Knob and tube wiring simply gets its name from the way it's installed. There are ceramic tubes for the cloth covered wires that run through the wood framing of the house and the knobs are used when the wires run alongside or next to the wood framing. The two wires (there is no ground wire) are separated approximately four inches apart. One wire is the black "hot" wire, and the other wire is the white "neutral" wire (most knob and tube wires do not have different color insulation jackets that we see in today's wiring). All the connections for knob and tube wiring are open and visible. The wires are simply spliced and soldered together with older style fibrous electrical tape around the splices. If this electrical tape is not cloth covered, consider it an unprofessional splice. These shiny taped splices need to be upgraded to include a junction box.
Knob and tube wiring was installed in houses up until about 1945, although in the rural areas up until about 1950. As I already stated, knob and tube wiring does not contain a ground wire. A ground is necessary if you are plugging in today's appliances that have the third prong in the plug. However, if the knob and tube wiring is limited to bedrooms, this is not necessarily a safety hazard. Plugging in a two prong lamp or clock is just as safe as a three prong grounded outlet. But with today's technology (and all the three prong appliances on the market) the average family will need at least one grounded receptacle installed per room in order to make the home a much safer and more convenient living area.
Knob and tube wiring is not necessarily dangerous. If it was installed properly and the insulation jacket around the wire is in good condition, and it was not abused with over splicing and connections, and it is not buried in house insulation, it can provide many more years of reliable service. It is the wiring that has been abused by homeowners that make this wiring a potential hazard. On its own, knob and tube wiring is not inherently a problem. If the knob and tube wiring is on top of the attic floor (which most are not due to insulation upgrades), it could be easily nicked or the insulation could be worn off, causing a safety hazard. If the knob and tube wiring is in a traveled area or even in an area for "just storage", I highly recommend that it be protected or replaced.
Although knob and tube is a workable system, and completely safe when installed and used properly, there are many concerns with this electrical system today:
-THERE'S NO GROUND WIRE (for more modern lifestyle requirements and safety). See "Ungrounded Outlets".
-A fear exists that the hot and neutral wires can make contact with each other (a potential fire and safety hazard).
-The rubber and cloth insulation around the knob and tube wiring breaks down over time and becomes brittle (a potential fire and safety hazard).
-It would be too costly to maintain or even install this type of wiring today.
-More importantly perhaps, mortgage companies are now refusing to finance houses and insurance companies are refusing to provide home owners insurance on houses with existing knob and tube wiring that is active and live today.
-Knob and Tube wiring can not be run in or under insulation. This often happens when outside walls or attics are insulated. Older wiring was installed in open spaces so that it would stay cool. The insulation around the wires is made of rubber that burns at a relatively low temperature. If surrounded by house insulation, the knob and tube wires will not cool enough and could heat up enough to burn. It is important that if an old house is to be insulated, that all knob and tube wiring be removed and re-wired with today's Romex wiring before insulation is added. This is one reason why insurance companies will not insure homes with knob and tube wiring. They are afraid that home owners will bury the wiring when upgrading their attic or wall insulation.
-Many appliances that we plug in today contain a three prong plug. The large round prong is the ground. Many homeowners (with knob and tube wiring) are utilizing those little three to two prong adapters for today's appliances. Those three to two prong adapters bypass the ground safety circuit. This is a particularly bad idea if you are plugging in a computer or television because they dissipate harmful static electricity through the ground wire. Without a true ground, you're putting yourself and your equipment at risk.
-Using knob and tube wiring for wall outlets is where hazards are more likely to occur. Everyday items in the bathroom can quickly overload a 15 amp circuit. A 1500 watt hair dryer and a 300 watt curling iron plus a light bulb will definitely exceed 15 amps.
-A cost effective strategy in the older homes is to abandon any of the electric outlets wired with knob and tube and have a licensed Electrician install new grounded outlets with modern Romex wiring. You can keep the current knob and tube overhead lighting wiring in service. However, I still recommend eventually updating and replacing all of this old wiring due to issues that I have already discussed.
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I recently accompanied David Valley during his inspection of the house my son was buying. Having purchased a number of houses during my lifetime, I considered myself very knowledgeable and thorough. However, he surpassed me greatly, picking up on important details I would have missed. He frequently suggested ways on how something could be fixed or improved. He displayed broad knowledge of house structure, heating and cooling system, plumbing, electrical, etc. David is the best home inspector I have ever employed and I recommend him implicitly. Thank you for a great home inspection.