What is Knob and Tube Wiring?

Knob and Tube Wiring

Knob and tube wiring was the most common method for electricians in the United States from the 1880’s to the late 1930’s. Many homeowners looking to upgrade their electrical system or complete a renovation are discovering “hidden” k&t wiring in their homes and wondering how to proceed.

Knob and tube wiring received its name from the ceramic knobs used to hold wires in place and ceramic tubes that act as protective case for wires running through wall studs or floor joists. Instead of the three wires found in modern electrical installations, knob and tube wiring has only two — a hot, black, wire and a neutral, white, wire. This means there is no ground wire in the system for excess charge or in the event of a short

Knob and Tube Wiring

Dangers and Disadvantages of Knob and Tube Wiring

  • Insulation companies cannot insulate your home with knob and tube wiring. The wiring is run between joists with air space to dissipate heat. When insulation, whether it’s blown in or standard pink insulation is surrounding the old wiring, it can overheat and potentially cause a fire hazard. The insulation company will require an electrician to inspect all wiring to confirm that there is no concealed knob and tube. If knob and tube wiring is found; it must be rewired to the standard set forth in the 2014 NEC (National Electrical Code).
  • Knob and tube was never meant to last 100+ years. Homeowners sometimes take out the proper 15 amp or 20 amp fuse and install a 30 amp fuse because they do not have enough power. Yes, this will prevent your fuse from blowing but it creates a fire hazard. The knob and tube wiring that is now over-fused was not meant to carry such a high current and the wire insulation can begin to break down.
  • Squirrels, mice and other critters seem to love the insulation on this knob and tube wiring style. These pests can can leave bare copper wire exposed.
  • Knob and tube wiring system does not incorporate a ground. The ground wire on any circuit is the most important wire, as it is there for safety. Also, some electronic equipment requires a ground to operate correctly.
  • Switched neutrals. Electricians in the early 1900’s would switch the neutral feeding 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 would still be present at the fixture. Making a dangerous situation for a homeowner that is trying to change the light fixture.
  • Insulation becomes very brittle and breaks down when heated too much. Older light fixtures had no insulation in them to prevent overheating. These older light fixtures were usually not marked with a maximum wattage for light bulbs. Homeowners could be installing 100 watt bulbs in fixtures that were never meant to have such a high wattage bulb, the wiring above the ceiling fixture could break down, and sometimes the insulation would disintegrate.
  • Some insurance companies are not insuring homes with existing knob and tube wiring.

Electricians starting in 1903 had the choice to use either knob and tube wiring, or BX wiring (armored cable). The interesting fact is that electricians continued to use knob and tube wiring instead of BX cable because it was a cheaper, material wise. The BX wiring was without a doubt easier to install, but at the time electrical labor was cheap and electricians opted for the cheaper K+T wiring.

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

60% of KES customers are having their knob and tube replaced due to having insulation installed. 30% due to an insurance issue and the remainder because they want an up to date safe and grounded electrical system.

Knob and Tube Wiring is generally found 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 typically already rewired.
  • Second floor wiring including any older outlets in the baseboards and sometimes lights and switches on this floor if there is a finished third floor
  • The second floor, if there is a finished 3rd floor, is the hardest floor to rewire because there is no unfinished space above or below
  • If there is a finished third floor, that is original to the house, this space often has some degree of knob and tube wiring left

What is the cost to replace knob and tube Wiring?

Home owners with knob and tube wiring are offered two options for replacement cost:

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. These circuits will be shut off at the electrical panel. A walk through the home will be done to identify all the lights and outlets that are no longer active. An estimate will be created off the findings, with an easy to understand “adjustments” section of the estimate.

Meaning – If there is any modern wiring, originally thought to be knob and tube wiring listed in the estimate, a credit will be due. Any additional knob and tube wiring found that is not listed on the estimate will be considered an extra cost.

Transparent Knob and Tube Replacement Estimate:
A “cost per point” method of estimating is used 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 $200.00 each, if you decide to add two outlets in a bedroom, the homeowner will 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 while others are quite difficult and more time consuming

Whole home wiring evaluation and mapping

KES offers a whole home wiring evaluation and 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, unfortunately some holes must be made in your existing walls and ceilings to fish the new wiring around. We pride ourselves on getting creative, while making the least amount of holes possible. To learn more about the installation process, please 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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