Category: Electrical

Knob-and-tube wiring that is intact, untouched and lightly loaded is not an emergency, but it is not a system you should plan to keep. It has no ground wire, its rubber-and-cloth insulation has spent its whole life drying out above ceilings and under floors, and in many houses that still have it, decades of add-ons and amateur splices have been hung off the original runs. California’s electrical code no longer allows it to be installed new, restricts burying it in insulation, and requires a permit for any repair or extension. The practical answer for most owners: have a licensed electrician evaluate it now, keep loads and insulation away from it until then, and replace it — ideally at the same time as any remodel that opens walls or ceilings.

We are Green Design and Build, a licensed residential general contractor based in Van Nuys (CSLB #1110975), working across Los Angeles, Orange, Ventura and the western edges of Riverside and San Bernardino counties. Electrical work on our projects is done by licensed C-10 electrical contractors we schedule and supervise, usually as part of a kitchen, bathroom, addition or whole-home renovation in Los Angeles. This guide deliberately has no price list: the cost of replacing old wiring depends on the house far more than on the wire, so we explain what moves the number and give you the number itself after a site visit.

What knob-and-tube wiring is, and how to tell if you have it

Knob-and-tube (often written K&T) is one of the oldest wiring methods still found in American houses. Instead of a single cable carrying several wires in one jacket, it runs the hot and neutral conductors as two separate wires, usually several inches apart. Porcelain knobs nailed to joists and studs hold the wires off the wood; porcelain tubes carry them through framing members; and where a wire enters a box or crosses another wire, it is often sleeved in a flexible woven tube called loom. The conductors are copper wrapped in rubber and a cloth braid, and because the two wires are separated and surrounded by air, the system sheds heat into the open space around it — which is exactly why insulation and knob-and-tube do not mix.

Where to look in a Los Angeles house

• The attic. With a flashlight, look along the ceiling joists for pairs of single wires stretched between white porcelain knobs.
• The crawlspace. Many older LA houses sit on raised wood floors. Under the floor you may see the same paired wires running through porcelain tubes in the floor joists.
• The panel. A screw-in fuse panel does not prove you have knob-and-tube, but it is a strong hint that the original wiring was never fully replaced.
• The outlets. Two-prong receptacles throughout the house are a clue, not proof. Some later cable systems also lacked a ground wire, and some K&T houses had their outlets swapped for three-prong ones without a ground ever being added.

A house can show modern cable in the attic and still have live knob-and-tube inside the walls, because earlier owners often replaced only what was easy to reach. Only an electrician tracing circuits from the panel can tell you how much is still energized.

Is knob-and-tube wiring safe?

Short answer: the original design was sound for the loads it was built for. The danger comes from what has happened to it since — age, alteration, overloading and insulation — and from what it cannot do, which is provide a ground or work with modern protective devices. An intact, original run carrying a few lights is a lower risk than a spliced, overfused run feeding a space heater in a bedroom.

These are the conditions that decide how urgent replacement is:

Condition	Why it matters	What the electrician looks for
Brittle or missing insulation	The rubber and cloth covering dries and cracks with age and heat, leaving bare copper near wood.	Insulation that flakes when touched; exposed copper at knobs and tubes
Amateur splices and add-ons	Later owners often tapped new outlets or lights into old runs with taped splices outside any box.	Splices not in junction boxes; modern cable spliced onto K&T
Overfusing	A bigger fuse stops the blowing by letting the wire carry more current than it was sized for, so the wire heats instead.	Fuse sizes larger than the wire can safely carry
Buried in insulation	K&T is designed to cool in open air. Blown-in or batt insulation packed around it traps heat.	Insulation in contact with or covering the conductors in attics and wall cavities
No ground	There is no path for fault current from appliance cases and metal fixtures, and surge protectors cannot do their job without a ground.	Two-prong outlets, or three-prong outlets with no ground behind them
Not enough circuits	A system laid out for lighting and a few outlets now feeds microwaves, hair dryers and air conditioners.	Extension cords everywhere, fuses that blow, lights that dim when something switches on

Warning signs that mean “call an electrician this week”

• A burning or hot-plastic smell near outlets, switches or the ceiling, with no obvious source
• Outlets or switch plates that are warm to the touch, discolored or scorched
• Fuses that blow repeatedly, or anyone having installed a larger fuse “to stop it blowing”
• Buzzing, crackling or flickering on an old circuit

Until it has been evaluated, keep high-wattage appliances such as space heaters and window air conditioners off old circuits, do not let anyone blow insulation into the attic, and never replace a fuse with a larger one.

What California’s electrical code says about knob-and-tube

The California Electrical Code (Title 24, Part 3) is the National Electrical Code with California amendments. Its 2025 edition took effect on January 1, 2026, according to the California Building Standards Commission. Concealed knob-and-tube is covered in Article 394, and three points matter to homeowners:

1. New installations are not allowed. As the City of Menlo Park’s building division summarizes it, new knob-and-tube is no longer permitted except by special permission of the building official. Article 394 limits it to extending existing installations. Existing knob-and-tube in service is not automatically illegal; the code does not order you to tear it out.
2. Any work on it needs a permit. Repairs, alterations or extensions of an existing knob-and-tube system require permits and inspections.
3. It may not be buried in insulation — with one California exception. Section 394.12(5) prohibits concealed knob-and-tube in hollow spaces of walls, ceilings and attics where loose, rolled or foamed-in-place insulation envelops the conductors.

The California exception for insulating over knob-and-tube

California amended Section 394.12 so that an older house with live knob-and-tube can still be insulated, but only if every condition is met. As quoted in the City of Berkeley’s knob-and-tube safety report form, which cites the 2025 code:

• The wiring is surveyed by an electrical contractor licensed by the State of California, who certifies it is in good condition with no evidence of deterioration, improper overcurrent protection, or improper connections and splices. Repairs found necessary need permits and inspections.
• The certification is filed with the building department, with a copy to the owner.
• Accessible areas where insulation surrounds the wiring are posted with a caution notice in English and Spanish.
• The insulation is noncombustible as defined in the California Building Code, and contains no electrically conductive material.
• The building department may still require permits and inspections for the insulation itself.

In practice, owners planning an attic insulation upgrade often find it simpler to rewire the attic runs first: the survey, the certification and the noncombustible-only rule add cost and limit choices, and you still own the old wiring afterward.

Knob-and-tube and homeowners insurance

There is no California law that says an insurer must, or must not, cover a house with knob-and-tube wiring. Each carrier sets its own underwriting rules, and they differ: an insurer may ask about wiring type and age on the application, ask for an electrician’s evaluation, or attach conditions such as proof of replacement. A policy you already have may come up for review at renewal or after an inspection. Because the answer depends entirely on your carrier, the only reliable approach is to ask directly.

Questions worth putting to your agent or broker, in writing:

• Do you write or renew policies on homes with any active knob-and-tube wiring?
• Would a licensed electrician’s evaluation letter satisfy underwriting? What must it say?
• If I commit to replacing it, how long do I have, and what proof do you need — the finaled permit, an invoice, an electrician’s letter?

If you are buying, ask before you remove your inspection contingency. A finaled electrical permit for the rewire is the cleanest evidence you can hand an insurer later.

Two-prong outlets: the GFCI stopgap and its limits

Swapping a two-prong outlet for a three-prong one without adding a ground is a hazard, because it tells every plug that a ground exists when it does not. The electrical code allows a different fix: an ungrounded receptacle can be replaced with a GFCI receptacle, or with a receptacle protected by an upstream GFCI device, as long as it is marked “No Equipment Ground” (and, where protection comes from an upstream device, “GFCI Protected”). The North Carolina Office of State Fire Marshal’s interpretation of NEC 406.4(D)(2) sets out those marking requirements clearly; the same section appears in the model code California adopts.

What that buys you, and what it does not:

• It adds shock protection at that outlet and anything downstream of it, which is a real improvement in a kitchen, bathroom or laundry.
• It does not create a ground. Surge protectors and equipment that expects a ground still lack one.
• It does nothing for the wiring itself — brittle insulation, bad splices, overfusing and buried runs are all still there.

Treat GFCI protection as a sensible interim step, done by a licensed electrician, while you plan replacement — not as the replacement.

Your options, compared

Option	What it involves	When it makes sense	The catch
Evaluate and monitor	A licensed electrician traces circuits, checks insulation, splices and fuse sizes, and fixes immediate hazards under permit	Original, intact runs on light loads, while you plan and budget for replacement	A holding pattern, not a solution. May not satisfy your insurer.
GFCI protection on ungrounded outlets	GFCI receptacles or breakers, outlets marked “No Equipment Ground”	Interim shock protection, especially in wet areas	No ground, no change to the wiring’s condition
Partial replacement	New circuits for the rooms being remodeled, old runs in those areas disconnected	A kitchen, bath or addition is opening walls anyway	The rest of the house still has K&T. Be clear with your insurer about what remains.
Full rewire	Every old circuit replaced with grounded cable, usually with new boxes and devices, often alongside a panel upgrade	Insurance pressure, a sale or purchase, an attic insulation project, or evidence of damage	The most disruption; plaster walls need careful access and patching

When should you rewire?

Rewire when any of these is true. Several are moments when walls, ceilings or the attic are already open, which is when replacement costs and disrupts least.

• You are remodeling. A kitchen remodel or a bathroom remodel is rewired as part of the job anyway, and extending it into neighboring rooms while walls are open is far cheaper than coming back later.
• You are opening up the structure. Removing a load-bearing wall or a seismic retrofit in the crawlspace puts trades exactly where the old wiring runs.
• You are adding load. An EV charger, a heat pump, an induction range or an addition can trigger an electrical panel upgrade, and a new panel feeding old wiring leaves the weakest part of the system in place.
• You want to insulate the attic (see the code section above).
• Your insurer is asking and has set a deadline.
• The evaluation found damage — brittle insulation, overfusing, rodent damage or a tangle of amateur splices.

What rewiring an old house involves

Rewiring an occupied house is mostly a problem of access: getting new wire from the panel to every outlet, switch and light without tearing the house apart. A typical sequence:

1. Evaluation. The electrician traces each circuit, maps what is still knob-and-tube, checks the panel and service, and looks at attic and crawlspace access.
2. Scope. You decide where outlets go, which rooms need their own circuits and whether lighting changes now. Plan for future loads rather than replacing circuit-for-circuit.
3. Permit. In the City of Los Angeles, LADBS lists “rewiring or adding devices to existing electrical circuits” as express-permit work in its information bulletin P/GI 2020-003, meaning no plan check is needed for non-engineered residential wiring. Cities outside LA, and unincorporated county areas, have their own building departments and procedures. Our guide to LADBS permit costs explains how fees are worked out.
4. Fishing the new wire. With a usable attic above and a crawlspace below, much of the new cable can be fished through walls from those spaces through small access openings.
5. Disconnecting the old system. Old circuits are de-energized and disconnected at the panel. Accessible runs in the attic and crawlspace are often removed; runs sealed inside finished walls are usually left in place, dead.
6. Rough inspection. The inspector checks the new wiring before access openings are closed.
7. Patching and final inspection. Openings are repaired, devices go in, and the permit is finaled. Keep the record; your insurer and your eventual buyer will want it.

Plaster walls are the hidden variable

Many of the older houses that still have knob-and-tube also have lath-and-plaster walls. Plaster cracks if it is cut carelessly, is harder to patch invisibly than drywall, and in older homes the paint on it may contain lead, so ask how dust will be contained. A careful crew plans access holes where they are easiest to patch — inside closets, behind baseboards, at the top of walls near the ceiling line — rather than wherever is quickest.

What drives the cost of rewiring a house

We do not publish a rewiring price, because two homes of the same size can differ widely depending on access, walls and what else is happening in the project. These are the factors that set the number:

Factor	How it moves the cost
House size and number of circuits	More rooms, outlets, switches and fixtures mean more wire, more boxes and more labor. Adding circuits the old system never had adds to all three.
Attic and crawlspace access	A walkable attic and a crawlspace with good clearance let most wire be fished from above and below. Low attics, no crawlspace, slab floors or a second story mean more openings in finished walls.
Plaster versus drywall	Plaster takes longer to open, and more skill to patch so it does not show.
Patching and painting	Whether repairs stop at patched and primed, or include full repainting of affected rooms, changes the finish scope considerably.
Panel and service	If the existing panel is a fuse panel or too small for the new circuits, a panel upgrade, and sometimes utility coordination, becomes part of the project.
Permit and inspections	Permit fees depend on the scope of work and the jurisdiction. Inspections also set the schedule, because walls cannot be closed until the rough inspection passes.
Combined with a remodel	When walls are already open for a kitchen, bath or renovation, much of the access and patching cost disappears into work you were doing anyway.

The honest number comes from a site visit that sees your attic, crawlspace, panel and walls. For a larger project, our whole-home renovation cost guide shows how electrical work fits into a full renovation budget, and our financing options page covers ways to spread the cost.

Rewiring alongside a bigger project

Knob-and-tube is very often found during other work, which is the most efficient time to deal with it:

• Kitchens and baths. The old circuits in a fully remodeled room go regardless; extending the rewire into adjacent rooms while the crew is there is the cheap upgrade.
• Additions. A room addition adds load and often requires a larger panel; extending old knob-and-tube to serve new rooms makes no sense, so the plan should address the house as a whole.
• Garage conversions and ADUs. A garage conversion or a new unit from an ADU builder in Los Angeles often means a panel upgrade or subpanel at the main house, which is a natural point to reassess its wiring.
• Fire and water damage. When a house is opened up for fire damage repair and rebuilding, any surviving knob-and-tube in the affected areas should be replaced, not reconnected.

Our electrical services page covers the electrical work we coordinate on residential projects.

Who should do the work, and what to ask them

Electrical work in California is a licensed trade. The Contractors State License Board defines the C-10 Electrical Contractor classification as the license for placing, installing or connecting electrical wiring and equipment. On a stand-alone rewire you may hire one directly; as part of a remodel, a general contractor typically schedules and supervises the electrician alongside the other trades. Either way, verify the contractor’s license on the CSLB website before you sign anything.

Questions that separate careful contractors from fast ones:

• Who pulls the permit, and is it in the contract?
• How will you get wire to each room — where will the access openings be, and who patches them?
• What happens to the old wiring: removed, or disconnected and left in place?
• Is the panel adequate for the new circuits, and if not, is the upgrade in this price?
• Will you give me a written letter at the end confirming what was replaced, for my insurer?

Knob-and-tube wiring FAQs

Is knob-and-tube wiring illegal in California?

No. Existing knob-and-tube that is in service is not illegal, and the California Electrical Code does not order owners to remove it. New installations are not allowed except by special permission, any repair or extension needs a permit and inspection, and it may not be enveloped in insulation unless California’s certification exception in Section 394.12 is met.

Can I buy or sell a house with knob-and-tube wiring?

Yes. Buyers should have a licensed electrician evaluate the wiring and call their insurance agent during the inspection period, because the insurance answer can affect the deal. Sellers who have replaced it should keep the finaled permit to show buyers.

Will homeowners insurance cover a house with knob-and-tube?

It depends on the carrier. There is no statewide rule in California; each insurer sets its own underwriting standards. Carriers differ in what they ask for, from a simple disclosure to an electrician’s evaluation or proof of replacement. Ask your agent in writing before you buy or before your renewal.

Can I add insulation to an attic that has knob-and-tube wiring?

Only under California’s exception to Section 394.12: a licensed electrical contractor certifies the wiring as sound, the certification is filed with the building department, caution notices are posted in English and Spanish, and the insulation is noncombustible with no conductive material.

Can I just replace two-prong outlets with three-prong outlets?

Not without adding a ground wire. A three-prong outlet with no ground behind it is a hazard. The code-compliant alternative is a GFCI receptacle, or GFCI protection upstream, with the outlet marked “No Equipment Ground”. That adds shock protection but does not create a ground or fix the wiring.

Does rewiring a house mean opening every wall?

No. In a house with a usable attic and crawlspace, electricians fish much of the new cable through the walls from above and below, using small access openings that are patched afterward. Two-story homes, slab floors and plaster walls need more openings and more careful patching.

Do I need to move out while the house is rewired?

Often not, for a rewire on its own. Expect circuits to be off while crews work, some dust from access openings, and rooms cleared in sequence. As part of a larger renovation, the renovation decides whether you can stay.

How long does it take to rewire a house?

It depends on the size of the house, the access and the inspection schedule. In the City of Los Angeles a residential rewire generally qualifies for an express permit with no plan check, so the permit is rarely the bottleneck. Expect a written schedule with the proposal.

Talk to us about old wiring in your house

If your inspection report says “knob-and-tube”, your insurer is asking questions, or you are planning a remodel in an older house, we can look at the whole picture — what is still live, what your panel can carry, and how to sequence the work so walls are opened once. A site visit takes about two hours, we reply within one business day, and written proposals follow within 48 hours. You can read what homeowners say in our client reviews, learn more about Green Design and Build, or contact us to schedule a site visit.