Guide
Knob-and-tube wiring: what it is, and what insurers think
Identification first, then the underwriting question — because for most people reading this, the letter from the carrier came before the curiosity.
Written by HyreElectrical Research Desk Primary-source research and fact checking
The short version
Knob-and-tube is an open wiring method used in American homes from roughly the 1880s to the 1940s: single insulated conductors run separately through porcelain knobs where they cross framing and porcelain tubes where they pass through it, relying on air around the conductor to carry heat away.
It is not illegal, and existing installations are not required to be removed. NFPA 70 still carries it as Article 394, and the model code permits existing concealed knob-and-tube to remain.
What it is not is grounded — there is no equipment grounding conductor, so no three-prong outlet on that circuit is doing what its third slot implies.
The practical problem is usually insurance, not code. Multiple carriers list knob-and-tube among ineligible risks outright, and unlike aluminum wiring — where at least one carrier publishes a remediation route — the guides reviewed here state no path back to eligibility.
The other live issue is thermal insulation: NEC 394.12 generally prohibits burying knob-and-tube in loose, rolled or foamed-in insulation, though at least one state publishes a conditional exception. HyreElectrical does not perform, supervise or warrant electrical work.
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What it actually is
Knob-and-tube is a two-wire open wiring method. The hot and neutral conductors are not bundled in a common sheath the way they are in the cable used today.
They run as separate single conductors, spaced apart, held off the framing on porcelain knobs and threaded through porcelain tubes wherever they pass through a joist or stud.
That separation is the design, not sloppiness: the method depends on free air around each conductor to shed heat. Almost everything that later goes wrong with knob-and-tube follows from something interfering with that assumption.
- Knobs
- Porcelain insulators nailed to framing that hold a conductor away from the wood as it runs along or across a member.
- Tubes
- Porcelain sleeves set into a drilled hole so a conductor can pass through a joist or stud without touching it.
- Loom
- Flexible insulating sleeving over the conductor where it enters a box or fixture, or where it runs in a tight space.
- No equipment ground
- The method has two conductors and no grounding conductor. This is the single most consequential fact about it, and it does not change with age or condition.
- Article 394
- The NFPA 70 article covering concealed knob-and-tube. Its continued existence is why an installation is permitted to remain rather than being outlawed outright.
Read that chart carefully. It shows when homes were built, not how they are wired. Most homes of this age were rewired decades ago, many in stages. No federal source counts homes with active knob-and-tube, so neither do we.
How to recognize it, without opening anything
Knob-and-tube is visible in the places where a house is unfinished — an open basement ceiling, an unfloored attic, the back of a closet in an ell. You do not need to remove a cover, a panel front or a fixture to look, and you should not.
Roughly spool-shaped, with a nail through the center and the conductor sitting in the groove.
A short ceramic sleeve where a wire passes through a joist. This is the clearest single indicator.
Not a single gray or white jacketed cable. Two wires taking their own paths is the signature.
Modern thermoplastic insulation stays flexible. Fabric-covered rubber does not.
Consistent with an ungrounded system, though it is evidence, not proof — other ungrounded methods look the same at the outlet.
Original splices were soldered and taped in the open. Any splice you can see should be in a box today.
This is the condition NEC 394.12 addresses, and the one most likely to matter to both an inspector and an underwriter.
Later modification by non-professional hands is a more common source of trouble than the original work.
Where to stop. Scorching, a smell of hot plastic or varnish, lights that dim and brighten, or a breaker or fuse that will not hold is no longer an identification question. Stop and have a licensed electrician determine the cause.
The insurance problem, which is usually the real question
Most people arrive at this subject because a carrier raised it — at renewal, at purchase, or after an inspection report. So it belongs here rather than at the bottom.
Source fact. The two underwriting documents below were read directly. Both name knob-and-tube among ineligible risks. The distinction worth noticing is against aluminum wiring: one of these same carriers publishes a remediation route for aluminum — properly pigtailed, with proof from a licensed electrician — and neither publishes an equivalent route for knob-and-tube.
HyreElectrical analysis. Two guides are a sample, not a market survey, and we are not going to tell you what "most insurers" do from a sample of two.
What the sample does establish is that outright ineligibility is a real published position rather than a contractor talking point, and that the absence of a stated remedy is a meaningful difference from how the same documents treat aluminum.
| Carrier | Document and date | What it says | Stated route back |
|---|---|---|---|
| Orion180 Insurance Company | Texas Flex Program Underwriting Guide, Homeowners Program 13 January 2025 | Lists “Knob and tube wiring” among ineligible risks, alongside aluminum wiring, double-tapped wires and named unsafe panel brands. | None stated. The risk is ineligible for the program. |
| Homeowners of America Insurance Company | Texas Homeowners underwriting guidelines, ineligible risks no revision date published on the page; retrieved 6 September 2026 | Lists as ineligible “Risks with Aluminum or knob and tube wiring.” | The published remediation route is stated for aluminum, not for knob-and-tube. |
Source facts, quoted from each carrier’s own underwriting guide, retrieved 6 September 2026. Underwriting rules vary by carrier, program and state, and they change.
Ask your own carrier in writing what they require and what evidence they accept, before commissioning any work — a scope built to satisfy an unwritten requirement cannot be enforced later. See the panels and wiring insurers decline for the wider picture.
The four ways it actually fails
Correctly installed, undisturbed, and still carrying the load it was designed for, knob-and-tube is not the emergency it is often described as. Three of the four failure modes below are things done to it after installation, not properties of the method — the distinction a good electrician assesses and a generic article flattens.
1. Insulation contact
The method needs free air to shed heat. Bury it in blown-in cellulose, rolled batts or foam and the conductor runs hotter under the same load.
This is what NEC 394.12 addresses, and it is the most common serious finding, because attic insulation was retrofitted into millions of older homes by people with no reason to think about the wiring under it.
2. Overfusing
These circuits were sized for the loads of their era. Replacing a blown 15-amp fuse with a 20- or 30-amp one — historically easy in a screw-base fuse panel — removes the protection the conductor depends on while leaving everything looking normal.
Washington’s own rule addresses this directly by requiring Type S fuses or breakers and protection per the 60 °C column of NEC Table 310.16.
3. Deteriorated conductor insulation
Rubber-and-cloth insulation embrittles with heat and age. It tends to fail first where it has been hottest and most handled: at fixtures, at switches, and in the last few inches entering a box.
A system can be sound in the open runs and unsound at every termination, which is why a condition assessment is not a glance from the basement.
4. Later modification
A century of extensions, splices and "just this one outlet" — modern cable spliced to original conductors, splices left outside boxes, grounded three-prong receptacles fitted to an ungrounded circuit so the plug fits.
HyreElectrical analysis: this is the failure mode most often mistaken for the original wiring being dangerous, because the mess and the method share a cavity.
Insulation over knob-and-tube: the rule, and the exception nobody mentions
Source fact. Under NFPA 70, Article 394.12 generally does not permit concealed knob-and-tube in hollow spaces of walls, ceilings and attics where the space is insulated with loose, rolled or foamed-in material that envelops the conductors.
That is the rule almost every consumer page reports, and it is correct as a general statement of the model code.
What those pages leave out is that the model code is not the adopted code. States amend it.
Washington has published a conditional pathway in its own administrative code — WAC 296-46B-394, filed 21 February 2024 and effective 1 April 2024 — under which loose or rolled insulation may be installed in spaces containing existing knob-and-tube provided every condition below is met. The text was read from the state legislature’s own site.
| Condition | What Washington requires |
|---|---|
| Survey and certification | An appropriately licensed electrical contractor must survey the wiring and certify in writing to the department that it is in good condition — no improper overcurrent protection, no conductor insulation failure or deterioration, no improper connections or splices. |
| Inspection of any work | The electrical inspector must inspect all repairs, alterations or extensions to the electrical system. |
| Insulation specification | The insulation must meet Class I specifications as identified in the Uniform Building Code, with a flame spread factor of 25 or less tested to ASTM E84-81a. |
| Foam | Foam insulation may not be used with knob-and-tube. The exception does not extend to it. |
| Overcurrent protection | All knob-and-tube circuits must be protected in compliance with NEC Table 310.16, 60 °C column, using either circuit breakers or Type S fuses. |
Source fact: Washington Administrative Code 296-46B-394, "Concealed knob-and-tube wiring", read from app.leg.wa.gov and retrieved 6 September 2026.
HyreElectrical recommendation. The Washington conditions are not permission anywhere else, and the general prohibition is not the last word in your own state. What governs your house is the NFPA 70 edition your jurisdiction adopted and how it amended it. Our NEC edition lookup is the starting point; the building department is the authority.
What to actually do about it
In order, and the first two cost nothing.
Look in the unfinished spaces for knobs, tubes and separated conductors. Two-prong outlets alone are not proof. Photograph what you find.
Their answer determines the scope. Get the requirement and the acceptable evidence in writing before anyone quotes work — a scope built on an unwritten requirement cannot be enforced later.
The party assessing the wiring is very often the party selling the rewire. Paying for an independent assessment buys you a scope you can hand to several bidders on the same terms.
Original knob-and-tube may be sound in open runs and unsound at terminations, or fine everywhere except an insulated attic. "Whole-house rewire" is sometimes the right answer and sometimes the easiest one to sell.
Rewiring is permitted work, and when you need an electrical permit sets out what the model code exempts and what unpermitted work does at sale and at claim time.
The inspection is the only step where somebody with no financial stake in the job checks it — including, in Washington, the certification the insulation exception depends on.
Knob-and-tube has no equipment ground. Ask what the scope does about existing three-prong receptacles on ungrounded circuits, because leaving them is a hazard the rewire was supposed to remove.
Related: whole-house rewiring covers the work itself, and aluminum wiring covers the other wiring type carriers ask about — which, unlike this one, does have a published remediation route.
What we could not establish
How many American homes still have active knob-and-tube. No federal source publishes it. The Census records when a home was built, not how it is wired. We publish the construction-era figure and label it as a weak proxy rather than converting it into a count somebody will quote back as fact.
A national fire statistic isolating knob-and-tube. We did not find one that separates it from other pre-1950 wiring and from the later modifications made to it. Numbers of this kind circulate widely; we could not trace one to a source that measured what the sentence claims, so this page publishes none.
What most insurers do. Two named guides establish that outright ineligibility is a published position. They do not establish a market share, and we will not extrapolate one from them.
Verbatim NEC wording. NFPA 70 is copyrighted and the full text of Article 394 was not retrievable from a free primary source on the retrieval date. Article 394 is therefore cited by provision and characterized here, never quoted. The Washington conditions are quoted, because state administrative code is public law.
Questions
Is knob-and-tube wiring illegal?
Do I have to rewire my house if it has knob-and-tube?
Can I put insulation over knob-and-tube wiring?
Why will my insurer not cover a house with knob-and-tube?
Is knob-and-tube grounded?
Written and audited by
HyreElectrical Research Desk
Primary-source research and fact checking
We read the model code, the federal safety notice, the municipal fee sheet or the utility tariff ourselves, and we publish the figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify. HyreElectrical does not perform, supervise or warrant electrical work. Authorship is organizational: this desk, not a named persona.
- 16
- original research studies published
- 5
- states with verified license records
- 16,369
- electrical companies in those records
- 74%
- of the store is Florida — disclosed first, not footnoted
How this desk works
- Primary sources only. Code statements come from NFPA 70 as the model code. Safety statements come from CPSC. Permit fees come from the city or county fee sheet. Utility charges come from the filed tariff. We do not cite a blog that cites a source; we open the source.
- Our contractor store is five states, and 74% of it is Florida. Any figure built on that store is titled to those states and names the concentration in the first screen. It is not a national sample of electricians.
- Load calculations on this site are a published simplification in the shape of NEC 220.82 — first 10 kVA at 100%, remainder at 40%, HVAC at 100%, EVSE at 125%. They are labeled as not a stamped calculation. A licensed electrician using the adopted edition does that work.
- No national price for a panel upgrade is shipped. Labor rates, the adopted code edition, whether the utility is involved, and the state of the existing wiring move the invoice too far for a roundup to help. The cost pages publish named permit fees and the variables that move the price instead.
- We do not perform electrical work, and we take no payment for placement, ranking or a favorable mention. Nobody buys a position on this site.
Data as of 6 September 2026. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
Get a condition assessment before you get a quote
The party assessing century-old wiring is often the party selling the rewire. An independent assessment gives you one scope to put to several bidders.
HyreElectrical does not perform, supervise or warrant electrical work, and takes no payment for placement, ranking or favorable mention. Underwriting rules vary by carrier, program and state and change without notice; nothing here is insurance or legal advice.