You cannot see most of your electrical system. It is inside walls, and it was installed to the standards of whenever it was installed.
That makes construction era one of the more useful things you can know about a house, because wiring methods changed in fairly well-defined phases. Knowing roughly when a home was wired tells you what to look for, and what the likely failure modes are.
This is a guide rather than a diagnosis. Homes get modified, partially rewired, added to. But it is a good starting point.
Before roughly 1950: knob-and-tube
Conductors run individually through porcelain knobs and tubes, relying on air circulation around them for cooling. There is no equipment ground.
What tends to go wrong: the insulation ages and becomes brittle. More importantly, knob-and-tube becomes hazardous when buried in insulation added decades later, which is extremely common, because the retrofit insulation industry did not always account for it. The system depends on air circulation it no longer has.
Many insurers will not cover homes with active knob-and-tube, and will ask whether it has been abandoned in place or is still energised. Those are very different situations.
1950 to about 1964: early cable, limited grounding
Cable-based wiring arrives. Service capacity is typically 60 to 100 amps, which was ample for the appliance load of the era and is not now. Grounding is limited or absent.
What tends to go wrong: ungrounded circuits, two-prong outlets, and service capacity that modern households outgrow. A frequent and dangerous shortcut here is three-prong outlets fitted to ungrounded two-wire circuits. They look correct and provide no ground at all.
About 1965 to 1973: the aluminium branch wiring window
For a period, aluminium was widely used for branch circuits, not just for service conductors.
What tends to go wrong: aluminium expands and contracts more than copper. Connections at outlets and switches loosen over time, and a loose connection under load generates heat. The failure is at the terminations rather than in the conductor itself.
This is remediable. Approved connectors at every device and junction is a recognised approach and considerably cheaper than full replacement. Standard outlets and switches are not rated for direct aluminium connection.
One clarification worth making: this concerns aluminium branch circuits. Aluminium service entrance conductors are still standard practice and entirely normal.
1974 to about 1990: grounded copper becomes standard
Grounded copper cable is the norm. Service capacity typically 100 to 150 amps. GFCI requirements begin appearing, initially for a narrow set of locations and expanding over subsequent code cycles.
What tends to go wrong: generally sound, but capacity is often marginal for a household that has since added air conditioning, electric cooking, and now vehicle charging. GFCI coverage is far narrower than current requirements.
1991 to about 2005: GFCI expansion
Service capacity commonly 150 to 200 amps. GFCI protection required across most wet and damp locations. AFCI requirements have not yet expanded.
What tends to go wrong: relatively little structurally. The gap is AFCI protection, which addresses arcing fire risk that GFCI does not.
Post-2005: AFCI, and now EV readiness
200 amp service is common. AFCI requirements expand considerably across living areas. More recent construction increasingly anticipates vehicle charging.
What tends to go wrong: mostly capacity planning for loads that were not anticipated even at this stage.
What to do with this
Two practical points.
First, era predicts the question, not the answer. A home built in 1968 might have been fully rewired in 1995. What matters is what is physically present, which an electrician can determine from the panel, accessible junction boxes, and outlets.
Second, code compliance is not the same as safety. Codes apply at the time work is performed. A home wired in 1978 to 1978 standards was compliant then and was never required to be updated. It is not “in violation” now. What matters is whether the installation has deteriorated, whether it has been modified badly, and whether its capacity suits how you actually live.
If you are buying an older home, a dedicated electrical inspection is worth considerably more than the electrical portion of a general home inspection, which is, by design, a fairly limited look.
