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Wikipedia about high and low voltages
The electric code makes a distinction between "high" voltages (100 volts and higher), and "low" voltages below that, and which have two different safety classifications and regulations. For circuits defined as low voltage, in some jurisdictions, there no requirement for licensing, training, or certification of installers, and no inspection of completed work is required, for either residential or commercial work. Low voltage cabling run in the walls and ceilings of commercial buildings is also typically excluded from the requirements to be installed in protective conduit.
The precise reasoning for the selection of 100 volts as the division between high and low is not clearly defined, but appears to be based on the idea that a person could touch the wires carrying low voltage with dry bare hands, and not be electrocuted, injured, or killed. This is generally true for 12 volt systems, but becomes more ambiguous as the voltage increases to 100.
The meaning also varies when alternating current is used, as there is the more commonly known root mean square voltage (120v) but also a peak wave voltage (170v). Telephones for example use low voltage cabling, but the ringing voltage from the central office is approximately 90 volts peak AC, and which has an RMS voltage of 63v.
In more recent terms, the upper cutoff for what is considered low is approximately 50 volts, with most computer network equipment operating at 48 volts DC or lower, and not requiring special training to connect or use.
Although low voltage cabling does not require inspection or training to install in some jurisdictions, it is still important for installers to be aware of specific electric code safety rules such as how to correctly penetrate building fire barriers and use firestop putty (intumescents) to prevent a low voltage cable from reducing building fire protection and increasing the risk of injury or death for building occupants. Access to such safety information is typically restricted and limited access by the electrical industry itself so as to only permit licensed professionals to learn the NEC rules and educate themselves.
Power cables and their history
Early telegraph systems used the first forms of electrical cabling, transmitting tiny amounts of power. Gutta-percha insulation used on the first submarine cables was, however, unsuitable for building wiring use since it deteriorated rapidly when exposed to air.
The first power distribution system developed by Thomas Edison in 1882 in New York City used copper rods, wrapped in jute and placed in rigid pipes filled with a bituminous compound. Although vulcanized rubber had been patented by Charles Goodyear in 1844, it was not applied to cable insulation until the 1880s, when it was used for lighting circuits. Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls power project.
Mass-impregnated paper-insulated medium voltage cables were commercially practical by 1895. During World War II several varieties of synthetic rubber and polyethylene insulation were applied to cables.
Typical residential and office construction in North America has gone through several technologies:
Early bare and cloth-covered wires installed with staples
Knob and tube wiring, 1880s-1930s, using asphalt-saturated cloth or later rubber insulation
Armored cable, known by the genericized trademark "Bx" - flexible steel sheath with two cloth-covered, rubber-insulated conductors - introduced in 1906 but more expensive
Rubber-insulated wires with jackets of woven cotton cloth (usually impregnated with tar), waxed paper filler - introduced in 1922
Early two-wire PVC-insulated cable, known by the genericized trademark "Romex", 1950s
Aluminum wire was used in the 1960s and 1970s as a cheap replacement for copper, but this is now considered unsafe due to corrosion
Asbestos was used as an Insulator (electricity) in some cloth wires from the 1920s to 1970s.
Modern three-wire PVC-insulated cable, also known as "Romex" - see thermoplastic-sheathed cable
Computer programs useful in electrical
Electrical installations, especially those which will be installed in buildings with complicated structures often require the creation of a very difficult project. Special computer programs enable electricians much easier to plan where and how they will run the wire, where will be located branch and so on. In this way, more and more electricians may remind us of specialists, because the design of electrical networks at the present time is done properly only through the computer. Special programs also allow control of parameters during operation of the installation.