Is it safe?
Most codes still recognize the knob and tube method, although it is not used for new installations. Several issues make the method problematic, though.
• Knob and tube relies on the free flow of air around the wiring to reduce heat buildup. This means that thermal insulation cannot be installed in direct contact with the wiring. Installing insulation in attics and crawl spaces containing this wiring would require extra care.
• Grounding conductors were not installed as part of this method. See grounding.
• The old insulation on the wires is often degraded - sometimes to the point of complete failure. If the insulation has become brittle, the wiring is unsafe and should be replaced
• Knob and tube wiring can create electrical "noise" due to something called inductive coupling. Because the wires are run separately, any current flow through them will create a magnetic field. This magnetic field can induce unwanted currents in adjacent wires, creating electrical "noise".
• Many insurance companies will not cover properties that contain knob and tube wiring
• Knob and tube relies on the free flow of air around the wiring to reduce heat buildup. This means that thermal insulation cannot be installed in direct contact with the wiring. Installing insulation in attics and crawl spaces containing this wiring would require extra care.
• Grounding conductors were not installed as part of this method. See grounding.
• The old insulation on the wires is often degraded - sometimes to the point of complete failure. If the insulation has become brittle, the wiring is unsafe and should be replaced
• Knob and tube wiring can create electrical "noise" due to something called inductive coupling. Because the wires are run separately, any current flow through them will create a magnetic field. This magnetic field can induce unwanted currents in adjacent wires, creating electrical "noise".
• Many insurance companies will not cover properties that contain knob and tube wiring
What should a homeowner do?
We recommend, at the least, that heavy appliance loads be removed from knob and tube circuits. This is done by installing new wiring outlets as needed. This will relieve some of the loading that can cause destructive heating as well as provide equipment grounding. Whenever a space is renovated, knob and tube should be removed. It is usually less expensive to install new wiring when walls and ceilings are accessible. Keep in mind that codes usually prohibit the extension of new wiring from existing wiring that doesn't include a ground conductor.
While knob and tube is not in and of itself a dangerous wiring method, it is old. The early cable systems that replaced knob and tube are actually more prone to failure. However, like other building systems, electrical wiring does have a finite useful life after which it should be replaced. In the intervening 80 or so years since the wiring was installed, it has likely been repaired, modified and added to by electricians with a range of competency. It is usually better to plan an upgrade than to be forced into one due to sudden failure.
While knob and tube is not in and of itself a dangerous wiring method, it is old. The early cable systems that replaced knob and tube are actually more prone to failure. However, like other building systems, electrical wiring does have a finite useful life after which it should be replaced. In the intervening 80 or so years since the wiring was installed, it has likely been repaired, modified and added to by electricians with a range of competency. It is usually better to plan an upgrade than to be forced into one due to sudden failure.
The rules and methods for wire installation have changed over the years. The earliest method of wiring, called knob and tube, is still in use in many older homes. In this method, wiring is installed as individual strands; where the wires must pass through wood framing, it is run through a porcelain "tube" which is inserted into a bored hole. The "knobs" are porcelain fittings that allow wires to be attached parallel to framing timbers. The wire is sleeved in a flexible fiber jacket when it must be fished through a space or enters a junction box.
