Real Estate Resources

Knob & Tube Wiring.

Having misgivings about old wiring and insurance ?

One of the most frequently asked questions during a home inspection is what the electrical service is. As professionals we often take for granted that everyone purchasing a home is aware of the various types of electrical service and types of wiring.

Let’s examine two types of electrical issues that exist in many older homes.

60 Amp Service: 60 amp service in itself does not create an unsafe or hazardous situation. It becomes unsafe only when the homeowner places more demand on the existing service. This will depend on the size of the family as well as there lifestyle. Typically, 60 amp panels become a fire and safety concern when occupants use larger fuses than the ones specified. 60 amp service supplies the home with the average number of lights and receptacles and one stove. Typically household circuits are on a 15 amp rating.

The increase risk occurs when one or more of the following are added:

• Modifications to increase the number if circuits, receptacles or lights.
• The installation of a major electrical appliance in addition to the electric stove.
• Overfusing 15 amp fuses with larger ones.
• The additional demand on supply due to appliances or the size of family. Insurance companies work on the law of averages. A 60 amp service with knob and tube wiring my lead to an increased risk of fire if abused. New houses are supplied with a minimum of 100 amps.

You are required to upgrade your service if you consider installing one or more of the following appliances to an existing 60 amp service:

• Central air conditioner
• Electric cloths dryer
• Whirlpool/hot tub Note that the addition of numerous small appliances such as microwave ovens, computers, stereo equipment and the like will contribute to a significant draw on the system.

Although a 60 amp service is legal, some jurisdictions require that the service be upgraded to at least 100 amps, especially if the meter is located indoors as with older installations.

Knob and Tube Wiring: This type of wiring was installed in houses up until about 1950. This system consists of two wires, one black or hot wire and the other white or neutral to create a circuit. These two wires are held in place with ceramic knobs and tubes. Knobs are used to clamp the wire to structural members, while tubes are placed in holes in the structural members to prevent the wire from chaffing.

In modem household wiring, these two wires are bundled together with a ground wire in a single plastic sheathing cable that runs through holes in the structural members and is held in place with clamps. While knob and tube wiring is not inherently dangerous, it is old and its insulation may no longer be intact. Much of this wiring is concealed behind walls, ceilings and insulation where its condition cannot be completely evaluated. In addition to the wiring being deteriorated, these wires are connected by soldering together and wrapped in electrical tape. After time, this tape either falls off or deteriorates. Knob and tube wiring is usually associated with older installations consisting of 60 amp service. The wire is fused with 15 amps. This installation handled a total of 12 circuits, thus the houses have fewer receptacles than modem houses. To prevent fuses from constantly blowing, home owners put in higher rated fuses such as 20 or 30 amps!

Given that the wire was not intended to carry this additional current, the insulation becomes brittle exposing more wiring, or worse, overheating to the point of causing fires. Knob and tube wiring does not have a ground conductor. This is identified by two prong receptacles as opposed three pronged receptacles. A ground conductor is necessary if you are plugging in appliances that have a three prong plug. Modem receptacles also have one prong slightly larger than the other. This is necessary to prevent reverse polarization. Limitations with knob and tube wiring:

• Usually restricted to a maximum of 60 amp service
• The wire is old, and its insulation my no longer be intact
• It is not a grounded system making it more hazardous than modem wiring
• Two pronged receptacles, restricting the use of small kitchen appliances

It is true that in bedrooms, living and dining rooms, plugging in a TV or lamp poses very little risk. The opposite is true in areas where you may come in contact with water such as bathrooms, kitchen, basement, laundry and outdoors. An ungrounded system in these areas could be potentially hazardous. In fact, a good building practice is to install GFCI (Ground Fault Circuit Interrupters) receptacles in these areas. Furthermore, newer stereo equipment, computer and appliances are affected by incorrect polarization something that this type of wiring cannot prevent. Many manufactures of appliances and equipment will not honor warranties if used with ungrounded circuits.

Safety Tips:

• Never replace blown fuses with larger amp fuses
• Never cut the grounding (third) prong off a plug to fit into a two hole receptacle
• Do not use outlet multiplier plugs to connect several appliances to one receptacle. Knob and tube wiring becomes a problem when it is abused. If you have such wiring, it would be wise to have it evaluated by a licensed electrician. If necessary, it should be upgraded to modem wiring.

Written in in August 2003
Ray Kornelson, RHI Pillar to Post Professional Home Inspection