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Electrical wiring in general refers to conductors used to carry electricity, and their accessories. This article describes general aspects of electrical wiring as used to provide power in or to buildings and structures, commonly referred to as building wiring. Electrical wiring practices vary greatly by locality. This article is intended to describe common features of electrical wiring that should apply worldwide.
Wiring safety codes
Electrical codes arose in the 1880's with the early commercial introduction of electrical power, since many conflicting standards existed for the selection of wire sizes and other design rules for electrical installations. Regulations may be set by local (city, provincial/state) legislation, perhaps by amendments to a model code produced by a technical standards-setting organization, or by a national standard electrical code. The intention of wiring safety codes is to provide technical, performance and material standards that will allow efficient distribution of electrical energy and communication signals, at the same time protecting persons in the building from electric shock and preventing fire or explosion.
Country specific
North America
The first electrical codes in the United States originated in New York in 1881 to regulate installations of electric lighting. The U.S. National Fire Protection Association, a private nonprofit association formed by insurance companies, produced the first draft of the U.S. National Electrical Code in 1885.Since 1927, the Canadian Standards Association has produced the Canadian Safety Standard for Electrical Installations, which is the basis for provincial electrical codes.
Although these two national standards deal with the same physical phenomena and broadly similar objectives, they differ significantly in technical detail. As part of the NAFTA program, US and Canadian standards are slowly converging towards each other, in a process known as harmonization.
In the United States local governing bodies such as counties or cities often include the National Electrical Code in their local building codes by reference along with any local differences.
Europe
In European countries, an attempt has been made to harmonize national wiring standards in an IEC standard, IEC 60364 Electrical Installations for Buildings. However, this standard is not written in such language that it can readily be adapted as a national wiring code. Neither is it designed for field use by electrical tradesmen and inspectors for verification of compliance to national wiring standards. National codes, such as the NEC or CSA C22.2, exemplify the common objectives of IEC 60364, and provide rules in a form that allows for guidance of persons installing and inspecting electrical systems.The 2006 edition of the Canadian electrical code references IEC 60364 and states that the code addresses the fundamental principles of electrical protection in Section 131. The Canadian code reprints Chapter 13 of IEC 60364 and it is interesting to note that there are no numerical criteria listed in that chapter whereby the adequacy of any electrical installation can be assessed.
Germany
DKE - German Commission for Electrical, Electronic & Information Technologies of DIN and VDE - is the German organisation responsible for the elaboration of electrical standards and safety specifications.United Kingdom
In the United Kingdom wiring installations are regulated by the produced by the IEE Requirements for Electrical Installations: IEE Wiring Regulations, BS 7671: 2001 which is now in its 16th edition.Wiring methods
Materials for wiring interior electrical systems in buildings vary depending on:
- Rating of the circuit
- Type of occupancy of the building
- Type of electrical system
- National and local regulations
- Conditions in which the wiring must operate.
Early wiring methods
The very first interior power wiring systems used conductors that were bare or covered with cloth, which were secured by staples to the framing of the building or on running boards. Where conductors went through walls, they were protected with cloth tape. Splices were done similarly to telegraph connections, and soldered for security. Underground conductors were insulated with wrappings of cloth tape soaked in pitch, and laid in wooden troughs which were then buried. Such wiring systems were unsatisfactory due to the danger of electrocution and fire, and due to the high labor cost for installation.Knob and tube
The earliest standardized method of wiring in buildings, in common use from about 1880 to the 1930s, was knob and tube wiring: single conductors ran directly through walls and ceilings, with ceramic tubes forming protective channels through joists and ceramic knobs acting to support the weight and tension of the wires. New installations of knob and tube wiring have been forbidden by code in most areas for many years; existing installations are typically permitted to remain, but if the wiring has been damaged it may need to be upgraded to use more modern materials.
Other historical wiring methods
Other methods of securing wiring that are now obsolete include:- Re-use of existing gas pipes for electric lighting. Insulated conductors were pulled into the pipes feeding gas lamps.
- Wood moldings with grooves cut for wires. These were eventually prohibited in North American electrical codes by the 1930s, but may still be permitted in other regions.
Cables
The first cables for building wiring were introduced in 1922. These were two or more solid copper wires, with woven cloth and paper insulation, sometimes impregnated with tar as a protection from moisture wicking. By the 1940s, the cost and other advantages of cable resulted in a decline in new knob-and-tube installations.Insulation of these cables was rubber. Old rubber-insulated cables may become brittle over time, so they must be handled with care, and should be replaced during renovations. When switches, outlets or light fixtures are replaced, the simple act of tightning connections may cause insulation to flake off the conductors.
From the mid-1960s to the mid-1980s, many buildings used asphalt-impregnated wiring, which appears to have retained the flexibility of its insulation. Asphalt-impregnated wiring will typically have a cloth-like appearance on the outer jacket, but must be checked for markings like "Al" or "Cu".
Aluminum wiring was common in North American residential wiring from the late 1960s to mid 1970s, due to the rising cost of copper. The outer jacket should be checked for markings like "Al" or "Cu". Due to the greater resistivity of aluminum, aluminum wiring will typically use one wire gauge larger conductors than would be required of copper - instead of 14 awg (American wire gauge) for most lighting circuits, aluminum wiring would typically be 12awg on a typical 15 amp circuit, though local building codes may vary.
Aluminum conductors were originally used with wiring devices intended for copper wires. Because aluminum conductors slowly creep under the pressure of wiring device terminals, a few residential installations of aluminum wiring failed early, causing fires due to poor connections. Later wiring devices were designed compatible with both aluminum and copper wiring, but because aluminum screw connection joints require more care and attention in installation, aluminum wiring became unpopular and, with moderation of copper prices, small solid aluminum wiring was no longer used in residential construction.
Aluminum conductors are still used for power distribution because they cost less than copper wiring, especially in large sizes needed for heavy current loads. Proper installation techniques are required to prevent oxidation and heating of terminations of aluminum conductors.
From the mid-1970s, asphalt cables were replaced with nylon or PVC-jacketed cables which are still in use.
The simplest form of cable is two insulated conductors twisted together to form a unit; such unjacketed cables with two or three conductors are used for low-voltage signal and control applications such as doorbell wiring. In North American practice an overhead cable from a transformer on a power pole to a residential electrical service is three twisted | triplexed wires, often with one being a bare grounded neutral.
Modern wiring materials
Modern nonmetallic sheathed cables (NMC), like (US and Canadian) Type NM, consist of two to four thermoplastic insulated wires and a bare wire for grounding (bonding) surrounded by a flexible plastic jacket.Rubber-like synthetic polymer insulation is used in industrial cables and power cables installed underground because of its superior moisture resistance.
Insulated cables are rated by their allowable operating voltage and their maximum operating temperature at the conductor surface. A cable may carry multiple usage ratings for applications, for example, one rating for dry installations and another when exposed to moisture or oil.
Generally single conductor building wire in small sizes is solid wire, since the wiring is not required to be very flexible. Building wire conductors larger than #10AWG (or about 6 square millimetres) are stranded for flexibility during installation.
Industrial cables for power and control may contain many insulated conductors in an overall jacket, with helical tape steel or aluminum armor, or steel wire armor, and perhaps as well an overall PVC or lead jacket for protection from moisture and physical damage. Cables intended for very flexible service or in marine applications may be protected by woven bronze wires. Signal cables, such as Ethernet cables, that must be run in air-handling spaces (plenums) of office buildings may be required to be fire-resistant and made with Teflon or other materials that produce little toxic fumes or smoke.
For some industrial uses in steel mills and similar hot environments, no organic material gives satisfactory service. Cables insulated with compressed mica flakes are sometimes used. Another form of high-temperature cable is a mineral insulated cable, with individual conductors placed within a copper tube, and the space filled with magnesium oxide powder. The whole assembly is drawn down to smaller sizes, which compresses the powder. Such cables are fireproof and can be used up to 200 °C, but are costly to purchase and install, and have little flexibility.
Because conductors in a cable are in contact and so cannot dissipate heat as easily as single insulated conductors, they usually are rated at a lower current carrying capacity or "ampacity". Tables in electrical safety codes give the maximum allowable current for a particular size of conductor, for the voltage and temperature rating of the insulation, and for a given physical environment. The allowable current will be different for wet or dry, for hot ( attic) or underground (cool) locations. In a run of cable through several areas, local electrical codes will determine the proper rating of the overall run.
Cables usually are secured by special fittings where they enter electrical apparatus; this may be a simple screw clamp for jacketed cables in a dry location, or a rubber-gasketed cable connector that mechanically engages the armor of an armored cable and provides a water-resistant connection. Special cable fittings may be applied to prevent explosive gases from flowing in the interior of jacketed cables, where the cable passes through areas where flammable gases are present. To prevent loosening of the connections of individual conductors of a cable, cables must be supported near their entrance to devices and at regular intervale through their length. In tall buildings special designs are required to support the conductors of vertical runs of cable. Usually, only one cable per fitting is allowed.
Special cable constructions and termination techniques are required for cables installed in ocean-going vessels; in addition to electrical safety and fire safety, such cables may also be required to be pressure-resistant where they penetrate bulkheads of a ship.
Conduits, ducts, wire ways, cable trays
Insulated wires may be run in one of several forms of raceway between electrical devices. This may be a rigid steel or aluminum pipe, called a conduit, or in one of several varieties of metal or non-metallic tubing. Wires run underground, for example, may be run in plastic tubing encased in concrete, but metal elbows may be used in severe pulls. Wiring in exposed areas, for example factory floors, may be run in raceways or cable trays. For protection from flame spread, fire stopping material of silicone may be used. However, the thickness of such material along the cable must be held to a minimum, else this becomes a heat limit. Special fittings are used for wiring in potentially explosive atmospheres.Cable trays are used in industrial areas where many insulated cables are run together. Individual cables can exit the tray at any point, simplifying the wiring installation and reducing the labor cost for installing new cables. Power cables may have fittings in the tray to maintain clearance between the conductors, but small control wiring is often installed without any intentional spacing between cables.
Since wires run in conduits or underground cannot dissipate heat as easily as in open air, and adjacent circuits contribute induced currents, wiring regulations give rules to establish the ampacity.
Bus bars, bus duct
See also main article on Bus bars
For very heavy currents in electrical apparatus, and for heavy currents distributed through a building, bus bars can be used. Each live conductor of such a system is a rigid piece of copper or aluminum, usually in flat bars (but sometimes as tubing or other shapes). Open bus bars are never used in publicly- accessed areas, but are used in manufacturing plants and power company switch yards to gain the benefit of air cooling.
In industrial applications, conductor bars are assembled with insulators in grounded enclosures. This assembly, known as bus duct, can be used for connections to large switchgear or for bringing the main power feed into a building. A form of bus duct known as plug-in bus is used to distribute power down the length of a building; it is constructed to allow tap-off switches or motor controllers to be installed at definite places along the bus. The big advantage of this scheme is the ability to remove or add a branch circuit without removing voltage from the whole duct.
Bus duct may have all phase conductors in the same enclosure (non-isolated bus), or may have each conductor separated by a grounded barrier from the adjacent phases (segregated bus). Likewise, for conducting large currents between devices, cable bus is used. For very large currents in generating stations or substations, where it is difficult to provide circuit protection, isolated-phase bus is used. Each phase of the circuit is run in a separate grounded metal enclosure. A fault in any phase jumps to ground. This type of bus can be rated up to 50,000 amperes and up to hundreds of kilovolts, but is not used for building wiring in the conventional sense.
Wiring tools
- Lineman's pliers are heavy-duty pliers for general use in cutting, bending, crimping and pulling wire; electricians commonly use the tool as a hammer, as well.
- Needle-nose pliers share the basic design of lineman's, but feature a namesake long, tapered gripping nose and are of more various size, generally smaller and for finer work (including very small tools used in electronics wiring).
- wire strippers come in many sizes and designs, but those intended for electric power wiring feature special blades to cut wire insulation on American Wire Gauge (AWG) #12 and #14 while leaving the conductor wire intact. Some wire strippers include cable strippers among their multiple functions, for removing the outer jacket of NM cable (also known as Romex).
- cable cutters are highly-leveraged pliers for cutting cable larger than #10/3.
- rotosplit is a brand-name tool designed to assist in breaking the spiral jacket of metallic-jacketed cable (MC cable).
- multimeter is a small, battery-powered instrument for basic electrical testing and troubleshooting; features voltage-, resistance-, and current-reading settings.
- step-bit is a metal-cutting drill bit with stepped-diameter cutting edges, generally at 1/8-inch intervals, for conveniently drilling holes to specification in stamped/rolled metal up to about 1/16" thick; for example, to create custom knock-outs in a breaker panel or junction box.
- fishes include fish wire, cord or rope, fish-tape and fish-sticks, the principle in each case being a long 'lead' device for pulling the installed wire or cable into or through an inaccessible cavity, such as in the stud-bay or joist-bay of a finished wall or in a floor or ceiling.
- Other, general-use tools with applications in electric power wiring include screwdrivers, crimpers, hammers, reciprocating saws, drywall saws, metal punches, flashlights, chisels, adjustable pliers (for example, channel-lock pliers), drills.
External links
- Electrical wiring FAQ, oriented to US/Canadian practice
- Several good guides for doing home wiring
- Residential troubleshooting strategy and tips
See also
- Electric power transmission
- Electricity distribution
- Electrical wiring (U.S.)
- Electrical wiring (UK)
- Domestic AC power plugs and sockets
- Industrial and multiphase power plugs and sockets
- Unusual and obsolete plugs and sockets
Sources
- Electric Codes, (n.d.). Retrieved Nov. 14, 2005, from Electric Codes Web site: www.http://codecheck.com/eleccode.htm.
- Hirst, E. (n.d.). Retrieved Nov. 14, 2005, from Electric Utilities and Energy Efficiency Web site: http://ornl.gov/info/ornlreview/rev28_2/text/uti.htm.
- Bodman, G. (n.d.). Retrieved Nov. 14, 2005, from Electrical Systems for Agricultural Buildings (Recommended Practices) Web site: http://ianrpubs.unl.edu/farmbuildings/g845.htm
- Valcourt, Robert. Personal Interview. 3 October 2005
- National Electrical Code 2005. 2005 ed. Quincy: National Fire Protection Association, 2004.
- Sorge, Harry. Residential Wiring. 2002 ed. : Thomson Delmar Learning, 2002.
- Mullin, Ray. Residential Wiring. 2005 ed. : Thomson Delmar Learning, 2002.
References
- Insuring a home with knob and tube insulation
- History of Residential Wiring Methods - U.S.
- * NEMA comparison of IEC 60364 with the US NEC
"Electrical wiring"