Knob-and-tube is the wiring method used in North American houses from roughly the 1880s to the 1950s. It is named for its hardware: ceramic knobs holding conductors away from the timber, and ceramic tubes lining the holes where a wire passes through a joist.
The design was deliberate
The two conductors run separately, several inches apart, suspended in open air rather than bundled in a sheath. That spacing is the insulation strategy — air does the work, and the ceramic keeps the wire from touching anything that could carry current or burn.
For a house with a few lights and no appliances, running cool in open joist bays, it was a sound system. Plenty of it is still intact a century later, which is not nothing.
What went wrong was later
Blown-in or batt insulation added decades afterwards buries conductors that were designed to shed heat into open air. The wire now sits in a thermal blanket, which is the single most common way an intact knob-and-tube circuit becomes dangerous.
Then there are the extensions. A circuit built for two lamps gets spliced into for a kitchen, a window air conditioner, a basement workshop. The original conductors were never sized for that, and the splices are frequently made outside a box.
There is no ground
The system predates equipment grounding entirely, so there is no third conductor to bond anything to. That is why an older house has two-prong receptacles: not cost-cutting, simply nothing to connect a third pin to.
The failure mode to look for is a three-prong receptacle installed on one of these circuits anyway. It accepts grounded plugs, looks modern, and its ground pin goes nowhere — which quietly disables every surge protector plugged into it.
It is not illegal to have
Existing installations are generally permitted to remain. What tends to force the issue is insurance rather than code: many insurers now ask directly, and some decline or surcharge.
The honest assessment is condition-dependent. Intact, uninsulated, unextended runs are a different proposition from a circuit that has been buried and spliced into four times, and only looking will tell you which you have.
How to spot it in your house
- In an unfinished basement or attic, look for white ceramic knobs on joists with a single wire running through each.
- Ceramic tubes where a conductor passes through a joist are the other half of the signature.
- Two conductors running several inches apart rather than in one sheathed cable is the giveaway even where the ceramics are hidden.
- Two-prong receptacles throughout the house suggest ungrounded circuits, which often means this or early cable.
The next question
Older-home wiring — knob-and-tube and aluminumRelated terms
- Aluminum wiringBranch wiring run in aluminum rather than copper, common in Canadian homes built between the mid-1960s and late 1970s.
- Grounding and bondingThe system that gives stray current a safe path back instead of through you: grounding ties the system to the earth, bonding ties metal parts together so they sit at the same voltage.