Knob and tube wiring is not dangerous simply because it is old. It becomes dangerous through what has happened to it: cracked insulation, loose connections, later splices, oversized fuses, too much load and insulation packed around it. Most of that is hidden inside walls and ceilings, so the condition of your wiring decides the risk.
LA Knob and Tube replaces knob and tube wiring in Los Angeles, Pasadena, Glendale and South Pasadena. This guide explains the mechanisms behind the risk, the signs that justify switching a circuit off today, and the limits of what anyone can tell you without reaching the wiring. If you are new to the system itself, the knob and tube wiring guide covers how it works and why it was a sound design when it went in.
How the safety agencies frame the risk
The U.S. Consumer Product Safety Commission and the Electrical Safety Foundation International both describe old residential wiring as a hazard mainly through its condition: worn or damaged insulation, loose connections, overloaded circuits and protection that does not match the wire. Neither treats a particular wiring method as unsafe by date alone. That framing matters, because it turns “is it dangerous?” into a question with an answer you can actually find out.
It also explains why the evidence on fires is less dramatic than some insurance conversations suggest. San Francisco’s fire department said in 2025 news coverage that it had not seen trends in residential fires tied to knob and tube. That does not make any one house safe. It does mean the useful question is what condition your wiring is in, not whether the system is old.
How knob and tube actually fails
The porcelain knobs and tubes can stay intact long after other parts have aged. Porcelain does not burn, conduct or rot. The weak points are the insulation on the wire, the connections, the protection at the fuse box and the conditions around the run.
Insulation that cracks and falls away
Each conductor is wrapped in rubber under a cloth braid. Heat and time make the rubber brittle. In still air, untouched, it can hold together for decades. Bend it, brush it with a cable being pulled through, step on it in an attic or let a rodent chew it, and it can crack and drop off, leaving bare copper on a live conductor.
Bare copper on one wire is not an instant short, because the two conductors of a knob and tube circuit run several inches apart. The danger comes when that exposed section touches something it should not: the other conductor, a metal pipe, a nail or a metal box. That contact can arc, and an arc is hot enough to ignite century-old framing, dust and debris nearby. It is also why no one should handle the wiring to see whether the insulation is sound.
Connections that loosen
Original knob and tube splices were twisted, soldered and taped, usually out in the open between knobs. Done well, they last. The connections that cause more trouble tend to be at devices: screw terminals on old switches, sockets and outlets that have been worked loose by decades of heating and cooling, plugging and unplugging. A loose connection has more resistance than a tight one, so it heats under load. The heat makes it looser, and the cycle repeats until the terminal discolors, the plate warms up or the connection begins to arc.
Splices and modifications added later
Over a century, knob and tube in many older Los Angeles houses has been extended. New outlets were tapped into old runs, fixtures were swapped, and modern cable was tied into original conductors. Some of that work was careful. Some was done without a junction box, with tape instead of a proper connector, or with a splice buried in a wall where nobody would see it again.
Those later splices are a common weak point, often weaker than the original installation. A poorly made splice can loosen and arc exactly the way a loose terminal does, but it sits inside a wall cavity or under attic insulation, out of sight. That is why mixed-era wiring often carries more risk than the visible original layout, and why the guide to active and abandoned knob and tube spends so much time on where old and new wiring meet.
Fuses that no longer match the wire
A fuse protects the wire behind it by opening before the wire overheats. Knob and tube branch circuits were typically protected at 15 amps. When a fuse kept blowing because the circuit carried too much load, a familiar fix in many houses was to screw in a 20 or 30 amp fuse instead. The blowing stopped, and so did the protection. The wire could now carry far more current than it was rated for, heating along its whole length.
The same mismatch can happen when a fuse box is replaced with a breaker panel and old circuits are given breakers sized for modern cable. The panel upgrade vs rewiring guide explains why the panel and the branch circuits have to be planned together.
Too much load on too few circuits
A house wired in the 1920s might have had a handful of circuits for the whole house. Space heaters, window air conditioners, hair dryers and a home office can each draw a large share of a circuit’s capacity. When several run at once on an old circuit, the conductors run hot. On wiring with aging insulation, that extra heat is what speeds up the cracking described above.
Insulation packed around the conductors
Knob and tube was designed to cool in open air. When blown or loose-fill insulation is packed around it, the wire can no longer shed heat, and the rubber ages faster. The California Electrical Code restricts insulating over knob and tube for this reason, and the attic insulation page explains the certification path and the alternative of replacing the attic circuits first.
Warning signs that justify switching a circuit off
Not every sign of old wiring is an emergency. Some symptoms point to heat or arcing right now; others are reasons to plan an inspection.
Switch the circuit off and call an electrician
- A burning, hot-plastic or fishy smell near an outlet, switch or fixture
- Scorch marks, browning or melting on a cover plate or device
- An outlet, switch or plate that feels warm to the touch
- Sparks, buzzing or sizzling when something is plugged in or switched on
- A mild shock or tingle from a fixture, appliance or faucet
- A fuse that blows again after being replaced with the correct size
If you see any of these, turn that circuit off at the fuse box or panel, unplug what you can from it, and leave the wiring itself alone. Do not open the box or pull the device out to look. And never replace a blowing fuse with a larger one.
Worth an inspection soon, not tonight
- Lights that dim when an appliance starts, or flicker without an obvious bulb problem
- Two-prong outlets throughout the house, or three-prong outlets you are not sure are grounded
- Extension cords and power strips used as permanent wiring
- Insulation visible around old wiring in the attic
- A remodel or insulation job planned near the old runs
The three-prong outlets guide explains why a grounded-looking outlet may have no ground behind it, which is a shock-protection issue rather than a fire issue.
What nobody can know without access
A house can have clean porcelain and intact insulation in every visible stretch of attic and still have a failing splice inside a wall. Visible condition is evidence, but it is not proof of what is hidden. Symptoms are similar: a flickering light can come from a loose terminal, a failing splice, an overloaded circuit or a bulb, and no single symptom points to one cause.
A knob and tube inspection maps the panel, matches outlets to circuits, traces the accessible runs in the attic and crawlspace, checks how each circuit is protected and tests for grounding. What it cannot do is look inside closed walls and ceilings. A responsible report says which parts were seen, which were tested and which could not be verified. Anyone who calls an entire knob and tube house safe without that caveat is claiming more than the evidence supports.
Lowering the risk while you decide
If replacement is months away, a few habits take strain off the old circuits without anyone touching the wiring. Keep space heaters and window air conditioners off knob and tube circuits. Do not store boxes on attic runs or add insulation over them. Do not drill, cut or fish new cable near walls where old wiring may run. AFCI and GFCI protection installed by an electrician can add a layer of protection on circuits that stay, though neither device fixes damaged insulation.
From risk to scope
Once the condition is known, the answer becomes a scope. A single bad splice on an otherwise sound circuit can be repaired. Circuits that carry the heaviest loads, sit near water or run through areas due for insulation are candidates for partial replacement. When most of the house still runs on original wiring, or an insurer wants none left, whole-house rewiring is the cleaner answer. Either way, routes usually run through the attic and crawlspace first, with openings at boxes and where framing has to be crossed, and the quote should say who patches them. The LA cost guide explains what each path tends to cost.
If you have noticed one of the warning signs above, switch that circuit off and call. If not, an inspection is the way to turn a general worry into a specific answer for your house, and the guide to identifying knob and tube can help you describe what you have seen when you get in touch.