Issues with knob and tube wiring

Since January 2003 the Electrical Safety Authority has received an increasing number of  questions about the safety of knob and tube wiring. In particular, purchasers or owners of older homes are finding that many insurers will not provide or renew coverage on such properties. In some cases, the insurance companies are requiring a total replacement of this wiring prior to providing insurance coverage.

Knob and tube wiring, more recently referred to as open wiring, was a wiring method used in the early 1900s to 1940s in the residential sector. Over the years wiring installation practices have changed in the residential sector and knob and tube wiring is no longer installed; however, parts continue to be available for maintenance purposes.

Diagram F1- Typical knob and tube installation

 

Existing knob and tube conductors concealed in walls, floor spaces, etc; supplying general lighting and receptacle circuits are permitted to remain in place if:

• They are protected by a 15 ampere fuse or circuit breaker; and
• No additional outlets have been added to the original installation so as to overload the circuit; and
• The conductors, where visible, appear to be in good condition.

If your home has knob and tube wiring, we recommend that you follow these guidelines:

 

• Have a Robert Redford Electrical Services check the “knob and tube” conductors in your existing installations for sign of deterioration and damage; or request a general inspection from ESA. The General Inspection report will identify visible electrical safety concerns in your electrical wiring.

 

• “Knob & tube” conductors should be replaced where exposed conductors show evidence of mechanical abuse and or deterioration, poor connections, overheating, alterations that result in overloading, or if changes to wiring contravene any section of the Electrical Safety Code.

Homes with knob and tube wiring may not have the electrical capacity to meet today’s needs. As a result, homeowners have modified their electrical system with what the Electrical Safety Authority classifies as unsafe practices:

• Improper use of extension cords – using improperly rates extension cords, or using extension cords as permanent wiring;
• Improper fuse replacement – using 20 or 30 amp fuses to replace15 amp;
• Improper connections - adding receptacles and outlets on existing circuits or improperly connecting to the knob and tube wiring (this work should be done by a licensed electrician);
• Removing ground pins – ground pins on power bars or electrical equipment should not be removed to accommodate the two pin receptacles used in knob and tube wiring (2 pin to 3 pin are not permitted)
• Improper replacement of two pin receptacles. If you require a three prong receptacle, only use a GFCI receptacle.

Homeowners who are planning to modify their knob and tube wiring, or any other electrical wiring, should have the work performed by a licensed electrical contractor or electrician

Receptacles in existing knob and tube installations

Where grounding type receptacles (three pin) are installed in existing knob and tube installations to replace the ungrounded type (two pin) receptacle, special caution must be exercised.

Diagram F2-Two and three pin receptacle configuration

 

Two Pin (ungrounded)                     Three Pin (Grounded)

Rule 26-700(7) requires the installation of a bond conductor to bond the receptacle to ground. This is permitted to be an external bonding conductor that is connected to either the system ground conductor or a metallic cold water pipe that is bonded to ground. This method may be difficult to accomplish.

As an alternative to bonding, Rule 26-700(8) of the Code also states "grounding type receptacles without a bonding conductor shall be permitted to be installed provided each receptacle is protected by a ground fault circuit interrupter of the Class A type that is an integral part of this receptacle; or supplied from a receptacle containing a ground fault circuit interrupter of the Class A type; or supplied from a circuit protected by a ground fault circuit interrupter of the Class A type (a GFCI breaker in the panel, or either a GFCI receptacle or a GFCI dead front mounted in an outlet box next to the panel). Where this option is used, no bonding conductor is permitted between outlets unless that bond conductor is in turn connected to ground. GFCI protection of the receptacles does not provide a ground reference to the U-ground slot of the receptacles. Some appliances require a bond be connected to the U-ground slot in order to function properly. For example, surge protective devices for computer or entertainment equipment will not function without a ground reference.