Overview
Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring that is used to provide power in buildings and structures, commonly referred to as building wiring. The article describes common features of electrical wiring that should apply worldwide.
Wire Sizes
The international standard wire sizes are given in the IEC 60228 standard of the International Electrotechnical Commission. In North America, the American Wire Gauge is used.
Wiring Safety Codes
Wiring safety codes are intended to protect people and buildings from electrical shock and fire hazards. A city, county, province, state or Congress may legislate regulations. These entities sometimes adopt (in amended form) a model code produced by a technical standards-setting organization, or by a national standard electrical code.
Electrical codes arose in the 1880s with the commercial introduction of electrical power. Many conflicting standards existed for the selection of wire sizes and other design rules for electrical installations.
The first electrical codes in the United States originated in New York in 1881 to regulate installations of electric lighting. In 1897 the U.S. National Fire Protection Association, a private nonprofit association formed by insurance companies, published the National Electrical Code (NEC). States, counties, or cities often include the NEC in their local building codes by reference along with local differences. The NEC is modified every three years. It is a consensus code that considers suggestions from interested parties. Committees of engineers, tradesmen, manufacturer representatives, fire fighters, and other invitees study the proposals.
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 occasionally in technical detail. As part of the North American Free Trade Agreement (NAFTA), U.S. and Canadian standards are slowly converging, in a process known as harmonization.
In European countries, an attempt has been made to harmonize national wiring standards in an IEC standard, IEC 60364 Electrical Installations for Buildings. National standards follow an identical system of sections and chapters. 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 testing compliance with 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 those installing and inspecting electrical systems.
The 2006 edition of the Canadian electrical code references IEC 60364 and states that the code addresses the basic principles of electrical protection in Section 131. The Canadian code reprints Chapter 13 of IEC 60364. There are no numerical criteria listed in that chapter, however, whereby the adequacy of any electrical installation can be assessed.
DKE - the German Commission for Electrical, Electronic and Information Technologies of DIN and VDE - is responsible for the promulgation of German electrical standards and safety specifications. DIN VDE 0100 is the German wiring regulations document harmonized with IEC 60364.
The Institution of Engineering and Technology regulates wiring installations in the United Kingdom. The U.K.'s Requirements for Electrical Installations: IEE Wiring Regulations, BS 7671: 2001 are harmonized with IEC 60364. The previous edition (16th) was replaced by the current 17th Edition in January 2008. The 17th edition includes new sections for microgeneration and solar photovoltaic systems. The first edition was published in 1882.
AS/NZS 3000 is an Australian/New Zealand standard, commonly known as the "wiring rules," which specifies the requirements for the selection and installation of electrical equipment and the design and testing of such installations. The standard is mandatory in both New Zealand and Australia; therefore, all electrical work covered by the standard must comply.
Wiring Methods
Installing electrical wiring "the old way" by cutting into the bricks of the building
Another picture of this outdated cutting method of installing wires in buildings
Materials for wiring interior electrical systems in buildings vary depending on:
- intended use and amount of power demand on the circuit
- type of occupancy and size of the building
- national and local regulations
- environment in which the wiring must operate
Wiring systems in a single family home or duplex, for example, are simple. Such systems have relatively low power requirements, infrequent changes to the building structure and layout (usually with dry, moderate temperatures), and noncorrosive environmental conditions. In a light commercial environment, more frequent wiring changes can be expected, large apparatus may be installed, and special conditions of heat or moisture may apply. Heavy industries have more demanding wiring requirements, such as very large currents and higher voltages, frequent changes of equipment layout, and corrosive, wet, or explosive atmospheres.
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 because of the danger of electrocution and fire and the high labor cost for such installations.
Knob and tube
The earliest standardized method of wiring in buildings, commonly used in North America from about 1880 to the 1930s, was knob and tube (K&T) wiring. This wiring used single conductors that were run through cavities between the structural members in walls and ceilings. Ceramic tubes formed protective channels through joists, and ceramic knobs were attached to the structural members to provide air between the wire and the lumber and to support the wires. Because air was free to circulate over the wires, smaller conductors could be used than required in cables. By arranging wires on opposite sides of building structural members, some protection was afforded against short circuits that can be caused by driving a nail into both conductors simultaneously. By the 1940s, the labor cost of installing two conductors rather than one cable resulted in a decline in new K&T installations.
Metal-Sheathed Wires
In the United Kingdom, an early form of insulated cable introduced in 1896 consisted of two impregnated-paper-insulated conductors in an overall lead sheath. Joints were soldered, and special fittings were used for lamp holders and switches. These cables were similar to underground telegraph and telephone cables of the time. Paper-insulated cables proved unsuitable for interior wiring installations because very careful workmanship was required on the lead sheaths to ensure moisture did not affect the insulation.
A system invented in the UK in 1908 used vulcanized-rubber-insulated wire enclosed in a strip-metal sheath. The metal sheath was bonded to each metal wiring device to ensure continuity.
Other historical wiring methods
Other methods of securing wiring that are now obsolete include:
- Reuse of existing gas pipes for electric lighting. Insulated conductors were pulled into the pipes feeding gas lamps.
- Wood moldings with grooves cut for single conductor wires. These were eventually prohibited in North American electrical codes by the 1930s, but may still be permitted in other regions.
Cables
The first polymer-insulated cables for building wiring were introduced in 1922. These were two or more solid copper wires with rubber insulation and woven cotton cloth over each conductor for protection of the insulation. The cables also had an overall woven jacket, usually impregnated with tar as a protection from moisture. Waxed paper was used as a filler and separator.
Rubber-insulated cables become brittle over time because of exposure to oxygen, so they must be handled with care. The cables should also be replaced during renovations. When switches, outlets, or light fixtures are replaced, the mere act of tightening connections may cause insulation to flake off the conductors. Rubber was hard to separate from bare copper, so copper was tinned, causing slightly more resistance.
About 1950, PVC insulation and jackets were introduced, especially for residential wiring. About the same time, single conductors with a thinner PVC insulation and a thin nylon jacket became common.
Aluminium wire was common in North American residential wiring from the late 1960s to mid 1970s, because of the rising cost of copper. Because of its greater resistivity, aluminium wiring requires larger conductors than with copper. For instance, instead of 14 AWG (American wire gauge) for most lighting circuits, aluminium wiring would typically be 12 AWG on a typical 15 amp circuit. Local building codes may vary, however.
Aluminium conductors were originally used with wiring devices intended for copper wires. This could cause defective connections unless all devices (breakers, switches, receptacles, splice connectors (i.e., wire nuts), etc.) were designed to address certain problems. These included problems with junctions between dissimilar metals, oxidization on metal surfaces, and mechanical effects that occur as different metals expand at different rates with increases in temperature. Because of improper design and installation, some junctions to wiring devices overheated under heavy current load and caused fires. Revised standards for wiring devices (such as the CO/ALR "copper-aluminum-revised" designation) were developed to reduce these problems.
Aluminium conductors are still used for power distribution and large feeder circuits because they cost less than copper wiring, especially in the large sizes needed for heavy current loads. Aluminum conductors must be installed with compatible connectors.
The simplest form of cable is composed of 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 America, an overhead cable from a transformer on a power pole (for residential electrical service) consists of three twisted (triplexed) wires. Often, one wire will be a bare neutral, and the other two will be insulated for the line voltage.
Modern wiring materials
An electrical power cable found commonly in modern European houses. The cable consists of 3 wires (2 wires + 1 grounding) and is double insulated.
Modern, nonmetallic sheathed cables (NMC), like (U.S. 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. Many call this "Romex (tm)" cable because it was the first of its type, by Rome Cable. (Southwire has owned the trade name since 2006.)
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, because the wiring is not required to be very flexible. Building wire conductors larger than 10 AWG (or about 6 mm²) 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 or steel-wire armor, as well as 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. Power or communications cables (e.g., computer networking) that are routed in or through air-handling spaces (plenums) of office buildings are required under the model code to be either encased in metal conduit or rated for low flame and smoke production.
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, thereby compressing the powder. Such cables have a certified fire resistance rating, are more costly than non-fire rated cable, and have less flexibility.
Mineral insulated cables at a panel board
Because multiple conductors bundled in a cable cannot dissipate heat as easily as single insulated conductors, those circuits are always rated at a lower "ampacity." Tables in electrical safety codes give the maximum allowable current for a particular size of conductor and for the voltage and temperature rating at the surface of the conductor for a given physical environment, including the insulation type and thickness. The allowable current will be different for wet or dry and for hot (attic) or cool (underground) locations. In a run of cable through several areas, the most severe area will determine the appropriate 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 polymer-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 inflammable 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 intervals 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 unless the fitting is otherwise rated.
Special cable constructions and termination techniques are required for cables installed in ocean-going vessels. In addition to electrical and fire safety, such cables may be required to be pressure resistant where they penetrate bulkheads of a ship.
Raceways
Electrical Conduit risers, seen inside fire-resistance-rated shaft, as seen entering bottom of a firestop. The firestop is made of firestop mortar on top, rockwool on the bottom. Raceways are used to protect cables from damage.
Insulated wires may be run in one of several forms of a raceway between electrical devices. This may be a pipe, called a conduit, or in one of several varieties of metal (rigid steel or aluminum) or nonmetallic (PVC or HDPE) tubing. Rectangular cross-section metal or PVC wire troughs (North America) or trunking (U.K.) may be used if many circuits are required. Wires run underground may be run in plastic tubing encased in concrete, but metal elbows may be used in severe pulls. Wiring in exposed areas such as factory floors may be run in cable trays or rectangular raceways with lids.
Where wiring, or raceways that hold the wiring, must traverse fire-resistance-rated walls and floors, the openings are required by local building codes to be firestopped. In cases where the wiring has to be kept operational during an accidental fire, fireproofing must be applied to maintain circuit integrity in compliance with a product's certification listing. The nature and thickness of any passive fire protection materials used together with wiring and raceways has a quantifiable impact on the ampacity derating.
A cable tray can be used in stores and dwellings.
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; however, small control wiring is often installed without any intentional spacing between cables.
Because 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 current capacity (ampacity).
Special fittings are used for wiring in potentially explosive atmospheres.
Bus bars, bus duct, cable bus
Topside of firestop with penetrants consisting of electrical conduit on the left and a bus duct on the right. The firestop consists of firestop mortar on top and rockwool on the bottom, for a 2-hour fire-resistance rating.
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 accessible areas, although they are used in manufacturing plants and power company switchyards to gain the benefit of air cooling. A variation is to use heavy cables, especially where it is desirable to transpose or "roll" phases.
In industrial applications, conductor bars are assembled with insulators in grounded enclosures. This assembly, known as a 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 called a plug-in bus is used to distribute power down the length of a building. The plug-in is built 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 ducts 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). For conducting large currents between devices, a cable bus is used. For very large currents in generating stations or substations, where it is difficult to provide circuit protection, an isolated-phase bus is used. Each phase of the circuit is run in a separate grounded metal enclosure. The only fault possible is a phase-to-ground fault, because the enclosures are separated. This type of bus can be rated up to 50,000 amperes and up to hundreds of kilovolts (during normal service, not just for faults), but is not used for building wiring in the conventional sense.
Electrical Panels
Electrical panels, cables, and firestops in an electrical service room at St. Mary's Pulp and Paper, a paper mill in Sault Ste. Marie, Ontario, Canada.
Electrical panels are easily accessible junction boxes used to reroute and switch electrical services.
External Links
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Electrical wiring."
