Key takeaways
- Knob and tube was the standard residential method from roughly the 1880s into the 1940s. It has no equipment grounding conductor, which is the single fact that drives most of the trouble it causes today.
- NEC 394.12 prohibits concealed knob and tube in hollow spaces filled with insulation. Blown-in attic insulation over live knob and tube is the most common way a house fails an inspection on this.
- The wiring is not automatically unsafe. Original, undisturbed, correctly loaded knob and tube can be in serviceable condition. What makes it dangerous is age-embrittled insulation and decades of amateur splices.
- In San Antonio it is uncommon rather than absent. The metro's housing is mostly postwar, so knob and tube shows up in the pockets that predate 1940 rather than across the city.
Knob and tube is the wiring method that predates the cable in your walls. Two separate conductors, a hot and a neutral, run through open framing on ceramic knobs and pass through joists inside ceramic tubes. It was the residential standard from roughly the 1880s into the 1940s, and in houses that were never rewired, some of it is still energized.
Most of what gets said about it is half right. It is not automatically a fire waiting to happen, and it is also not fine just because the lights still work. The useful version is more specific than either.
What the system actually is
The hot and the neutral are run separately, several inches apart, supported on porcelain knobs and held clear of framing. That separation is the design. Air is the insulation, and the conductors were meant to shed heat into open space rather than into anything touching them.
The conductors themselves are copper, wrapped in rubber and then in a cotton or asphalt-saturated cloth braid. That braid is how most people recognise it, and it is also the part that ages worst.
There is no third conductor. Knob and tube has no equipment grounding conductor at all, because the concept was not part of residential practice when it was installed. That single absence drives most of what follows.
The rule that catches people
If you take one code citation from this post, take NEC 394.12. It lists the places concealed knob and tube is not permitted, and one of them is hollow spaces and enclosures filled with thermal insulation.
That is the collision. A house with original wiring in the attic gets an energy retrofit, somebody blows cellulose or fiberglass across the whole floor, and conductors designed to run in open air are now packed in insulation that holds their heat against them. The wiring was compliant before the insulation truck arrived and is not after.
It is a common finding because the two trades rarely talk. An insulation crew is not looking at wiring methods, and the homeowner buying an attic upgrade has no reason to know the interaction exists.
Why the absent ground matters more than the age
Every three-prong outlet in a knob and tube house is fed by two wires. Whatever is behind the faceplate, there is no equipment grounding conductor in that circuit, which means the outlet is reporting a protection it does not have. That is the same condition, arrived at from a different direction, that we cover in what an open ground reading means and in two prong outlets in older San Antonio homes.
The practical consequence is the same as it is on any ungrounded circuit. A fault inside an appliance that energizes its metal case has no low resistance path back to the panel, so the breaker does not clear it and the case stays live.
What actually makes a given house a problem
- Embrittled insulation. Rubber that is eighty years old cracks when it is moved, and moving it is exactly what happens when somebody works in the attic.
- Splices outside boxes. Original knob and tube used soldered and taped splices in open air, which was the accepted method. Later additions were often spliced the same way by people who were not electricians.
- Modern load on original circuits. These circuits were sized for lighting and a small appliance, not a window unit and a space heater.
- Insulation contact, as above.
- Mixed vintages. A house rewired in part can leave original runs feeding a back room, so a tester in the kitchen tells you nothing about the bedroom.
Where it turns up here
San Antonio is not Boston. The metro's housing stock is overwhelmingly postwar, so the great majority of homes in this market were wired long after knob and tube stopped going in. Treating it as a citywide problem would be inventing a problem.
Where it is real is the housing that predates about 1940, which in this metro means specific older pockets rather than whole suburbs. The same neighbourhoods that carry other pre-war conditions are the ones worth checking, and we wrote about that housing in panel upgrades in Monte Vista historic homes and electrical problems in Alamo Heights older homes.
Even in those, partial is more likely than total. Houses that old have usually had at least one round of work, and what survives is whatever nobody had a reason to open up.
What we would do about it
Start by establishing scope, because that is the number that decides everything else. How many circuits are original, what do they feed, and is the insulation intact where it can be seen. That is an assessment, not a quote.
If the answer is a few circuits feeding lighting in one part of the house, staged replacement is reasonable and the order should follow risk. If the answer is the whole house, the conversation is a rewire and the signals are laid out in signs your San Antonio home needs a rewire, with the money side in what it costs to rewire a house.
If the pressure is coming from an insurer rather than from a symptom, the letter they want is usually about condition and load rather than about removal, and that is worth establishing before committing to anything. We covered that conversation in what it means when your insurer flags your wiring. An electrical inspection is how we put a number on scope, and whole-home rewiring is what it turns into when the answer is everything.