Saturday, 21 January 2012

Knob and Tube Wiring




Knob and tube wiring was the original internal wiring system installed in homes across North America.

The following information will provide insight into:

  • ·         The Main Components
  • ·         Buyer Considerations
  • ·         Knob and Tube Advice
The Main Components
Knob and Tube (K&T) Wiring can be found in buildings built in the late 1800’s and first half of the 1900’s. It is sometimes referred to as “Open Wiring” because it consists of two wires, live and neutral, suspended throughout the structure with the use of insulating “knobs” for surface mounting and “tubes” for passing through wooden beams and other flammable barriers.  The copper strands are insulated with rubberized or asphalt saturated cloth. These systems were designed for a much smaller electrical draw (ampacity) than our usage requirements are today.  The original fuse type panel boxes were typically rated for 60 amps or less.

 Some potential problems occur when the system is improperly tapped into with additional circuits, the cloth wrapping material around the wires deteriorates or is broken, or walls and ceilings are insulated covering the components thereby trapping excessive heat in the wires causing damage.

Knob and tube wiring was eventually displaced from interior wiring systems because of the high cost of installation compared with multi-strand power cables, which combined both power conductors of a circuit in one run (and which later included grounding conductors). With most conversions, all traces of the K&T wiring is removed. Sometimes however, the old system is disconnected and left in place. This can be deceptive and should be tested for live current.

Considerations for buildings with K&T wiring
As existing K&T wiring gets older, insurance companies may deny coverage due to a perception of increased fire risk. Several companies will not write new homeowners policies at all unless all K&T wiring is replaced, or an electrician certifies that the wiring is in good condition. Also, many institutional lenders are unwilling to finance a home with limited ampacity i.e. 60 amp service (which, as noted above, often goes hand-in-hand with K&T wiring), unless the electrical service is upgraded.



Advice for those with K&T wiring:
  • Have the system evaluated by a qualified electrician. Only an licensed professional should confirm that the system was installed and modified correctly.
  • Do not run an excessive amount of appliances in the home, as this can cause a fire.
  • Where the wiring is brittle or cracked, it should be replaced. Proper maintenance is crucial.
  • K&T wiring should not be used in kitchens, bathrooms, laundry rooms or outdoors. Wiring must be grounded in order to be used safely in these locations.
  • Rewiring a house can take weeks and cost thousands of dollars, but unsafe wiring can cause fires, complicate estate transactions, and make insurers skittish.
  • Homeowners should carefully consider their renovation options before deciding whether to rewire their house.
  • The homeowner or an electrician should carefully remove any blown-in insulation that is found surrounding K&T wires.
  • Prospective home buyers should get an estimate of the cost of replacing K&T wiring. They can use this amount to negotiate a cheaper price for the house.
 Hopefully this information will help you and your clients better understand some of the factors involved with buildings where Knob and Tube wiring is present. If you suspect that a prospective property has live Knob and Tube wiring, have your home inspector verify its presence during the Home Inspection and discuss recommended next steps.