Knob-and-tube wiring was a method of branch wiring found throughout homes and buildings in North America between the late 1800s through the 1930s. These circuits consist of an insulated copper wire that passes through porcelain tubes when penetrating a stud or joist, and is secured to the framework with porcelain knob clamps. If knob-and-tube wiring is installed correctly, is not modified, and is used within its load limits then it could be a very reliable form of branch wiring in a house.
As a home inspector, I almost always report knob-and-tube wiring as a safety hazard. This is an evaluation made on many different factors. First, knob-and-tube circuits were not designed to support today’s levels of power use. When these systems were installed a whole house might only contain 4-8 circuits for the whole house. Even today’s simplest homes easily have double or triple this amount of circuits.
Second, we often deem knob-and-tube wiring unsafe due to building upgrades and modifications that might cause damage to the circuits. We often find that residents in these older houses will increase comfort by insulating walls and attic spaces. This is a big red flag if found because the wiring has an intentional air gap so the circuit can dissipate heat that builds up during power consumption. Also, we often find knob-and-tube wiring spliced or tied together without being in junction boxes or using safe splicing devices. Even though this was regular practice 85-135 years ago, when the circuits have been modified or have deteriorated from age these splicing points could be problem areas if circuits are overloaded.
The last factor to mention is the fact that knob-and-tube circuits do not contain a grounding circuit. A grounding circuit is often found throughout homes aging back to some homes built 50 years ago. The ground conductor in an electrical circuit gives electricity somewhere to go if the hot/neutral circuit would fail. This means that if you come into contact with a malfunctioning circuit a ground conductor can keep the full current from attempting to ground out though your body. Also, grounding conductors make it possible to use GFCI and AFCI devices, which help create safer electrical systems.
In conclusion, knob-and-tube wiring was an effective branch wiring method used to build homes and buildings, but is far outdated, often modified or overloaded due to current power demands from residents. A faulty knob-and-tube wiring circuit can cause malfunctions that could lead to shorted circuits, arching, and electrical fires. Knob-and-tube wiring should be inspected by a residential electrician when found to ensure that the wiring can support the current circuit. Lastly, knob-and-tube branch wiring does not usually use a ground conductor, so these circuits are not usually grounded, GFCI or AFCI protected.