Knob and tube wiring was the standard in North American homes from about 1880 to the 1940s, and some of it is still energized behind walls and in attics today. It isn’t automatically dangerous, but it has no ground wire, its insulation turns brittle, and it can overheat when it’s buried in insulation or overloaded. Below you’ll find how to spot it, whether it’s safe, how insurers treat it, and what replacement involves, including what drives the price. We’ll also mark where a homeowner can look for themselves and where a licensed contractor has to take over.
Key Takeaways
- Knob and tube wiring runs single copper wires on porcelain knobs and through porcelain tubes, and it was common from about 1880 to the 1940s.
- It has no ground wire, so it can’t protect against faults the way modern wiring does, and it overloads easily when many appliances run at once.
- The biggest risks come from age, later changes to the system, and insulation packed around the wires.
- Some insurers decline or cancel policies on homes with knob and tube wiring, so ask your insurer before you buy, renew or remodel.
- Cities set their own rules. In Menlo Park, repairs or changes to an existing system need permits and inspections, and insulation over it needs a licensed contractor’s certification.
- Replacement cost depends on house size, circuit count, access, the panel, and permits, so get at least three written estimates and ask whether each is hourly or fixed.
On This Page
- What Is Knob and Tube Wiring?
- Is Knob and Tube Wiring Safe?
- Knob and Tube Wiring Insurance: What to Expect
- Knob and Tube Wiring Replacement: Your Options
- Knob and Tube Wiring FAQs
- Next Steps for Your Knob and Tube Wiring
What Is Knob and Tube Wiring?
Knob and tube wiring is an early way of running electricity through a house. Single copper conductors were held off the framing by porcelain knobs and passed through porcelain tubes where they crossed joists and studs. Fire Engineering notes that the conductors had rubber insulation under a cloth jacket, and that the system was safe and reliable as installed, unless a remodel ignored the original details.
That caveat is the heart of the issue. The original method worked when it was left alone, and trouble tends to start when later changes ignore how it was meant to work.
You can often spot it without touching anything. In an attic, basement or crawl space, look for single wires running along joists on porcelain knobs, with porcelain tubes where they pass through framing. Two-prong outlets throughout a house are a clue, not proof, because outlets get swapped while old wiring stays behind them.
Looking with a flashlight is fine. Opening walls, touching wires or cutting into a circuit isn’t, because this wiring is often still energized.
Is Knob and Tube Wiring Safe?
Intact wiring that nobody has altered can keep working, but it falls short of modern wiring in ways that matter. The Center for Energy and Environment points to three problems:
- It has no ground wire, so there’s no protection when a fault occurs.
- It was built for light loads, so it overloads easily in a modern home with several appliances running at once.
- Heat from the wires makes the rubber coating brittle, and it can break and leave live wire exposed.
Insulation adds a fourth problem. InterNACHI warns that insulation around the wires causes heat to build up and creates a fire hazard. That’s why blown-in attic insulation over live knob and tube wiring is a risk and not a bonus.
Some warning signs are easy to see. Our page on the signs you need to rewire a house lists flickering lights, a breaker that keeps tripping, and discolored or burnt-looking switches and faceplates. The table adds a few that matter with older wiring.
| What You Notice | What It Can Mean | What to Do |
|---|---|---|
| Lights flicker or a breaker keeps tripping | An overloaded or failing circuit | Stop adding load to it and have it checked |
| Switches or faceplates look discolored or burnt | Heat at a connection | Stop using that switch and get it looked at |
| A burning smell or a warm outlet | Overheating | Turn off that circuit at the breaker and call a licensed contractor. If you see smoke or flames, call 9-1-1 |
| Old wires sitting under attic insulation | Heat that can’t escape | Don’t add more insulation until someone has surveyed the wiring |
| Two-prong outlets throughout the house | Older wiring, or just older outlets | Have someone confirm what’s behind them |
None of these confirms knob and tube wiring on its own. A hands-on inspection does. Our page on preventative maintenance for electrical systems makes the same point: melting insulation and corrosion are why a physical inspection isn’t optional.
Knob and Tube Wiring Insurance: What to Expect
Some insurers decline or cancel policies on homes with knob and tube wiring. In March 2025, ABC7 in San Francisco reported that insurers were canceling and denying homeowners policies over it, and a broker in the story said that disclosing it on an application can get a policy declined.
The report adds that without insurance, a lender won’t approve a mortgage unless the buyer pays for the expensive California FAIR Plan. That makes this a buying decision as much as a wiring one.
Keep it in proportion. The same report says San Francisco Fire Department investigators haven’t seen trends in residential fires from knob and tube wiring, and most of the insurance claims come from a broker and the owner of a replacement company. Treat it as a warning, not a rule, because policies differ by insurer.
Before you buy, renew or start a remodel:
- Ask your insurer or broker in writing whether they cover homes with knob and tube wiring, and on what conditions.
- Ask what they’d want to see, such as an inspection report, a partial replacement or a full one.
- Keep the invoice and permit paperwork from any replacement. The Center for Energy and Environment suggests the invoice can serve as proof for insurers and insulation contractors.
Knob and Tube Wiring Replacement: Your Options
Replacing knob and tube wiring isn’t an all-or-nothing choice. Most homeowners end up in one of three places:
- Keep it and have it inspected, which fits best when the wiring is intact, unaltered and not buried in insulation.
- Replace part of it, such as circuits that are overloaded or sit under attic insulation.
- Replace all of it, which makes sense during a major remodel, before adding insulation, or when an insurer requires it.
Rules depend on your city, so check yours before any work starts. As one Peninsula example, the City of Menlo Park says any repair, alteration or extension of an existing system needs permits and inspections, and new knob and tube wiring isn’t allowed under the California Electrical Code except by special permission. If it turns up during construction, the city recommends removing it entirely or in part. For insulation, it requires a survey and certification by an electrical contractor licensed by the State of California.
Knob and Tube Wiring Replacement Cost: What Drives the Price
No single figure fits every house, and we’d rather not guess at one. The Center for Energy and Environment says cost varies with the size of the house, how much wiring there is, how easy it is to reach, whether you add outlets, and whether the panel needs an upgrade. In practice the price comes down to:
- How many circuits need new wire.
- Access to attics, crawl spaces and framing, versus finished walls and ceilings that need patching afterward.
- Whether the existing panel can handle the new circuits.
- Permits and inspections.
- Whether the job is partial or whole-house.
Get at least three written estimates, as that guide recommends, and ask whether each price is hourly or fixed and what it includes. Expect part or all of the house to be without power during some phases of the work.
This isn’t DIY work. California requires a license from the Contractors State License Board for any contractor job totaling $1,000 or more in labor and materials, and you can check a license on the CSLB site before you hire. Our electrical services page lists rewiring, panel replacement and electrical inspections among the work we take on.
Knob and Tube Wiring FAQs
How Do I Know If I Have Knob and Tube Wiring?
Look in the attic, basement or crawl space for single wires running along joists on porcelain knobs, with porcelain tubes where they pass through framing. Two-prong outlets are a clue but not proof. Only someone who can see the wiring can confirm it, so have a licensed contractor check it instead of opening walls yourself.
Can You Put Insulation Over Knob and Tube Wiring?
Usually not without extra steps. The California Electrical Code prohibits concealed knob and tube wiring in hollow spaces filled with loose, rolled or foamed-in-place insulation that envelops the conductors. Menlo Park also requires a State-licensed electrical contractor to survey and certify the wiring before other insulation goes in.
Can I Insure a House With Knob and Tube Wiring?
Sometimes, but some insurers decline or cancel policies over it. A San Francisco broker told ABC7 that listing it on an application can get a policy declined, and buyers who can’t find coverage may end up on the California FAIR Plan. Ask your insurer in writing what they require before you buy, renew or remodel.
How Much Does Knob and Tube Wiring Replacement Cost?
Price comes down to the size of the house, how many circuits need new wire, how easy the wiring is to reach, whether the panel needs an upgrade, and permit costs. Get at least three written estimates, and ask whether each price is hourly or fixed and what it includes.
How Long Does Knob and Tube Wiring Replacement Take?
The schedule depends on the size of the home and how easy the wiring is to reach, so ask for it in writing with your quote. Expect part or all of the house to be without power during some phases of the work, and plan around it.
Do I Need a Licensed Contractor to Replace Knob and Tube Wiring in California?
For practical purposes, yes. California requires a CSLB license for contractor jobs totaling $1,000 or more in labor and materials, and rewiring a house is likely to pass that. Repairs and alterations to existing wiring also need permits and inspections in cities like Menlo Park. Check any contractor’s license before you hire.
Next Steps for Your Knob and Tube Wiring
Knob and tube wiring comes down to one decision: leave it, fix part of it, or replace it. The right answer depends on what’s behind your walls, so start by finding out what you have and what your insurer expects.
If you’d like someone to look at the wiring in your home, schedule a visit with Proper Hour Home Service. We’re a licensed contractor, CSLB #1131330, serving homeowners across the South Bay and the Peninsula.
