# What is knob and tube wiring, and what should an LA homeowner do about it?

> Knob and tube wiring explained for LA homeowners: how it works, why it fails, California code and insulation rules, insurance, inspection and replacement options.

URL: https://laknobandtube.com/guide/knob-and-tube-wiring/

How the system works, why it was a sound design in its day, what goes wrong with it now, and how Los Angeles owners decide what to do.

Last Updated: October 25, 2026

[LA Knob and Tube](https://laknobandtube.com/) wrote this guide for owners of older Los Angeles houses who have just learned the word “knob and tube” from an inspector, an insurer or a contractor. Knob and tube is the electrical wiring system used in American houses from the late 1800s into the 1930s. Two separate wires run on porcelain knobs and through porcelain tubes, open to the air, with no ground wire. It is not dangerous by default, but most of it is now 90 to 120 years old, has often been altered, and was never designed for today’s loads or for being buried in insulation.

The rest of this guide explains how the system works, why it lasted as long as it did, what tends to go wrong, how California code and LA permits treat it, how insurance fits in, and what the options are.

Key takeaways

- Knob and tube is two separate conductors on porcelain insulators, with no ground wire
- Intact, unaltered runs are not automatically unsafe; age, alterations, overloading and insulation are the real risks
- California allows insulation over it only after a licensed electrical contractor certifies the wiring
- Insurance is often the deciding factor, and carriers differ
- A new panel does not remove knob and tube; only rewiring the branch circuits does

## How knob and tube wiring works

Every electrical circuit needs two paths: one carrying current out to the light or outlet and one bringing it back. Knob and tube runs those two paths as separate single wires, usually spaced several inches apart. Where the wires travel along a joist or stud, they are tied to small porcelain knobs nailed to the wood, which hold them off the surface. Where they pass through a joist or stud, they go through porcelain tubes set in drilled holes, so the wire never touches the framing. Where a wire had to enter a box or run near something it could rub against, installers slipped a flexible cloth sleeve called loom over it.

The wires themselves are copper, covered in rubber and then a cloth braid. Connections between wires were usually made by twisting them together, soldering the joint and wrapping it in tape, often out in the open between knobs rather than inside a box. Each circuit ran back to a fuse, and a house typically had only a handful of circuits, laid out for a few ceiling lights and a few outlets per room.

The guide to [identifying knob and tube](https://laknobandtube.com/guide/how-to-identify-knob-and-tube/) shows what each of these parts looks like from a safe distance.

### Why it was a sound design in its day

Knob and tube was not a cheap shortcut. Keeping the two conductors apart meant a damaged insulation spot on one could not easily short against the other. Running them in open air let the copper shed heat, which allowed relatively small wires to carry their load without overheating. Porcelain does not burn or conduct, and it lasts almost indefinitely. For the loads of the 1910s and 1920s, a few lights, a radio and an iron, it worked well, and plenty of it is still carrying current today.

Its limits came later, as houses changed around it.

## What goes wrong with knob and tube now

The porcelain usually outlasts everything else, so the parts that fail are the wire insulation, the connections, the protection at the panel and the conditions around the wiring. A symptom such as a warm outlet or a flickering light rarely points to one cause by itself, which is why an inspection maps the whole system rather than guessing from one room.

### Aging insulation

The rubber under the cloth braid becomes brittle with age and heat. Left alone in open air, it can stay intact for decades. Disturbed, during a remodel, an insulation job or a cable pulled past it, it can crack and fall away, leaving bare copper. That is one reason the system is often in worse condition near places where people have worked than in untouched stretches of attic. It is also the reason no one should handle suspect wiring to check it.

### Splices and later alterations

Most knob and tube in Los Angeles has been added to. Over a century, owners and electricians extended circuits, added outlets, tapped new wiring into old runs and replaced fixtures. Some of that work was done well, and some was done with splices hidden in walls, connections made without boxes, or modern cable tied into old conductors in ways that are hard to see. Mixed-era wiring is often a larger risk than the original layout, because the weak points are the ones nobody documented.

### Oversized fuses and breakers

A circuit’s wire can only safely carry so much current, and the fuse or breaker is what stops it from carrying more. When fuses kept blowing on a circuit carrying too much load, the familiar fix in many houses was a larger fuse. That removes the protection the wire depended on. The same problem appears when a fuse box is replaced with a breaker panel and old circuits are given breakers sized for modern wire. The [panel upgrade vs rewiring guide](https://laknobandtube.com/guide/panel-upgrade-vs-rewiring/) covers why the panel and the branch wiring have to be considered together.

### Too few circuits for modern loads

A 1920s bedroom might have had one outlet. Today the same room may carry a computer, chargers, a space heater and a window air conditioner, often through extension cords and power strips. Original circuits were laid out for a small fraction of that load. Overloading is not unique to knob and tube, but on a system with few circuits and aging insulation it adds heat where heat does the most harm.

### Insulation around the wiring

Knob and tube was designed to cool in open air. When loose-fill or blown insulation is packed around it in an attic or wall, the conductors can no longer shed heat the way they were meant to. This is the reason the California Electrical Code restricts insulating over knob and tube, covered below and on the [attic insulation page](https://laknobandtube.com/knob-and-tube-attic-insulation/).

### No ground wire

Knob and tube has no equipment grounding conductor. That is part of the original design, not a defect, but it means appliances with three-prong plugs have no ground path, and a three-prong outlet on a knob and tube circuit does not provide one unless a ground was added later. The guide to [three-prong outlets on knob and tube](https://laknobandtube.com/guide/knob-and-tube-three-prong-outlets/) explains the options, including GFCI protection, and their limits.

## Knob and tube in Los Angeles houses

Knob and tube turns up across older Los Angeles neighborhoods because so much of the city was built when it was the standard. The Census Bureau counts 296,223 homes in the City of LA built before 1940, 19.1% of the city’s housing. The share is far higher in the older neighborhoods: 40.7% in Highland Park’s 90042, 46.4% in Echo Park and Silver Lake’s 90026, 47.1% in West Adams’s 90018, and 51.6% in northwest Pasadena’s 91104.

### Raised floors, attics and plaster

Many pre-war LA houses, Craftsman and California bungalows, Spanish Revival houses and early duplexes, were built on raised wood floors over a crawlspace, with an attic above the ceilings. The original wiring usually runs through those two spaces, along the joists, and drops into the walls only where it has to reach an outlet or switch. That is good news for replacement: new cable can usually take similar routes, so most of the work happens above and below the rooms rather than inside the walls.

The city’s seismic retrofit records show how common raised floors are. Since January 2020, LADBS has issued 12,565 permits on one- and two-family houses for cripple-wall bracing and foundation bolting, the work done on raised foundations, with the most in Northeast LA and West Adams. Walls in houses of this era were typically finished in plaster over wood lath, which patches differently from drywall. The [plaster wall rewiring](https://laknobandtube.com/plaster-wall-rewiring/) page explains how routes are chosen to keep openings small.

### How often LA owners rewire

LADBS open data shows about 3,886 single-family rewire permits in the City of Los Angeles between January 2020 and October 2026, between about 540 and 640 a year. Nearly all were issued as express permits, without plan check. Only 96 permits since 1996 mention knob and tube by name, which says more about how permits are described than about how much work is done. Northeast LA, West Adams and Leimert Park, and Silver Lake and Echo Park lead the city. The [service areas](https://laknobandtube.com/service-areas/) page shows the figures by neighborhood.

## Is knob and tube wiring dangerous?

Knob and tube that is intact, unaltered, correctly fused and not covered in insulation is not dangerous simply because of its age. The U.S. Consumer Product Safety Commission and the Electrical Safety Foundation International both treat old wiring as a hazard mainly through its condition and alterations: damaged insulation, loose connections, overloaded circuits and missing protection.

The difficulty is that most of those conditions are hidden. A house can have perfect-looking porcelain in the attic and a bad splice inside a wall. That is why an honest answer to “is mine safe?” requires looking at the whole system, and why no one can certify a house as safe without inspecting concealed wiring. The [dangerous or not guide](https://laknobandtube.com/guide/is-knob-and-tube-wiring-dangerous/) goes through the warning signs that justify a call sooner rather than later.

## What California code says

### Existing knob and tube is not banned

No California rule requires an existing house to remove knob and tube. The California Electrical Code, which California adopts from the national electrical code with state amendments, treats existing wiring as an existing condition. New circuits are run with modern cable, and repairs, alterations or extensions to the system need permits and inspections from the local building department. The [is it legal in California guide](https://laknobandtube.com/guide/is-knob-and-tube-legal-in-california/) covers the details.

### Insulation over knob and tube: section 394.12

The code’s knob and tube article says concealed knob and tube shall not be used in hollow spaces of walls, ceilings and attics where loose, rolled or foamed-in-place insulation envelops the conductors. California adds an exception that allows insulation where knob and tube is present if several conditions are met:

1. The wiring is surveyed by an electrical contractor licensed by the State of California, who certifies that it is in good condition, with no deterioration, no improper overcurrent protection and no improper connections or splices.
2. The certification is filed with the local building department, and the owner gets a copy.
3. The insulation contractor posts caution notices, in English and Spanish, in every accessible area where insulation surrounds knob and tube.
4. The insulation is noncombustible and contains no electrically conductive material.

The California Building Standards Commission’s rulemaking for the current code edition, in force since January 1, 2026, carried the exception forward unchanged. The other route is to replace the attic circuits first, so the insulation can go in without conditions. The [attic insulation page](https://laknobandtube.com/knob-and-tube-attic-insulation/) explains how to choose.

### Arc-fault and ground-fault protection

When branch circuits in living areas are replaced or extended, the California Electrical Code requires arc-fault circuit interrupter protection, either at the breaker or at the first outlet. Kitchens, bathrooms, laundry areas and other wet locations get GFCI protection. New receptacles in dwellings are tamper-resistant. These requirements apply to the new work, which is part of why a rewired circuit ends up safer than the original even beyond the new cable. The [AFCI and GFCI guide](https://laknobandtube.com/guide/afci-gfci-knob-and-tube/) covers what each device does on old wiring that stays.

### Permits and licensing in Los Angeles

Rewiring is permitted work. In the City of Los Angeles, the electrical permit and inspections come from LADBS. LADBS allows an owner to pull an owner-builder electrical permit on a single-family house only with proof of ownership and proof that they live there; otherwise the permit is pulled by a contractor licensed by the California State License Board, typically in the C-10 electrical classification. Pasadena, Glendale and South Pasadena have their own building departments. The [Los Angeles rewiring permit guide](https://laknobandtube.com/guide/los-angeles-rewiring-permit/) covers what the inspector checks.

## Insurance and knob and tube in California

For an LA owner, insurance is what can turn knob and tube from a background fact into a deadline. Some carriers decline homes with active knob and tube when it appears on an application, and others ask for an electrician’s evaluation or a replacement plan before they will write or renew a policy. Bay Area news reporting in 2024 and 2025 described brokers saying knob and tube on an application led to declines. Carriers’ rules differ and change, so no one can promise that a given carrier will accept a house after a given scope of work.

The California FAIR Plan, the state’s insurer of last resort, has grown quickly: its residential policies rose from 236,515 in September 2021 to 621,234 in September 2025, according to the FAIR Plan’s own data. That growth reflects California’s wider insurance market, not knob and tube in particular, but it is the setting in which many LA owners are shopping for coverage.

The practical step is to ask your carrier or broker, in writing, exactly what it needs: whether partial replacement is acceptable, whether it wants an electrician’s letter, and what the letter has to say. The [California insurance guide](https://laknobandtube.com/guide/knob-and-tube-insurance-california/) covers the questions and paperwork in more detail.

## Buying or selling a house with knob and tube

In escrow, knob and tube usually appears in the buyer’s general home inspection, often with a recommendation for an electrician’s evaluation. A general inspection can usually note that knob and tube is present, but it cannot say how much is still live without circuit testing. Since January 1, 2026, California sellers must also give buyers a written electrical disclosure under Civil Code section 1102.6i. It says a buyer may want a qualified professional to inspect the panel and wiring, and warns that substandard or faulty wiring can be a fire risk and can make property insurance harder to obtain.

Buyers often need two answers before closing: how much of the system is still live, which drives cost, and whether their insurer will write the policy, which drives timing. The [knob and tube inspection](https://laknobandtube.com/knob-and-tube-inspection/) answers the first, and the guides to [buying](https://laknobandtube.com/guide/buying-a-house-with-knob-and-tube/) and [selling](https://laknobandtube.com/guide/selling-a-house-with-knob-and-tube/) cover the second.

## How knob and tube is assessed

An assessment answers four questions: which circuits still run on knob and tube, whether they are live, what condition the accessible parts are in, and how new cable could reach each room.

It starts at the panel, with the number of circuits and how each is protected. In the attic and crawlspace, the accessible runs are traced, along with splices, places where modern cable was tied in, and any insulation around the old wiring. Outlets and lights are matched to circuits, so the map shows which rooms depend on knob and tube today. Testing at outlets shows whether a ground path exists. The visit also measures access: crawlspace height, attic clearance, wall and ceiling finishes and where openings would be needed.

What an assessment cannot do is see inside closed walls. Concealed splices and damage are only found when a wall is opened, which is why a responsible assessment says what it could not verify. The guide to [active and abandoned knob and tube](https://laknobandtube.com/guide/active-vs-abandoned-knob-and-tube/) explains how live and dead runs are told apart.

## The options

### Leave it, with protection and limits

If the wiring is intact and lightly loaded, and nothing forces the issue, some owners leave it in place. GFCI protection can be added, heavy loads moved to new circuits, and oversized fuses corrected. This does not add a ground or change the age of the insulation, and it may not satisfy an insurer.

### Targeted repair

A single damaged splice or failed device on an otherwise sound circuit can be repaired. It fixes that point, not the system, and it is the option most easily misunderstood as solving more than it does.

### Partial replacement

[Partial replacement](https://laknobandtube.com/partial-knob-and-tube-replacement/) replaces the circuits that matter most now, usually kitchens, bathrooms, attic runs and heavily loaded rooms, and documents the ones that remain. It is common in houses where remodels already replaced some circuits.

### Full replacement

[Whole-house rewiring](https://laknobandtube.com/whole-house-rewiring/) replaces every knob and tube branch circuit and leaves no live knob and tube in the house. It answers an insurer’s question most completely, and it adds circuits and grounding throughout.

### Disconnection and removal

Replaced runs are disconnected at both ends. Whether to physically remove them, rather than leave them dead in place, depends on access and on what comes next, such as insulation or a remodel. The [removal page](https://laknobandtube.com/knob-and-tube-removal/) covers that choice.

## What replacement involves

A replacement job follows the assessment. The circuit map becomes a rewiring plan, which usually adds circuits where the old layout had too few. The permit is pulled. Work then moves circuit by circuit: new cable routed through the attic and crawlspace where possible, new boxes and devices, connections at the panel, and the old runs disconnected. Openings are made where routes have to cross framing or reach a box. The city inspector checks the new wiring before walls are closed and again at the end, and the openings are patched as the quote describes.

The [replacement process guide](https://laknobandtube.com/guide/knob-and-tube-replacement-process/) covers each stage, and [living at home during a rewire](https://laknobandtube.com/guide/living-at-home-during-rewiring/) covers power, dust and scheduling.

## What it costs

Whole-house knob and tube replacement typically costs $10 to $20 per square foot of living area in the LA area for the wiring, and a national budgeting range for a typical house is $12,000 to $35,000. Panel upgrades and plaster restoration are usually separate. The biggest swing factors are the number of live circuits and how open the attic and crawlspace are. The [Los Angeles cost guide](https://laknobandtube.com/guide/knob-and-tube-replacement-cost-los-angeles/) breaks the price down, and the [cost estimator](https://laknobandtube.com/tools/knob-and-tube-cost-estimator/) shows how the factors combine.

## Living with knob and tube in the meantime

If replacement is months away, a few habits reduce the strain on old circuits. Avoid running space heaters, window air conditioners or other heavy loads on circuits that are knob and tube. Do not add insulation, storage or boxes over attic runs. Do not open walls, drill or fish new cable near the old wiring without knowing where it runs. If you see scorching, smell hot insulation, feel a warm outlet or switch plate, or have a fuse that keeps blowing, stop using that circuit and call an electrician. And never replace a fuse with a larger one.

When you are ready to plan, the [knob and tube inspection](https://laknobandtube.com/knob-and-tube-inspection/) is the usual first step, and a call is the quickest way to start.

## Questions about this guide

### When was knob and tube wiring used?

It was the standard way to wire American houses from the late 1800s into the 1930s, and some installations continued into the 1940s. In Los Angeles, a house built before about 1940 was most likely wired this way originally, though later work may have replaced some or all of it.

### Can knob and tube wiring be grounded?

Not as it stands. The system has two conductors and no equipment ground. A circuit can be given GFCI protection, or replaced with modern cable that includes a ground, but the old conductors themselves cannot be made into a grounded circuit.

### What happens to the old wiring after replacement?

Replaced runs are disconnected at both ends so they cannot be energized. Runs that are easy to reach, usually in the attic and crawlspace, can be removed; runs inside closed walls are usually left in place, dead.

## More guides

- Learn [Is knob and tube wiring dangerous?](https://laknobandtube.com/guide/is-knob-and-tube-wiring-dangerous/)How the risk develops in old wiring, which signs mean a circuit should go off today, and what nobody can know about your house without seeing the wiring.
- Learn [Is knob and tube wiring legal in California?](https://laknobandtube.com/guide/is-knob-and-tube-legal-in-california/)Existing wiring, new work, the insulation rule, permits and licensing, and how the state code is enforced in Los Angeles and nearby cities.
- Learn [How does knob and tube affect home insurance in California?](https://laknobandtube.com/guide/knob-and-tube-insurance-california/)What carriers tend to ask, how to get their answer in writing, what an electrician's letter can and cannot say, and where the FAIR Plan fits.
- Learn [How do you identify knob and tube wiring?](https://laknobandtube.com/guide/how-to-identify-knob-and-tube/)A look-only guide to the clues in attics, crawlspaces, fuse boxes and rooms, and why what you can see is never the whole answer.
- Cost Guide [How much does knob and tube replacement cost in Los Angeles?](https://laknobandtube.com/guide/knob-and-tube-replacement-cost-los-angeles/)The ranges, what moves a house within them, and the parts of the price that come from public figures.