A brief history of residential electrical wiring buying a house built in the 70's
70 percent of American homes were connected to the electrical utility grid by 1935, thanks to electrical service that began in the late 1890s.Several important innovations aimed at improving the safety of electrical systems have been seen in the following 200 years.
The knob-and-tube was the main system of installation between 1890 and 1910.A surprising number of American homes still have knob-and-tube wiring functioning, where it is often found alongside more modern updates.
In knob-and-tube wiring, the individually conducting wires are protected by a rubberized cloth fabric and held in place by porcelain knob insulators on the sides of the framing members.The hot wires and neutral wires were separated for safety.The system allowed long circuit runs to be constructed.The insulation was stripped back, a new wire was wrapped around the exposed bare wire, and the splice was soldered together.The wire was exposed and there was no ground wire.
When knob-and-tube wiring is still functioning, it is living on borrowed time since the cloth insulation used on the wires has an expected lifespan of 25 years before it begins to crack and break down.The knob-and-tube wiring is in need of an upgrade.It doesn't mean you are in danger if you see knob-and-tubes in a wall or floor.When a home was rewired, it was common for old wiring to be left in place.It's possible that the porcelain insulators and wires are remnants of an earlier wiring installation.An electrician can tell you the truth.
Flexible armored cable was used in electrical installations in the 1920s to 1940s.Flex was a great addition to the home wiring because it allowed for flexible metal walls to protect the wires from damage, and it also offered a metal pathway that could ground the system when properly installed.The wiring method had some problems.Although the individual wire conductors are protected, the flexible outer metal jacket serves as a proper ground only when the metal pathway is complete.There is no ground wire in these installations.
A quicker installation method was developed in the 1930s.The hot and neutral wire were run together in this one sheath, much like a knob and tube wiring.Due to the lack of a ground wire, it had some drawbacks, but its development would eventually lead to major innovation.There is an expected lifespan of 25 years for early sheathed cable, and where it is still in use, it needs to be upgraded.
The metal conduit of the 1940s was very old.The invention allowed users to pull wires from the same enclosure.An insulated green wire can be pulled through the conduit if the system allows it.When wiring needs to be run along the face of basement masonry walls or in exposed locations, conduit is still the recommended method.Most homes have some areas where conduit is used, but it is sometimes made with plastic instead of metal.