Knob & Tube Wiring

When purchasing an older home it is important to know that there is a possibility that it still has knob & tube wiring.   Some of you may be familiar with this type of wiring but for those of you who aren't knob & tube wiring was the standard form of electricity used in residential houses before 1950.  It consists of two wires (a black and a white) running independently of each other and clamped onto porcelain insulator knobs with ceramic tubes used to line the holes where the wires pass through wood floor joist and beams, with no grounding wire.  Where conductors entered a wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth insulating sleeving called a loom.


 Knob & tube wiring could be made with high current carrying capacity. However, most existing residential knob & tube installations, dating to before 1950, have fewer branch circuits than is desired today. While these installations were adequate for the electrical loads at the time of installation, modern households use a range and intensity of electrical equipment unforeseen at the time.  As such modern home buyers often find that existing knob & tube systems lack the capacity for today's levels of power use and often first-generation wiring systems became susceptible to abuse by homeowners who would replace blown fuses with fuses rated for higher current. This over fusing of the circuits subjects wiring to higher levels of current and risks heat damage.

Some of the other problems and risks are:

1. This type of wiring system has no ground wire.

2. Given their age, wires are highly susceptible to wearing and exposure, thus presenting a serious safety hazard.

3. Unintentional contact of the hot and neutral wires may potentially cause an electrical fire.

Some still believe this type of wiring to be safe so don't fret if your house currently has knob and tube.  Just be aware that you need to make sure that you are taking care of it properly.  By its very nature it is generally much more accessible than modern day wiring, but don't be tempted, call in a professional!

Although it may still be safe, insurance companies are leery of it. In recent months, a number of our clients have discovered that they can't obtain insurance for the house they are about to buy because it has knob & tube wiring and it is mandatory for them to update the wiring within a certain time frame (usually 30 or 60 days).  However, don’t despair there are a few insurance companies out there that still insure this type of wiring and the cost is not prohibitively expensive.  Insurance companies are constantly changing their policy requirements so if you find yourself in that position please feel free to contact me directly for more information.

The overall moral to this blog is that if you are thinking about purchasing an older home, make sure you have the electrical system inspected before putting in an offer.  If it turns out the house still has knob & tube, adjust your offer to reflect the realistic costs of replacing the wiring.


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