Knob And Tube Wiring

By Pete Busch

HISTORY
Knob and tube, commonly referred to as K&T wiring was the first type of standardized wiring in homes and was used from the 1880’s into the 1940’s. By today’s standards it is considered obsolete but is still found in many homes today due to the level of difficulty and expense to replace it with modern wiring.  When K&T wiring was first installed it posed little danger or hazards. Over time improper modifications may have been made, the insulation has deteriorated, and modifications to the house such as the addition of insulation over the K&T wiring may have happened.

CONCERNS ASSOCIATED WITH KNOB AND TUBE WIRING

Attic Insulation over K&T
K&T wiring was designed to dissipate heat. Running K&T wiring in attics was a common practice. Original attic insulation was only several inches deep and the K&T wiring was ran above the insulation. It goes without saying that more insulation is better as we are all trying to reduce our energy costs. It is very common to find K&T wiring buried under added insulation. This is a fire hazard because not the wiring cannot dissipate heat and depending on the combustibles in the attic a fire may start.  To the right is a picture of K&T wiring buried in cellulose insulation that I found while performing a home inspection in Minneapolis Minnesota.  The yellow tool I’m holding is a non contact voltage tester, the red light at the end is indicating the wire is live. You can see the wire goes into the insulation.

No Ground Wire
Modern wiring has three wires, a hot, neutral, and a ground. K&T wire does not have the ground wire, only the hot and neutral. The purpose of ground wire is to reduce the risk of electric shock from current leaking into uninsulated metal parts of an electrical device. Surge protectors require a ground to function properly, without the ground they provide no protection of your electrical device plugged into it such as a computer.  Homes wired with K&T do not provide an option for a ground.
Deteriorated Insulation
The insulation on K&T wiring is very old and may have deteriorated exposing the live electrical wire inside. The insulation can become brittle and crack if the wire is bent. It also may have absorbed moisture causing it to deteriorate.  It is very common to find K&T wiring ran in the floor joists in the ceiling in a basement. This is very accessible for you to come in contact with it resulting in the possibility of an electrical shock.

Improper Modifications
The original K&T wire system in a typical home was very limited by today’s standards and is usually supplied by a 60 amp service. Today we have the need for a larger electrical system that typically results in modification to the original K&T wire. Its common to find modern wiring improperly spliced into the old K&T wiring. An improper wire connection may build heat increasing the risk of a fire. Extending a single branch circuit to far may result in voltage drops and or blown fuses. Constantly blowing fuses temps people to tamper with their fuse box to prevent this. I am purposely not going to give examples of “tricks to keep your fuses from blowing” to prevent you from doing so. If your fuses are blowing there IS A REASON!!!

Another thing to consider is that insurance company’s may refuse coverage or considerably increase rates on homes with K&T wiring. After reading the reasons listed above you can start to see why.  Exceptions may be made if the system was certified to be safe by a licensed electrician.  You may be wondering how did the previous owners obtain insurance. The reason is insurance companies are always changing their ratings based on statistics. As time goes on we are going to see more and more fire claims made from fires caused from old wiring. When the previous owners had their policy written their insurance provider did not consider it a risk and haven’t gone back to update old policies. When new policies are written they may or may not take K&T wiring into consideration.

If your considering buying a home with K&T wiring take into consideration the added expense of rewiring the home. It may be possible to negotiate this in the purchase price. If your already living in a home with K&T wiring consider having the system fully evaluated by a licensed electrician.

Pete Busch
Minnesota Home Inspector.