Knob and Tube Wiring-In Depth History and Issues
Updated: Mar 2
Over the years, I've seen many types of wiring, and many defects with them. I'm going to start with a very abbreviated History of modern Home wiring and wiring issues. Lets start with Knob and Tube Wire.
Knob and tube was the first “modern” wire system. It consisted of 2 wires (for each circuit) strung along on “knobs” (Porcelain insulators and spacers that held the wire away from the wood framing) and passed through “tubes” (Porcelain hollow insulators that were drilled through the wood house framing to allow the wires to safely pass through). The wire was known for running “hot” (The wire would heat when current was drawn through it) and the porcelain knobs and tubes protected the surrounding wood framing from heat and fire by keeping the wire spaced away from it. A thick (often asbestos containing) hollow cloth composite tube called "loom" was used to insulate wires where they cross over each other and enter light, outlet and switch boxes. The thick wires entering the outlet shown below are loom.
Knob and Tube wires were spliced together (by “wire wrap” or soldering) and taped with cloth electrical friction tape. The splices were in open air and had no boxes or enclosures. Over time, some of these connections form corrosion and resistance - generating more heat at the splice. Between the connections, the wire size and the age - this wire can sometimes cause a voltage drop (The voltage - e.g. 120 volts - would be reduced and often would be much lower at the outlet or light fixture. We have measured the voltage in faulty knob and tube wiring systems to be as low as 89 volts - A POTENTIAL HAZARD as the "lost" voltage has likely been given off as HEAT IN THE WALLS/WIRING SYSTEM!
(This is why we now offer detailed separate electrical inspections - with outlet voltage testing and reporting on every accessible outlet for our clients)
Often modifications or changes to the antiquated knob and tube wiring can cause even more potential hazards. Common modifications include splicing into the wire improperly such as shown below.
There were several variations of knob and tube including wood conduits (decorative wood trim which had 2 or 3 grooves for wires to run through with a thin wood decorative "cap" over the wires. This conduit became known as a potential fire and shock hazard and was phased out of use.
Another early knob and tube wire type was "Loom Wiring" - which involved running unfastened knob and tube wire through walls with no fasteners and no spacers. The wire was completely encased in the protective loom material along it's entire length. This wiring was also known for alot of fire and shock hazards and was quickly phased out.
(I'll add a picture here of loom wiring when I find one - many loom wired houses had electrical fires and there is not much left out there)
Early Knob and tube wire originally entered the house in attics often right next to the connection on the exterior to the street - and this is where the main fuses and sometimes the main were often found. The Early wood electrical box and cover shown below are lined with a woven sheet form of asbestos for "fire protection". The Hot AND the Neutral wires were often both fused (The two top center wires are the street power coming in - 1 Hot and 1 Neutral. Fusing both the neutral and offered no real extra electrical protection - the practice was discontinued shortly after - and the HOT wires only were typically fused from then on) Note that all the exposed connections and live contacts on the porcelain fuse holders were serious - and sometimes LETHAL shock hazards!
Some systems had main disconnects outside of any box or enclosure - often mounted to a board with asbestos backing. These were even greater shock hazards as one could simply lean against or fall onto them and be electrocuted.
Knob and Tube wire was commonly in use from the 1890s until the early 1920s but can be found as far back as 1880 and was still used in some rural areas until the 1940s!
The wire itself was a tin/nickel clad copper wire surrounded by nitrogenized rubber with a cloth (and occasionally asbestos cloth) covering. The wire was often run at amperages far too great for the wire size - resulting of dangerous overheating of the wires. For example: 14 Awg (15 Amp) wire would often be fused at 25 or 30 Amps! (Resulting in huge voltage drops and heating of wires)
Well, now that you've heard the history, What does it mean to you?
Knob and tube wire is ungrounded; This means that it should only feed 2-slot polarized (The Neutral slot is larger than the Hot slot in the outlet) outlets. (In some areas they allow 3-slot outlets to each have a sticker saying "No Equipment Ground" - but this is foolish in my opinion - as who reads the outlet before they stick a plug into it??) Ground wires are an essential part of electrical safety and ungrounded wiring should be rewired for safety where possible.
Houses with knob and tube wire cannot be safely insulated. Because knob and tube REQUIRES open air to cool - it is one of the few wire types in homes that cannot be installed in any insulation. If a house has blown in (or any) insulation in the walls then all knob and tube should be rewired for fire safety. Note: Many Homeowner insurance policies do not cover fires or damage from houses with knob and tube installled. Clients over the years have discussed fires that insurance companies refuse to cover because live knob and tube wire was present in the house. Consult your insurance company - many require you to get an extra cost "Knob and Tube Rider" to be properly covered.
Knob and tube voltage drops can damage equipment and electronics. Severe voltage drops on knob and tube seems like a "Brownout" (When the power grid has so much usage that the lights dim - a symptom of voltage drop) I have lived in a home with knob and tube earlier in life. I lost several appliances, power supplies, and a microwave before I realized the knob and tube's voltage drop hastened their demise.
Knob and tube is OLD. I mean REALLY OLD! The wire and it's insulation rubber are often brittle and long past it's expected life span!! This wire - although it may appear to function now, and has lasted so long - IS A SERIOUS POTENTIAL FUTURE FIRE RISK and should be rewired. A Note About "Rewiring": We have inspected MANY homes that were "rewired completely" or "totally rewired" - and live knob and tube was found in upper floors, attics, light fixtures etc. IF you rewire - DO IT COMPLETELY (And ask your electrician to verify all knob and tube has been removed - some of our clients had the electrician write them a letter stating that all knob and tube was completely rewired - to help protect against any insurance "You had knob and tube so no check" issues in the future)
Please stay tuned for more articles on electrical wiring history in homes. Written by Michael J Scaduto Home Inspection Lic# HI 213-1. Tri-Value Consultants 993 Summer Street Lynnfield MA 01940 - firstname.lastname@example.org - 781-334-3830
Some additional web links for knob and tube information:
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