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Mains electricity, utility power, grid power, domestic power, wall power, household current, or, in some parts of Canada, hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to homes and businesses through the electrical grid in many parts of the world. People use this electricity to power everyday items (such as domestic appliances, televisions and lamps) by plugging them into a wall outlet.
The voltage and frequency of electric power differs between regions. In much of the world, a voltage (nominally) of 230 volts and frequency of 50 Hz is used. In North America, the most common combination is 120 V and a frequency of 60 Hz. Other combinations exist, for example, 230 V at 60 Hz. Travellers' portable appliances may be inoperative or damaged by foreign electrical supplies. Non-interchangeable plugs and sockets in different regions provide some protection from accidental use of appliances with incompatible voltage and frequency requirements.
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In the UK, mains electric power is generally referred to as "the mains". More than half of power in Canada is hydroelectricity, and mains electricity is often referred to as "hydro" in some regions of the country. This is also reflected in names of current and historical electricity utilities such as Hydro-Québec, BC Hydro, Manitoba Hydro, Hydro One (Ontario), and Newfoundland and Labrador Hydro.
All of these parameters vary among regions. The voltages are generally in the range 100–240 Volt (always expressed as root-mean-square voltage). The two commonly used frequencies are 50 Hertz and 60 Hz. Single-phase or three-phase power is most commonly used today, although two-phase systems were used early in the 20th century. Foreign enclaves, such as large industrial plants or overseas military bases, may have a different standard voltage or frequency from the surrounding areas. Some city areas may use standards different from that of the surrounding countryside (e.g. in Libya). Regions in an effective state of anarchy may have no central electrical authority, with electric power provided by incompatible private sources.
Many other combinations of voltage and utility frequency were formerly used, with frequencies between 25 Hz and 133 Hz and voltages from 100 V to 250 V. Direct current (DC) has been displaced by alternating current (AC) in public power systems, but DC was used especially in some city areas to the end of the 20th century. The modern combinations of 230 V/50 Hz and 120 V/60 Hz, listed in IEC 60038, did not apply in the first few decades of the 20th century and are still not universal. Industrial plants with three-phase power will have different, higher voltages installed for large equipment (and different sockets and plugs), but the common voltages listed here would still be found for lighting and portable equipment.
U.S. residential sector electricity consumption by major end uses in 2021[1], How is electricity used in U.S. homes?, US Energy Information Administration, 3 March 2022, (retrieved 11 November 2022)
| Space cooling | 15% | |
| Space heating | 14% | |
| Water heating | 12% | |
| Refrigeration | 6% | |
| Clothes dryers | 4% | |
| Lighting | 4% | |
| Televisions and related equipment1 | 4% | |
| Computers and related equipment2 | 2% | |
| Furnace fans and boiler circulation pumps | 2% | |
| Freezers | 1% | |
| Cooking | 1% | |
| Clothes washers3 | 1% | |
| Dishwashers3 | 1% | |
| Other uses4 | 34% | |
| Total consumption | 100% |
Electronic appliances such as computers or televisions sets typically use an AC to DC converter or AC adapter to power the device. This is often capable of operation with a wide range of voltage and with both common power frequencies. Other AC applications usually have much more restricted input ranges.
| + ! Standard ! Earth/Ground wire ! Neutral wire ! Live/Phase 1 wire ! Live/Phase 2 wire (Optional) ! Live/Phase 3 wire (Optional) | |||||
| UK & Europe | green/yellow | blue | brown | black | gray |
| UK (Old) | green/yellow | black | red | yellow | blue |
| United States | green | white | black | red | blue |
| Australia | green/yellow | black | red | white | blue |
Portable appliances use single-phase electric power, with two or three wired contacts at each outlet. Two wires (neutral and live/active/hot) carry current to operate the device.[2] Electrical Inspection Manual, 2011 Edition], Noel Williams & Jeffrey S Sargent, Jones & Bartlett Publishers, 2012, p. 249 (retrieved 3 March 2013 from Google Books)[3] 17th Edition IEE Wiring Regulations: Explained and Illustrated], Brian Scaddan, Routledge, 2011, p. 18 (retrieved 6 March 2013 from Google Books) A third wire, not always present, connects conductive parts of the appliance case to earth ground. This protects users from electric shock if live internal parts accidentally contact the case.
In northern and central Europe, residential electrical supply is commonly 400 V three-phase electric power, which gives 230 V between any single phase and neutral; house wiring may be a mix of three-phase and single-phase circuits, but three-phase residential use is rare in the UK. High-power appliances such as , water heaters and household power heavy tools like may be supplied from the 400 V three-phase power supply.
Small portable electrical equipment is connected to the power supply through flexible cables terminated in a plug, which is inserted into a fixed receptacle (socket). Larger household electrical equipment and industrial equipment may be permanently wired to the fixed wiring of the building. For example, in North American homes a window-mounted self-contained Air conditioning unit would be connected to a wall plug, whereas the central air conditioning for a whole home would be permanently wired. Larger plug and socket combinations are used for industrial equipment carrying larger currents, higher voltages, or three phase electric power.
The terms "ground wires" and "earth wires" are used interchangeably, depending on regional differences, for a wire that connects to the earth or some conducting body that serves in place of the earth to take the electrical discharge. Grounding wires are installed as a safety feature in order to route electricity into another safe body that is not electrically conductive like the air or ground in order to prevent Short circuit or electrocution.
The United States and Canada use a supply voltage of 120 volts ± 6%. Japan, Taiwan, Saudi Arabia, North America, Central America and some parts of northern South America use a voltage between 100 V and 127 V. However, most of the households in Japan equip split-phase electric power like the United States, which can supply 200 V by using reversed phase at the same time. Brazil is unusual in having both 127 V and 220 V systems at 60 Hz and also permitting interchangeable plugs and sockets. Saudi Arabia and Mexico have mixed voltage systems; in residential and light commercial buildings both countries use 127 volts, with 220 volts at 60 Hz in commercial and industrial applications. The Saudi government approved plans in August 2010 to transition the country to a totally 230/400-volt 60 Hz system.
Many areas, such as the US, which use (nominally) 120 V, make use of three-wire, split-phase 240 V systems to supply large appliances. In this system a 240 V supply has a centre-tapped neutral to give two 120 V supplies which can also supply 240 V to loads connected between the two line wires. Three-phase systems can be connected to give various combinations of voltage, suitable for use by different classes of equipment. Where both single-phase and three-phase loads are served by an electrical system, the system may be labelled with both voltages such as 120/208 or 230/400 V, to show the line-to-neutral voltage and the line-to-line voltage. Large loads are connected for the higher voltage. Other three-phase voltages, up to 830 volts, are occasionally used for special-purpose systems such as oil well pumps. Large industrial motors (say, more than 250 hp or 150 kW) may operate on medium voltage. On 60 Hz systems a standard for medium voltage equipment is 2,400/4,160 V whereas 3,300 V is the common standard for 50 Hz systems.
In 2000, Australia converted to 230 V as the nominal standard with a tolerance of +10%/−6%, this superseding the old 240 V standard, AS 2926-1987. The tolerance was increased in 2022 to ± 10% with the release of AS IEC 60038:2022. The utilization voltage available at an appliance may be below this range, due to voltage drops within the customer installation. As in the UK, 240 V is within the allowable limits and "240 volt" is a synonym for mains in Australian and British English.
In the United StatesANSI C84.1: American National Standard for Electric Power Systems and Equipment – Voltage Ratings (60 Hertz) , NEMA (costs $95 for access) and Canada,CSA Group CAN3-C235-83: Preferred Voltage Levels for AC Systems, 0 to 50,000 V national standards specify that the nominal voltage at the source should be 120 V and allow a range of 114 V to 126 V (RMS) (−5% to +5%). Historically, 110 V, 115 V and 117 V have been used at different times and places in North America. Mains power is sometimes spoken of as 110 V; however, 120 V is the nominal voltage.
In Japan, the electrical power supply to households is at 100 and 200 V. Eastern and northern parts of Honshu (including Tokyo) and Hokkaido have a frequency of 50 Hz, whereas western Honshū (including Nagoya, Osaka, and Hiroshima), Shikoku, Kyushu and Okinawa operate at 60 Hz. The boundary between the two regions contains four back-to-back high-voltage direct-current (HVDC) substations which interconnect the power between the two grid systems; these are Shin Shinano, Sakuma Dam, Minami-Fukumitsu, and the Higashi-Shimizu Frequency Converter. To accommodate the difference, frequency-sensitive appliances marketed in Japan can often be switched between the two frequencies.
The world's first large scale central plant—Thomas Edison's steam powered station at Holborn Viaduct in London—started operation in January 1882, providing direct current (DC) at 110 V. The Holborn Viaduct station was used as a proof of concept for the construction of the much larger Pearl Street Station in New York, the world's first permanent commercial central power plant. The Pearl Street Station also provided DC at 110 V, considered to be a "safe" voltage for consumers, beginning 4 September 1882.
AC systems started appearing in the US in the mid-1880s, using higher distribution voltage stepped down via to the same 110 V customer utilization voltage that Edison used. In 1883, Edison patented a three–wire distribution system to allow DC generation plants to serve a wider radius of customers to save on copper costs. By connecting two groups of 110 V lamps in series more load could be served by the same size conductors run with 220 V between them; a neutral conductor carried any imbalance of current between the two sub-circuits. AC circuits adopted the same form during the war of the currents, allowing lamps to be run at around 110 V and major appliances to be connected to 220 V. Nominal voltages gradually crept upward to 112 V and 115 V, or even 117 V. After World War II the standard voltage in the U.S. became 117 V, but many areas lagged behind even into the 1960s. In 1954, the American National Standards Institute (ANSI) published C84.1 "American National Standard for Electric Power Systems and Equipment – Voltage Ratings (60 Hertz)". This standard established 120 volt nominal system and two ranges for service voltage and utilization voltage variations. Today, virtually all American homes and businesses have access to 120 and 240 V at 60 Hz. Both voltages are available on the three wires (two "hot" legs of opposite phase and one "neutral" leg).
In 1899, the Berliner Elektrizitäts-Werke (BEW), a Berlin electrical utility, decided to greatly increase its distribution capacity by switching to 220 V nominal distribution, taking advantage of the higher voltage capability of newly developed metal filament lamps. The company was able to offset the cost of converting the customer's equipment by the resulting saving in distribution conductors cost. This became the model for electrical distribution in Germany and the rest of Europe and the 220 V system became common. North American practice remained with voltages near 110 V for lamps.Thomas P. Hughes, Networks of Power: Electrification in Western Society 1880–1930, The Johns Hopkins University Press, Baltimore 1983 p. 193
In the first decade after the introduction of alternating current in the US (from the early 1880s to about 1893) a variety of different frequencies were used, with each electric provider setting their own, so that no single one prevailed. The most common frequency was Hz. The rotation speed of induction generators and motors, the efficiency of transformers, and flickering of carbon arc lamps all played a role in frequency setting. Around 1893 the Westinghouse Electric Company in the United States and AEG in Germany decided to standardize their generation equipment on 60 Hz and 50 Hz respectively, eventually leading to most of the world being supplied at one of these two frequencies. Today most 60 Hz systems deliver nominal 120/240 V, and most 50 Hz nominally 230 V. The significant exceptions are in Brazil, which has a synchronized 60 Hz grid with both 127 V and 220 V as standard voltages in different regions, and Japan, which has two frequencies: 50 Hz for East Japan and 60 Hz for West Japan.
Power quality can also be affected by distortions of the current or voltage waveform in the form of of the fundamental (supply) frequency, or non-harmonic intermodulation such as that caused by electromagnetic interference. In contrast, harmonic distortion is usually caused by conditions of the load or generator. In multi-phase power, phase shift distortions caused by imbalanced loads can occur.
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