Electrical wiring

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Electrical symbols for wiring WiringSymbols.png
Electrical symbols for wiring

Electrical wiring is an electrical installation of cabling and associated devices such as switches, distribution boards, sockets, and light fittings in a structure.

Contents

Wiring is subject to safety standards for design and installation. Allowable wire and cable types and sizes are specified according to the circuit operating voltage and electric current capability, with further restrictions on the environmental conditions, such as ambient temperature range, moisture levels, and exposure to sunlight and chemicals.

Associated circuit protection, control and distribution devices within a building's wiring system are subject to voltage, current and functional specification. Wiring safety codes vary by locality, country or region. The International Electrotechnical Commission (IEC) is attempting to harmonise wiring standards amongst member countries, but significant variations in design and installation requirements still exist.

Wiring codes of practice and regulations

Wiring layout plan for a house Architectural wiring diagram minihome.svg
Wiring layout plan for a house

Wiring installation codes and regulations are intended to protect people and property from electrical shock and fire hazards. They are usually based on a model code (with or without local amendments) produced by a national or international standards organisation, such as the IEC.

Australia and New Zealand

In Australia and New Zealand, the AS/NZS 3000 standard, commonly known as the "wiring rules", specifies 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.

Europe

In European countries, an attempt has been made to harmonise national wiring standards in an IEC standard, IEC 60364 Electrical Installations for Buildings. Hence national standards follow an identical system of sections and chapters. However, this standard is not written in such language that it can readily be adopted as a national wiring code. Neither is it designed for field use by electrical tradesmen and inspectors for testing compliance with national wiring standards. By contrast, national codes, such as the NEC or CSA C22.1, generally exemplify the common objectives of IEC 60364, but provide specific rules in a form that allows for guidance of those installing and inspecting electrical systems.

Germany

In Germany, DKE (the German Commission for Electrical, Electronic and Information Technologies of DIN and VDE) is the organisation responsible for the promulgation of electrical standards and safety specifications. DIN VDE 0100 is the German wiring regulations document harmonised with IEC 60364.

North America

The first electrical codes in the United States originated in New York in 1881 to regulate installations of electric lighting. Since 1897 the US National Fire Protection Association, a private non-profit association formed by insurance companies, has 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 considering suggestions from interested parties. The proposals are studied by committees of engineers, tradesmen, manufacturer representatives, fire fighters and other invitees.

Since 1927, the Canadian Standards Association (CSA) has produced the Canadian Safety Standard for Electrical Installations, which is the basis for provincial electrical codes. The CSA also produces the Canadian Electrical Code, the 2006 edition of which references IEC 60364 (Electrical Installations for Buildings) and states that the code addresses the fundamental principles of electrical protection in Section 131. The Canadian code reprints Chapter 13 of IEC 60364, but there are no numerical criteria listed in that chapter to assess the adequacy of any electrical installation.

Although the US and Canadian 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) program, US and Canadian standards are slowly converging toward each other, in a process known as harmonisation.

United Kingdom

In the United Kingdom, wiring installations are regulated by the Institution of Engineering and Technology Requirements for Electrical Installations: IEE Wiring Regulations, BS 7671: 2008, which are harmonised with IEC 60364. The 17th edition (issued in January 2008) included new sections for microgeneration and solar photovoltaic systems. The first edition was published in 1882. In 2018, the 18th edition of the wiring regulations BS7671:2018 was released and came into force in January 2019. Although BS 7671 is the standard to which the UK electrical industry generally adheres, compliance with BS 7671 is not a legal requirement.[ citation needed ]

Colour coding of wiring by region

Colour-coded wires in a flexible plastic electrical conduit found commonly in modern European houses ElectricWireGrounded.jpg
Colour-coded wires in a flexible plastic electrical conduit found commonly in modern European houses

In a typical electrical code, some colour-coding of wires is mandatory. Many local rules and exceptions exist per country, state or region. [1] Older installations vary in colour codes, and colours may fade with insulation exposure to heat, light and ageing.

Europe

As of March 2011, the European Committee for Electrotechnical Standardization (CENELEC) requires the use of green/yellow colour cables as protective conductors, blue as neutral conductors and brown as single-phase conductors. [2]

United States

The United States National Electrical Code requires a bare copper, or green or green/yellow insulated protective conductor, a white or grey neutral, with any other colour used for single phase. The NEC also requires the high-leg conductor of a high-leg delta system to have orange insulation, or to be identified by other suitable means such as tagging. Prior to the adoption of orange as the suggested colour for the high-leg in the 1971 NEC, it was common practice in some areas to use red for this purpose.[ citation needed ]

The introduction of the NEC clearly states that it is not intended to be a design manual, and therefore creating a colour code for ungrounded or "hot" conductors falls outside the scope and purpose of the NEC. However, it is a common misconception that "hot" conductor colour-coding is required by the Code.

In the United States, colour-coding of three-phase system conductors follows a de facto standard, wherein black, red, and blue are used for three-phase 120/208-volt systems, and brown, orange, and yellow are used in 277/480-volt systems. In buildings with multiple voltage systems, the grounded conductors (neutrals) of both systems are required to be separately identified and made distinguishable to avoid cross-system connections. Most often, 120/208-volt systems use white insulation, while 277/480-volt systems use grey insulation, although this particular colour code is not currently an explicit requirement of the NEC. [3] Some local jurisdictions do specify required color coding in their local building codes, however.

United Kingdom

The United Kingdom requires the use of wire covered with green/yellow striped insulation, for safety earthing (grounding) connections. [4] This growing international standard was adopted for its distinctive appearance, to reduce the likelihood of dangerous confusion of safety earthing (grounding) wires with other electrical functions, especially by persons affected by red-green colour blindness.

In the UK, phases could be identified as being live by using coloured indicator lights: red, yellow and blue. The new cable colours of brown, black and grey do not lend themselves to coloured indicators. For this reason, three-phase control panels will often use indicator lights of the old colours. [5]

Colours, fixed and flexible cable

Standard [lower-alpha 1] wire insulation colours
Flexible cable (e.g., extension, power, and lamp cords)
Region or countryPhasesNeutralProtective earth/ground
Argentina, European Union, South Africa (IEC 60446) Color wire brown.svg Color wire blue.svg Color wire green yellow.svg
Australia, New Zealand (AS/NZS 3000:2007 3.8.1, 3.8.3) Color wire brown.svg , Color wire red.svg (previously), "any colour other than green, yellow, green/yellow, black or light blue" Color wire light blue.svg , Color wire black.svg (previously) Color wire green yellow.svg ,
Color wire green.svg (previously)
Brazil (ABNT NBR 5410:2004 6.1.5) [6] Color wire black.svg , Color wire brown.svg , Color wire red.svg , Color wire orange.svg , Color wire grey.svg , Color wire white.svg

Any color may be used for flexible cable phases, excluding green and green/yellow striped.

Color wire yellow.svg For safety reasons, yellow should not be used when green/yellow striped cables are present.

Color wire light blue.svg Blue can be used for phases inside flexible cables when no neutral is present.

Color wire light blue.svg (light blue) Color wire green.svg , Color wire green yellow.svg
China (PRC) Color wire yellow.svg , Color wire green.svg , Color wire red.svg Color wire black.svg , Color wire blue.svg Color wire green yellow.svg
United States, Canada (120 V) Color wire black.svg
     metallic brass
Color wire white.svg
     metallic silver
Color wire green.svg , Color wire green yellow.svg ;
    ,      green/yellow striped
United States, Canada (split-phase 240 V) [7] Color wire black.svg , Color wire red.svg Color wire white.svg Color wire green.svg , Color wire green yellow.svg ;
    ,      green/yellow striped
Fixed cable (e.g., in-, on-, or behind-the-wall cables)
Region or countryPhasesNeutralProtective earth/ground
Argentina; China; European Union (IEC 60446) from April 2004; the United Kingdom from 31 March 2004 (BS 7671); Hong Kong from July 2007; Singapore from March 2009; Russia since 2009 (GOST R 50462); Ukraine, Belarus, Kazakhstan Color wire brown.svg , Color wire black.svg , Color wire grey.svg Color wire blue.svg Color wire green yellow.svg [lower-alpha 2]
India, Pakistan; United Kingdom, prior to 31 March 2004 (BS 7671); Hong Kong, prior to 2009; Malaysia and Singapore, prior to February 2011 Color wire red.svg , Color wire yellow.svg , Color wire blue.svg Color wire black.svg
  • Color wire green yellow.svg [lower-alpha 2]
  • Color wire green.svg (previously)
  • Color wire bare copper.svg no insulation (previously). Sleeved at the ends.
Australia, New Zealand (AS/NZS 3000:2018 3.8.1, table 3.4)
  • Color wire yellow.svg , Color wire light blue.svg , Color wire black.svg , Color wire green yellow.svg , Color wire green.svg
    To designate any Phase, the above colors are prohibited. While any other color is permitted, for single phase installations the "Line" color is usually Red and the "Switched Line" color is usually White [lower-alpha 3]
  • Color wire red.svg , Color wire brown.svg recommended for single phase
  • Color wire white.svg usually used for "Switched Line"
  • Color wire red.svg , Color wire white.svg , Color wire blue.svg recommended for multiphase
Color wire black.svg Color wire light blue.svg [lower-alpha 3] Color wire green yellow.svg (since about 1980 – Stranded Wire)
Color wire green.svg (since about 1966 – Stranded Wire)
Color wire bare copper.svg Stranded Wire – no insulation; sleeved at the ends (previously) [lower-alpha 4]
Brazil (ABNT NBR 5410:2004 6.1.5) Color wire black.svg , Color wire brown.svg , Color wire red.svg , Color wire orange.svg , Color wire grey.svg , Color wire white.svg

Any color may be used for fixed cable phases, excluding blue, green and green/yellow striped.

Color wire yellow.svg For safety reasons, yellow should not be used when green/yellow striped cables are present.

Color wire light blue.svg (light blue)

In installations where neutral also serves as protective ground, light blue wires with green/yellow striped terminal markings should be used.

Color wire green.svg , Color wire green yellow.svg
China (PRC) Color wire yellow.svg , Color wire green.svg , Color wire red.svg Color wire black.svg , Color wire blue.svg Color wire green yellow.svg
South Africa
  • Color wire red.svg , Color wire white.svg ; or
  • Color wire yellow.svg , Color wire blue.svg
Color wire black.svg Color wire green yellow.svg [lower-alpha 2]
United States [lower-alpha 5] Color wire black.svg , Color wire red.svg , Color wire blue.svg for 120, 208, or 240 V
Color wire brown.svg , Color wire orange.svg , Color wire yellow.svg for 277, or 480 V
     metallic brass
Color wire white.svg for 120, 208, or 240 V
Color wire grey.svg for 277, or 480 V
     metallic silver
Color wire green.svg     
Color wire bare copper.svg no insulation
Color wire green yellow.svg required for isolated systems
Canada [8] [lower-alpha 5] Color wire red.svg , Color wire black.svg for single-phase systems
Color wire red.svg , Color wire black.svg , Color wire blue.svg for three-phase systems
Color wire white.svg , Color wire grey.svg Color wire green.svg , Color wire green yellow.svg     
Color wire bare copper.svg no insulation
Color wire orange.svg , Color wire brown.svg for isolated single-phase systems
Color wire orange.svg , Color wire brown.svg , Color wire yellow.svg for isolated three-phase systems
Color wire green.svg , Color wire green yellow.svg for isolated systems
Boxes (e.g.,      translucent purple) denote markings on wiring terminals.
  1. The colours in this table represent the most common and preferred standard colours for wiring; however others may be in use, especially in older installations.
  2. 1 2 3 Cables may have an uninsulated PE[ clarification needed ] which is sleeved with the appropriate identifying colours at both ends, especially in the UK.
  3. 1 2 Except that in New Zealand domestic installations, the only permitted colour for Neutral is Black, Australian and New Zealand wiring standards allow both Australian and European colour codes. (However, TPS "Building Wire" to European colour codes is not generally available in Australia and New Zealand.) Australian-standard phase colours conflict with IEC 60446 colours, where IEC-60446 supported neutral colour (blue) is an allowed phase colour in the Australia/New Zealand standard. Care must be taken when determining the system used in any existing wiring.
  4. The protective earth conductor is now separately insulated throughout all cables.
  5. 1 2 Canadian and American wiring practices are very similar, with ongoing harmonisation efforts.

Wiring methods

Installing electrical wiring by "chasing" grooves into the masonry structure of the walls of a building Installing electrical wiring.jpg
Installing electrical wiring by "chasing" grooves into the masonry structure of the walls of a building

Materials for wiring interior electrical systems in buildings vary depending on:

Wiring systems in a single family home or duplex, for example, are simple, with relatively low power requirements, infrequent changes to the building structure and layout, usually with dry, moderate temperature and non-corrosive 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, corrosive, or wet or explosive atmospheres. In facilities that handle flammable gases or liquids, special rules may govern the installation and wiring of electrical equipment in hazardous areas.

Wires and cables are rated by the circuit voltage, temperature rating and environmental conditions (moisture, sunlight, oil, chemicals) in which they can be used. A wire or cable has a voltage (to neutral) rating and a maximum conductor surface temperature rating. The amount of current a cable or wire can safely carry depends on the installation conditions.

The international standard wire sizes are given in the IEC 60228 standard of the International Electrotechnical Commission. In North America, the American Wire Gauge standard for wire sizes is used.

Cables

Modern wiring materials

Modern non-metallic sheathed cables, such as (US and Canadian) Types NMB and NMC, consist of two to four wires covered with thermoplastic insulation, plus a bare wire for grounding (bonding), surrounded by a flexible plastic jacket. Some versions wrap the individual conductors in paper before the plastic jacket is applied.

Special versions of non-metallic sheathed cables, such as US Type UF, are designed for direct underground burial (often with separate mechanical protection) or exterior use where exposure to ultraviolet radiation (UV) is a possibility. These cables differ in having a moisture-resistant construction, lacking paper or other absorbent fillers, and being formulated for UV resistance.

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 10 AWG (or about 6 mm²) are stranded for flexibility during installation, but are not sufficiently pliable to use as appliance cord.

Cables for industrial, commercial and apartment buildings may contain many insulated conductors in an overall jacket, with helical tape steel or aluminium armour, or steel wire armour, 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. 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 building code to be either encased in metal conduit, or rated for low flame and smoke production.

Copper sheathed mineral insulated cables at a panel board Miccable.jpg
Copper sheathed mineral insulated cables at a panel board

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 and are more costly than non-fire rated cable. They have little flexibility and behave more like rigid conduit rather than flexible cables.

The environment of the installed wires determine how much current a cable is permitted to carry. 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 based on size of conductor, voltage potential, insulation type and thickness, and the temperature rating of the cable itself. The allowable current will also be different for wet or dry locations, for hot (attic) or cool (underground) locations. In a run of cable through several areas, the part with the lowest rating becomes the rating of the overall run.

Cables usually are secured with 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 armour of an armoured 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 intervals along their runs. In tall buildings, special designs are required to support the conductors of vertical runs of cable. Generally, only one cable per fitting is permitted, unless the fitting is rated or listed for multiple cables.

Special cable constructions and termination techniques are required for cables installed in ships. Such assemblies are subjected to environmental and mechanical extremes. Therefore, in addition to electrical and fire safety concerns, such cables may also be required to be pressure-resistant where they penetrate a vessel's bulkheads. They must also resist corrosion caused by salt water or salt spray, which is accomplished through the use of thicker, specially constructed jackets, and by tinning the individual wire stands.

US single-phase residential power distribution transformer, showing the two insulated "Line" conductors and the bare "Neutral" conductor (derived from the earthed center-tap of the transformer). The distribution supporting cantenaries are also shown. Power Transformer.jpg
US single-phase residential power distribution transformer, showing the two insulated "Line" conductors and the bare "Neutral" conductor (derived from the earthed center-tap of the transformer). The distribution supporting cantenaries are also shown.

In North American practice, an overhead cable from a transformer on a power pole to a residential electrical service usually consists of three twisted (triplexed) conductors, with one being a bare neutral conductor, with the other two being the insulated conductors for both of the two 180-degree out of phase 120 V line voltages normally supplied. [9] The neutral conductor is often a supporting "messenger" steel wire, which is used to support the insulated Line conductors.

Copper conductors

Electrical devices often contain copper conductors because of their multiple beneficial properties, including their high electrical conductivity, tensile strength, ductility, creep resistance, corrosion resistance, thermal conductivity, coefficient of thermal expansion, solderability, resistance to electrical overloads, compatibility with electrical insulators and ease of installation.

Despite competition from other materials, copper remains the preferred electrical conductor in nearly all categories of electrical wiring. [10] [11] For example, copper is used to conduct electricity in high, medium and low voltage power networks, including power generation, power transmission, power distribution, telecommunications, electronics circuitry, data processing, instrumentation, appliances, entertainment systems, motors, transformers, heavy industrial machinery and countless other types of electrical equipment. [12]

Aluminium conductors

Terminal blocks for joining aluminium and copper conductors. The terminal blocks may be mounted on a DIN rail. Transistive terminals for AL-CU junction.JPG
Terminal blocks for joining aluminium and copper conductors. The terminal blocks may be mounted on a DIN rail.

Aluminium wire was common in North American residential wiring from the late 1960s to mid-1970s due to the rising cost of copper. Because of its greater resistivity, aluminium wiring requires larger conductors than copper. For instance, instead of 14 AWG (American wire gauge) copper wire, aluminium wiring would need to be 12 AWG on a typical 15 ampere lighting circuit, though local building codes vary.

Solid aluminium conductors were originally made in the 1960s from a utility grade aluminium alloy that had undesirable properties for a building wire, and were used with wiring devices intended for copper conductors. [13] [14] These practices were found to cause defective connections and potential fire hazards. In the early 1970s new aluminium wire made from one of several special alloys was introduced, and all devices – breakers, switches, receptacles, splice connectors, wire nuts, etc. — were specially designed for the purpose. These newer aluminium wires and special designs address problems with junctions between dissimilar metals, oxidation on metal surfaces and mechanical effects that occur as different metals expand at different rates with increases in temperature.[ citation needed ]

Unlike copper, aluminium has a tendency to creep or cold-flow under pressure, so older plain steel screw clamped connections could become loose over time. Newer electrical devices designed for aluminium conductors have features intended to compensate for this effect. Unlike copper, aluminium forms an insulating oxide layer on the surface. This is sometimes addressed by coating aluminium conductors with an antioxidant paste (containing zinc dust in a low-residue polybutene base [15] ) at joints, or by applying a mechanical termination designed to break through the oxide layer during installation.

Some terminations on wiring devices designed only for copper wire would overheat under heavy current load and cause fires when used with aluminium conductors. Revised standards for wire materials and wiring devices (such as the CO/ALR "copper-aluminium-revised" designation) were developed to reduce these problems. While larger sizes are still used to feed power to electrical panels and large devices, aluminium wiring for residential use has acquired a poor reputation and has fallen out of favour.

Aluminium conductors are still heavily used for bulk power distribution and large feeder circuits with heavy current loads, due to the various advantages they offer over copper wiring. Aluminium conductors both cost and weigh less than copper conductors, so a much larger cross sectional area can be used for the same weight and price. This can compensate for the higher resistance and lower mechanical strength of aluminium, meaning the larger cross sectional area is needed to achieve comparable current capacity and other features. Aluminium conductors must be installed with compatible connectors and special care must be taken to ensure the contact surface does not oxidise.

Raceways and cable runs

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. Conduit firestop us.jpg
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 between electrical devices. This may be a specialised bendable pipe, called a conduit, or one of several varieties of metal (rigid steel or aluminium) or non-metallic (PVC or HDPE) tubing. Rectangular cross-section metal or PVC wire troughs (North America) or trunking (UK) 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, for example factory floors, may be run in cable trays or rectangular raceways having 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 safety-critical wiring must be kept operational during an accidental fire, fireproofing must be applied to maintain circuit integrity in a manner to comply with a product's certification listing. The nature and thickness of any passive fire protection materials used in conjunction with wiring and raceways has a quantifiable impact upon the ampacity derating, because the thermal insulation properties needed for fire resistance also inhibit air cooling of power conductors.

A cable tray can be used in stores and dwellings OrganizedElectricalWiring.jpg
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 labour 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.

Local electrical regulations may restrict or place special requirements on mixing of voltage levels within one cable tray. Good design practices may segregate, for example, low level measurement or signal cables from trays carrying high power branch circuits, to prevent induction of noise into sensitive circuits.

Since wires run in conduits or underground cannot dissipate heat as easily as in open air, and since adjacent circuits contribute induced currents, wiring regulations give rules to establish the current capacity (ampacity).

Special sealed fittings are used for wiring routed through 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. Conduit busduct.jpg
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 high currents in electrical apparatus, and for high currents distributed through a building, bus bars can be used. (The term "bus" is a contraction of the Latin omnibus – meaning "for all".) Each live conductor of such a system is a rigid piece of copper or aluminium, 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 switch yards 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 often pre-assembled with insulators in grounded enclosures. This assembly, known as bus duct or busway, 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 designated 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.

Busbars for distributing protective earth (ground) Busbars for earthing.JPG
Busbars for distributing protective earth (ground)

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.[ further explanation needed ]

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, since 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 a paper mill in Ontario, Canada Electrical panels st marys.jpg
Electrical panels, cables and firestops in an electrical service room at a paper mill in Ontario, Canada

Electrical panels are easily accessible junction boxes used to reroute and switch electrical services. The term is often used to refer to circuit breaker panels or fuseboxes. Local codes can specify physical clearance around the panels.

Degradation by pests

Rasberry crazy ants have been known to consume the insides of electrical wiring installations, preferring DC over AC currents. This behaviour is not well understood by scientists. [16]

Squirrels, rats, and other rodents may gnaw on unprotected wiring, causing fire and shock hazards. [17] [18] This is especially true of PVC-insulated telephone and computer network cables. Several techniques have been developed to deter these pests, including insulation loaded with pepper dust.

Early wiring methods

The 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, plus the high labour cost for such installations. The first Electrical codes arose in the 1880s with the commercial introduction of electrical power; however, many conflicting standards existed for the selection of wire sizes and other design rules for electrical installations, and a need was seen to introduce uniformity on the grounds of safety.

Knob and tube (US)

Knob-and-tube wiring (the orange cable is an unrelated extension cord) Splice-knob-corner.jpg
Knob-and-tube wiring (the orange cable is an unrelated extension cord)

The earliest standardized method of wiring in buildings, in common use in North America from about 1880 to the 1930s, was knob and tube (K&T) wiring: single conductors were run through cavities between the structural members in walls and ceilings, with ceramic tubes forming protective channels through joists and ceramic knobs attached to the structural members to provide air between the wire and the lumber and to support the wires. Since 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 knob-and-tube installations. However, the US code still allows new K&T wiring installations in special situations (some rural and industrial applications).

Metal-sheathed wires

Lead-cased electrical cable from a circa 1912 house in Southern England. Two conductors are sheathed in red and black rubber, the central earth wire is bare. These cables are dangerous because the sheath is prone to split if repeatedly flexed. Lead cased electrical wire from a circa 1912 house on Southern England.jpg
Lead-cased electrical cable from a circa 1912 house in Southern England. Two conductors are sheathed in red and black rubber, the central earth wire is bare. These cables are dangerous because the sheath is prone to split if repeatedly flexed.

In the United Kingdom, an early form of insulated cable, [19] 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 later invented in the UK in 1908 employed vulcanised-rubber insulated wire enclosed in a strip metal sheath. The metal sheath was bonded to each metal wiring device to ensure earthing continuity.

A system developed in Germany called "Kuhlo wire" used one, two, or three rubber-insulated wires in a brass or lead-coated iron sheet tube, with a crimped seam. The enclosure could also be used as a return conductor. Kuhlo wire could be run exposed on surfaces and painted, or embedded in plaster. Special outlet and junction boxes were made for lamps and switches, made either of porcelain or sheet steel. The crimped seam was not considered as watertight as the Stannos wire used in England, which had a soldered sheath. [20]

A somewhat similar system called "concentric wiring" was introduced in the United States around 1905. In this system, an insulated electrical wire was wrapped with copper tape which was then soldered, forming the grounded (return) conductor of the wiring system. The bare metal sheath, at earth potential, was considered safe to touch. While companies such as General Electric manufactured fittings for the system and a few buildings were wired with it, it was never adopted into the US National Electrical Code. Drawbacks of the system were that special fittings were required, and that any defect in the connection of the sheath would result in the sheath becoming energised. [21]

Other historical wiring methods

Armored cables with two rubber-insulated conductors in a flexible metal sheath were used as early as 1906, and were considered at the time a better method than open knob-and-tube wiring, although much more expensive.

The first rubber-insulated cables for US building wiring were introduced in 1922 with USpatent 1458803,Burley, Harry&Rooney, Henry,"Insulated electric wire",issued 1923-06-12, assigned to Boston Insulated Wire And Cable .[ citation needed ] These were two or more solid copper electrical wires with rubber insulation, plus woven cotton cloth over each conductor for protection of the insulation, with an overall woven jacket, usually impregnated with tar as a protection from moisture. Waxed paper was used as a filler and separator.

Over time, rubber-insulated cables become brittle because of exposure to atmospheric oxygen, so they must be handled with care and are usually replaced during renovations. When switches, socket outlets or light fixtures are replaced, the mere act of tightening connections may cause hardened insulation to flake off the conductors. Rubber insulation further inside the cable often is in better condition than the insulation exposed at connections, due to reduced exposure to oxygen.

The sulfur in vulcanized rubber insulation attacked bare copper wire so the conductors were tinned to prevent this. The conductors reverted to being bare when rubber ceased to be used.

Diagram of a simple electrical cable with three insulated conductors, with IEC colour scheme. Leitungsende Abisoliert en.svg
Diagram of a simple electrical cable with three insulated conductors, with IEC colour scheme.

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 (e.g. US Type THN, THHN, etc.) became common.[ citation needed ]

The simplest form of cable has two insulated conductors twisted together to form a unit. Such un-jacketed cables with two (or more) conductors are used only for extra low voltage signal and control applications such as doorbell wiring.

Other methods of securing wiring that are now obsolete include:

Metal moulding systems, with a flattened oval section consisting of a base strip and a snap-on cap channel, were more costly than open wiring or wooden moulding, but could be easily run on wall surfaces. Similar surface mounted raceway wiring systems are still available today.

See also

Related Research Articles

Electrical cable two or more wires running side by side and bonded, twisted, or braided together to form a single assembly

An electrical cable is an assembly of one or more wires running side by side or bundled, which is used to carry electric current.

Electrical wiring in North America follows regulations and standards for installation of building wiring which ultimately provides mains electricity.

National Electrical Code

The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment in the United States. It is part of the National Fire Code series published by the National Fire Protection Association (NFPA), a private trade association. Despite the use of the term "national", it is not a federal law. It is typically adopted by states and municipalities in an effort to standardize their enforcement of safe electrical practices. In some cases, the NEC is amended, altered and may even be rejected in lieu of regional regulations as voted on by local governing bodies.

As the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of grounding conductors and neutral conductors is carefully defined in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.

Electrical wiring in the United Kingdom is commonly understood to be an electrical installation for operation by end users within domestic, commercial, industrial, and other buildings, and also in special installations and locations, such as marinas or caravan parks. It does not normally cover the transmission of electrical power to them.

A power cable is an electrical cable, an assembly of one or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed.

Shielded cable electrical conductors enclosed by a conductive layer

A shielded cable or screened cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper, a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually this shield is covered with a jacket.

Ampacity is a portmanteau for ampere capacity defined by National Electrical Codes, in some North American countries. Ampacity is defined as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Also described as current-carrying capacity.

In an electrical installation, an earthing system (UK) or grounding system (USA) connects specific parts of that installation with the Earth's conductive surface for safety and functional purposes. The point of reference is the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary considerably among countries, though most follow the recommendations of the International Electrotechnical Commission. Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants.

IEC 60364Electrical Installations for Buildings is the International Electrotechnical Commission's international standard on electrical installations of buildings. This standard is an attempt to harmonize national wiring standards in an IEC standard and is published in the European Union by CENELEC as "HD 60364". The latest versions of many European wiring regulations follow the section structure of IEC 60364 very closely, but contain additional language to cater for historic national practice and to simplify field use and determination of compliance by electrical tradesmen and inspectors. National codes and site guides are meant to attain the common objectives of IEC 60364, and provide rules in a form that allows for guidance of persons installing and inspecting electrical systems.

Extra-low voltage (ELV) is an electricity supply voltage in a range which carries a low risk of dangerous electrical shock. There are various standards that define extra-low voltage. The International Electrotechnical Commission member organizations and the UK IET define an ELV device or circuit as one in which the electrical potential between conductor or electrical conductor and earth (ground) does not exceed 50 V a.c. or 120 V d.c.. EU's Low Voltage Directive applies from 50 to 1,000 V a.c. and from 75 to 1,500 V d.c.

Mineral-insulated copper-clad cable type of electrical cable

Mineral-insulated copper-clad cable is a variety of electrical cable made from copper conductors inside a copper sheath, insulated by inorganic magnesium oxide powder. The name is often abbreviated to MICC or MI cable, and colloquially known as pyro. A similar product sheathed with metals other than copper is called mineral insulated metal sheathed (MIMS) cable.

A thermoplastic-sheathed cable (TPS) consists of a toughened outer sheath of polyvinyl chloride (PVC) thermoplastic, covering one or more individual annealed copper conductors, themselves insulated with PVC. This type of wiring is commonly used for residential and light commercial construction in many countries. The flat version of the cable, with two insulated conductors and an uninsulated earth conductor, is referred to as twin and earth. In mainland Europe, a round equivalent is more common.

Knob-and-tube wiring Type of electrical wiring

Knob-and-tube wiring is an early standardized method of electrical wiring in buildings, in common use in North America from about 1880 to the early 1940s. It consisted of single-insulated copper conductors run within wall or ceiling cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators. Where conductors entered a wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth insulating sleeving called loom. The first insulation was asphalt-saturated cotton cloth, then rubber became common. Wire splices in such installations were twisted together for good mechanical strength, then soldered and wrapped with rubber insulating tape and friction tape, or made inside metal junction boxes.

An electrical code is a set of regulations for the design and installation of electrical wiring in a building. The intention of a code is to provide standards to ensure electrical wiring systems that are safe for people and property.

Pothead High-voltage electrical connection device

A pothead is a type of insulated electrical terminal used for transitioning between overhead line and underground high-voltage cable or for connecting overhead wiring to equipment like transformers. Its name comes from the process of potting or encapsulation of the conductors inside the terminal's insulating bushing.

Steel wire armoured cable, commonly abbreviated as SWA, is a hard-wearing power cable designed for the supply of mains electricity. It is one of a number of armoured electrical cables – which include 11 kV Cable and 33 kV Cable – and is found in underground systems, power networks and cable ducting.

High-voltage cable electric cable designed for use with high voltage (~ over 1 kV)

A high-voltage cable is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation, and is suitable for being run underground or underwater. This is in contrast to an overhead line, which does not have insulation. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, and alternating current (AC) and direct current (DC) power transmission. In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking. The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current. Cable joints and terminals must be designed to control the high-voltage stress to prevent breakdown of the insulation. Often a high-voltage cable will have a metallic shield layer over the insulation, connected to the ground and designed to equalize the dielectric stress on the insulation layer.

Copper conductor electrical wire or other conductor made of copper

Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor.

References

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  20. Croft
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  23. Croft, p. 143
  24. Croft, p. 136
  25. Croft, p. 137

Bibliography

Further reading