When installed correctly knob and tube wiring was, in some ways, superior to current wiring practices. Unfortunately, this system is rarely intact after 80 or so years of use. Things that happen well after the original installation can cause major problems. For this reason, knob and tube will normally need to be replaced, especially if the insurance company requires it!
Basically there are two ways to replace knob and tube wiring. The first method is to waste no time trying to preserve walls or ceilings and run the wires in the fastest and shortest route. After the electrical wiring is installed and inspected, the damage needs to be repaired. The cost for this type of electrical work is lower if it's completed faster. Although you will save on electrical costs, you need to factor in the cost of repairing and repainting the walls and ceilings. Just remember there will also be a lot of inconvenience due to dust and dirt. The second method is much slower and cleaner and involves looking for ways to fish the wiring through the walls and ceilings. Because this process is so time-consuming you will pay more for the electrician's time. The up side is that you'll have little or no damage and only minor disruptions to your home. The other question is the size of the main at the meter. Old K&T installations were typically undersized by today's standards [see "Is bigger really better?"].
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Time, heat, and hungry critters stripped insulation here. |
OK so knob and tube was a great wiring method in it's day but it really doesn't meet todays demands. All you have to do is check Article 394.12 (5), Uses not permitted: Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors. How many homeowners are willing to give up insulating said areas just to keep the good old K&T wiring? Not many! Most homes either have rolled insulation in the attic or 2 ft. of blown insulation. The reason most remodeling contractors don't remove the old sheetrock or plaster is because they can blow insulation materials into the walls. It's just real hard to keep this method and end up with an energy efficient home.
The Model-T Ford was a great car for it's day, but even old Henry Ford had to give up and produce a new model. But as we continue to remodel existing buildings in certain areas, we may never see the end of Knob and Tube . Some properly loaded and cared for, unmolested installations have been working for over a century and will continue to do so. Builders are often keen to leave certain architectural and interior finishes in place that are too hard or too expensive to replicate today. Often the decision on keeping older wiring in place must be weighed against the cost of maintaining those finishes, verses the safe loading and general condition of the circuitry they contain.
Our advice about improving the safety and reliability of knob and tube electrical wiring
Inspect the whole electrical: An expert should inspect the condition of the building electrical wiring, including the wires, connections, devices like receptacles, switches, and overcurrent protection by fuses or circuit breakers.
Replace bad circuits: Knob and tube circuits that have been modified, damaged, or covered with insulation should be replaced with a modern grounded electrical circuit.
Ground fault protection GFCI circuit protection and possibly arc fault protection can be added on two-wire un-grounded electrical circuits to reduce the chances of electrical shock or fire - steps that we recommend.
Thermal Insulation Knob & tube wiring is designed to dissipate heat to free air. The NEC® requires that attics and walls with this type of wiring not be insulated in such a manner that the insulation envelopes the wire. However, the California Electrical Code has amended section 324-4 to allow insulation in contact with knob & tube provided 6 conditions are met. These are: 1. A licensed electrical contractor must inspect the wiring and certify that it is safe. Any unsafe conditions, such as improper splicing or overfusing, must be corrected. All corrective work must be done with proper permits. 2. The certification must be filed with the local building department, and a copy furnished to the property owner. 3.Accessible areas where insulation has been installed over the wiring must be posted with a warning sign. In some jurisdictions, the sign must be in Spanish and English.
4. The insulation must be noncombustible. Paper-backed fiberglass would not qualify. 5. The insulation must be nonconductive. Foilfaced fiberglass would not qualify. 6. Normal permit requirements for insulation still apply. The insulation must meet standards of fire resistance per section 215.2 of Part 2 of Title 24. Even materials such as “Palco Wool” (redwood shavings) or crepe-type paper insulation are treated with materials to inhibit flame spread. Some materials, including loose cellulose, have failed flamespread tests after they were installed. Cellulose insulation is made of ground-up newsprint and a fireproofing chemical (usually a borate) mixed dry. It is possible the chemicals settle out of the insulation, reducing its fire resistance. Loose fiberglass does not have the same concerns regarding flammability. Additional hazards may be present if the walls have been insulated. Packing the insulation material around the knob & tube could damage the wiring in a concealed space.
Reporting Almost inevitably, other problems will be found when knob & tube wiring is present. Even if the system appears to be in pristine condition, there is a high likelihood that a recommendation will be made to refer to a qualified electrical contractor. If no thermal insulation is present, that by itself will be a concern to the client because of energy issues, and when installed, thermal insulation must comply with the California amendments to the NEC.
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