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Knob and tube wiring in Los Angeles

Knob and tube was the standard way to wire homes from the late 1800s into the 1940s: single wires run through ceramic tubes and along ceramic knobs, with cloth-wrapped insulation. Many older Los Angeles homes, especially craftsman and early 1900s houses, still have some of it in the walls and attic.

Is it dangerous?

Knob and tube isn't automatically unsafe, but it has real weaknesses. It has no ground wire, so outlets can't be properly grounded. The cloth and rubber insulation becomes brittle with age and can crack where it's been disturbed. Decades of amateur splices and added loads are common. And it was designed to shed heat into open air, which is why covering it with insulation is a problem.

Why insurers ask about it

Many home insurers decline policies, charge more, or require an electrical inspection or rewiring when they find knob and tube. Buyers' inspectors and lenders flag it too. If you're buying, selling or switching insurers, expect it to come up.

Insulating over knob and tube in California

The California Electrical Code (Article 394.12) does not allow concealed knob and tube in wall, ceiling or attic spaces filled with loose, rolled or foamed-in insulation that surrounds the wires. There is a narrow exception: a California-licensed electrical contractor inspects the wiring and certifies it's in good condition, the certification goes to the building department before insulating, and warning signs are posted at the attic access. Rules and forms vary by city, so check with LADBS or your own city's building department. In practice, many homeowners rewire first so the knob and tube is fully de-energized, and the insulation contractor asks for a letter confirming that.

What replacement costs

National cost guides put knob and tube replacement at roughly $10 to $20 per square foot, with many whole-house projects falling between $12,000 and $36,000. Homes with plaster walls, several floors, or tight attics and crawlspaces can go well beyond that, especially once wall repair is included. Labor in Los Angeles runs higher than the national average. Rewiring only the circuits that still have knob and tube can cost much less than a full rewire.

How to compare quotes

Give every electrician the same written scope: the number of outlets, switches and fixtures, whether knob and tube will be removed or disconnected and abandoned in place, AFCI and GFCI protection, the permit and inspections, wall patching, and a written letter confirming no knob and tube remains energized if your insurer or insulation contractor needs one. Quotes priced per opening, as a fixed total, or as time and materials can then be compared fairly. For time and materials work, ask for a not-to-exceed amount.

Signs to act sooner

Call a licensed electrician soon if you see scorched or crumbling insulation, outlets or switch plates that feel warm, flickering lights across rooms, breakers or fuses that trip often, or wiring that's been chewed by rodents.

Questions

Do I have to remove all of it?

Not always. Disconnecting it at the panel and abandoning it in place is common and cheaper than pulling it out. What matters is that none of it remains energized.

Can I just add a ground wire?

Knob and tube circuits have no ground conductor. The usual fix is new wiring. GFCI protection can make ungrounded outlets safer in the meantime, and an electrician can advise what's allowed.

Will rewiring damage my walls?

Electricians fish new cable through walls with small openings where they can, but older plaster homes usually need some cutting and patching. Ask whether patching is included in the quote.

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Updated 2026-10-21. General information only, not a substitute for an inspection by a licensed electrician.