in the baseboard. If the cellar is unfinished, getting to the outlets on the first floor is quite simple. So these floors are usually rewired already.
  • Second-floor wiring (including any older outlets in the baseboards) and lights and switches on this floor (if there’s a finished third floor)
  • If there is a finished third floor, the second floor is the hardest floor to rewire because there is no unfinished space above or below it.
  • If there is a finished third floor that was original to the house, this space often has some degree of knob and tube wiring left in it.

How Much Will it Cost to Replace Knob and Tube Wiring?

Homeowners with knob and tube wiring are offered two options for replacement costs:

Completely free in-home estimate

The KES team will come to your home and identify the active knob and tube wiring visible in the basement. We will shut these circuits off at the electrical panel, and we will conduct a walkthrough of the home to identify all the lights and outlets that are no longer active. We will then create an estimate of the findings with an easy-to-understand section that covers the adjustments of the estimate.

In other words, a credit will be due if there is any modern wiring that we originally thought was knob and tube wiring in the estimate.  On the flipside of this coin, any additional knob and tube wiring we find that is not listed on the estimate will be considered an extra cost.

Transparent Knob and Tube Replacement Estimate

We use a “cost per point” method to estimate jobs. A point is a switch, outlet, light, circuit from the panel, etc.. There is no hourly charge for knob and tube wiring replacement jobs.

Let’s say the cost per point is $225.00 each. Then if you decide to add two outlets in a bedroom, you’ll know exactly what the additional outlets will cost.

The cost per point for homes can vary, based on the type of home. Some homes are much easier to fish new wiring in, while others are quite difficult and time-consuming.

Whole Home Wiring Evaluation/Mapping

We also offer a whole home wiring evaluation/mapping service that will identify all your wiring types with an easy-to-read color-coded map. Please click here to learn more!

Careful Rewiring Techniques

Yes, it’s true. Unfortunately, we must make some holes in your existing walls/ceilings to fish the new wiring around.  But we pride ourselves in developing creative solutions and creating the smallest number of holes possible.  To learn more about the installation process, click here.

Knob and Tube Wiring Replacement

Check out our Knob and Tube Guide on Amazon!

A Homeowner’s Guide to Knob and Tube Wiring Replacement
Author: Jesse Kuhlman, Owner Kuhlman Electrical Services

Now available for digital download. This e-book is especially made for those who want to upgrade the old electrical wiring system of their home or want to know more about knob and tube wiring replacement.

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Homeowners Guide to Knob and Tube Wiring Replacement