Electrical service to American homes began in the late 1890s and blossomed from 1920 to 1935, by which time 70 percent of American homes were connected to the electrical utility grid. In the following 100-some years, the methods for installing wiring in those homes has seen several important innovations aimed at improving the safety of electrical systems.
Between 1890 and 1910, a wiring system known as knob-and-tube was the principal system of installation. It was quite a dependable system for the time, and 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, individually conducting wires protected by rubberized cloth fabric are installed in stud and joist cavities, held in place by porcelain knob insulators attached to the sides of framing members, and protected by porcelain tube insulators where the wires run through framing members. In this wiring system, hot wires and neutral wires were run separately for safety. The system also allowed long circuit runs to be constructed by splicing together lengths of wire. To do this, the insulation was stripped back, a new wire was wrapped around the exposed bare wire, and the splice was soldered together, then taped to cover the splice. The downfall was that the wire was exposed and there was no ground wire used. The rubber would also dry out and start to crack so the insulation could fall off when disturbed.
Where knob-and-tube wiring is still functioning, it is living on borrowed time, since the rubberized cloth insulation used on the wires has an expected lifespan of about 25 years before it begins to crack and break down. Electrical systems containing functioning knob-and-tube wiring are in critical need of an upgrade. But just because you see knob-and-tubes in some wall or floor cavities, doesn't necessarily mean you are in danger. It was common practice to simply leave old wiring in place when a home was rewired. It's possible that the porcelain insulators and wires you see are merely antique remnants of earlier wiring installation. An electrician can tell you for sure.
Flexible Armored Cable (Greenfield or BX)
In the 1920s to 1940s, electrical installations took a turn to a more protective wiring scheme—flexible armored cable. Flex, also known as Greenfield or BX, was a welcome addition to home wiring because the flexible metal walls helped to protect the wires from damage, and also offered a metal pathway that could ground the system when properly installed. Although it was an improvement, this wiring method had its troubles. Although the individual wire conductors are protected, the flexible outer metal jacket serves as a proper ground only when the metal pathway is complete all the way to the service entrance and grounding rod. There is still no separate ground wire in these installations.
In the early years of BX wiring, the rubberized fabric coating sheath was still being used (similar to that used in knob and tube wiring). This would dry out and crack over time. In a metal case undisturbed it would work fine, but when in ceiling or wall boxes, moving the wire to replace a device could often cause the insulation to fall off and then the exposed copper could cause a short circuit.
First-Generation Sheathed Cable
In the 1930s, a quicker installation method was developed. Nonmetallic-sheathed cable was born, which incorporated a rubberized fabric coating sheath, much like knob and tube wiring, but here the hot and neutral wire were run together in this one sheathing. It also had its drawbacks due to the lack of a ground wire, but its development would eventually lead to major innovation. Early sheathed cable, however, also has an expected lifespan of about 25 years, and where it is still in use, such installations need to be upgraded.
Ground wire was added in the mid-1940s to early 1950s, but it was only about a quarter of the size of the hot and neutral wires. This was also when the modern plastic-coated inner wire was introduced. This cable type was sometimes called Rat Wire because the coating was developed to have a bad taste to keep mice and rats away.
The 1940s brought the age of metal conduit. This invention allowed users to pull many individual conducting wires in the same rigid metal tube enclosure. The conduit itself is considered a viable grounding method, and the system can also allow another separate grounding wire (usually an insulated green wire) to be pulled through the conduit. Conduit has been in use ever since those days and is still the recommended method for wiring in certain applications, such as when wiring needs to be run along the face of basement masonry walls or in exposed locations. Most homes have some areas where conduit is used, though it is now sometimes made with rigid plastic PVC conduit rather than metal.
Modern NM Cable
The newest addition to wiring was introduced in around 1965. The form of NM cable was an update to older NM cable, incorporating the use of a bare copper grounding wire that joined the insulated hot and neutral wires contained within the sheathing. Instead of rubberized sheathing, modern NM cable uses a very tough and durable vinyl sheathing. This update made the NM cable inexpensive and very easy to install. It is a very flexible product and is used extensively in virtually every new home built.
Along with NM cable for interior use, a related type of cable was also developed for underground use. Underground feeder wire (UF) can be buried directly under the ground without the need for a protecting conduit. This type of wire has a hot, a neutral, and a ground wire embedded in a solid plastic vinyl sheath that protects it from moisture. This offers an inexpensive method for running power underground to outbuildings and yard lights. It is also sunlight-resistant, so it can run along the outside of a structure where damage isn't a concern.
Metals Used in Wires
Through most of the history of residential electrical service, the preferred metal used in the conducting wires has been copper, known as the best conductor of electrical current. In the mid-1960s, when copper prices were quite high, aluminum came into vogue as a material for electrical wiring. Residential installations between 1965 and 1974 sometimes used wires that were solid aluminum, or aluminum covered with a thin layer of copper. Aluminum (AU) or copper-coated aluminum (AL-CU) wiring is perfectly safe if connected to receptacles, switches, and other devices rated for use with aluminum, but it can pose problems when it's installed with devices intended for use with copper wiring only. Because of these issues, aluminum or copper-clad aluminum is no longer used in residential applications. If you have aluminum wiring, repairs are best made by a professional.
Copper wire conductors in NM sheathed cable or in rigid metal or PVC plastic conduit has been the norm since the mid-1970s, and there are currently no new innovations in the wiring materials themselves. Condos, tall apartment buildings, and other residential structures must be wired in a new form of armored cable, which has an aluminum case with three or four wires inside. This cable is similar to BX, but lighter-weight and easier to work with.
Recent safety improvements have involved the extended application of GFCI (ground-fault circuit interrupter) devices, and more recently, AFCI (arc-fault circuit interrupter) devices that help protect against fire and shock by sensing changes in current flow and shutting off power before problems occur.
But the history of residential wiring is one of the periodic innovations that can revolutionize the industry. It is possible that another such innovation looms on the near horizon.