If your home was built before about 1950, there is a fair chance some knob and tube wiring is still in service. It is common in older neighborhoods of San Francisco, Oakland, Berkeley, Pasadena, Sacramento and Los Angeles.
How to recognize it
Look in the attic or crawl space for single wires running separately through white ceramic knobs and tubes, with cloth-covered insulation. At the outlets, the giveaway is two-prong receptacles and no ground.
Why it is a concern
- No ground wire, so three-prong equipment is not properly protected.
- Insulation that has become brittle with age and heat.
- It was designed to cool in open air, and is often buried in attic insulation added later.
- Decades of amateur splices and added loads it was never sized for.
Knob and tube that has been left alone and lightly loaded can be in reasonable condition. The trouble is that few systems have been left alone.
Insurance
Many insurers in California decline to write or renew policies on homes with active knob and tube, or they require an electrician's report. Buyers often learn this in the middle of escrow. An electrical inspection before you list or make an offer avoids surprises.
What replacement involves
A whole house rewire replaces the old conductors with modern grounded cable, adds circuits for kitchens, baths and laundry, and usually includes a new panel. Good electricians fish new cable through attics, crawl spaces and wall cavities and keep openings small. Most people stay in the home while the work is done in sections.
Cost
For a typical single-family house in California, complete rewiring often falls between about $8,000 and $25,000 or more, depending on size, access and how many circuits are added. Phasing the work, starting with the highest-load areas, spreads the cost.
Aluminum wiring is a different problem
Homes from the mid-1960s to early 1970s may have aluminum branch wiring. The concern there is connections that loosen and overheat. Options include full replacement or approved copper pigtail repairs at every device.