"Knob & Tube" Wiring

Knob & tube wiring is a type of wiring which was in common use until the 1940's and sometimes used as late as the 1950's. The nick-name is derived from the ceramic knobs that are employed to insulate and secure the wiring runs and the ceramic tubes employed to protect the wires where they pass thru potentially abrasive materials (primarily wood joists, studs etc.) Unlike subsequent wiring systems where all the wires in a run are enclosed in a cable, the two wires (black/hot and white/neutral) run separately and only come together at a terminal (switch, receptacle, fixture, junction box etc.).

Knob & tube wiring does not provide a third wire for grounding and is therefore considered unsafe in kitchens, bathrooms, laundry rooms and outdoors by current construction standards. In other areas, knob & tube wiring that is in good condition with sheathing intact, properly protected from damage, and that hasn't been subjected to extended periods of overloading which can cause it to become brittle, should not pose an increased safety risk.

Aside from the preceding concerns, the primary risk with knob & tube wiring is its relative accessibility for amateur repairs, upgrades and maintenance. It is not uncommon to see a system with knob & tube wiring that has a history of amateur work (poorly joined connections, unfastened runs, unprotected wires, etc.). If a house does have knob & tube wiring, it should be inspected to ensure that it is properly installed and in good condition.

Many older homes with originally installed knob & tube wiring have had some of the wiring upgraded. While modern wiring is visible in many areas, much of the knob & tube wiring may still be in place and concealed beneath floors, above ceilings and behind walls.

If a few, but not all, of the circuits are in poor condition they can be replaced eventually without rewiring the whole house. However, if most or all of the circuits are in poor condition, it may be more economical to completely rewire the home. Rewiring can also allow the electrical system to be more convenient as the new circuits can be designed to accommodate the present lifestyle of the occupants of the home.