What is Knob and Tube Wiring?
Knob and tube wiring (K&T) is an early electrical wiring system used in homes built from the 1880s through the 1940s. The system uses individual insulated copper conductors run through porcelain knobs (which support the wires along structural members) and porcelain tubes (which protect wires passing through framing). K&T wiring was the standard for its era and is still legal in many jurisdictions as long as it is in good condition, but it presents significant concerns for modern use.
Why knob and tube is problematic
K&T wiring was designed for the electrical loads of its era -- a few lights and a radio. Modern homes draw far more power for appliances, electronics, HVAC, and other loads. K&T circuits typically run on 15-amp fuses without ground wires, making them inadequate and potentially unsafe for modern electrical demands. The lack of a ground wire means no ground fault protection, increasing shock and fire risk.
Insulation is the other major concern. Blown-in insulation installed over K&T wiring can cause the wires to overheat, creating fire risk. Many building codes prohibit adding insulation in contact with active K&T wiring.
Insurance and lending impact
Many insurance companies refuse to insure homes with active knob and tube wiring or charge significant surcharges (50-100% premium increases). Some lenders will not fund mortgages on homes with K&T. These restrictions directly affect both your ability to insure an investment property and your buyer pool at disposition.
Rewiring costs
Full rewiring of a home with knob and tube typically costs $8,000-$25,000 depending on home size, accessibility, and local labor rates. The work involves running new Romex (NM-B) cable through walls and ceilings, installing a modern electrical panel, adding grounded outlets and GFCI protection, and disconnecting the K&T circuits. Permits and inspections are required.
Investor strategy
K&T wiring is common in pre-1950 homes in the Northeast, Midwest, and West Coast. Properties with active K&T are discounted due to insurance and lending restrictions. Budget rewiring into your repair estimate and verify that the discount you receive exceeds the rewiring cost plus hassle factor. Post-rewiring, the property is fully insurable and financeable, opening it to the full buyer market.