View source for Mains electricity

{{short description|Type of lower-voltage electricity most commonly provided by utilities}}
{{Use dmy dates|date=August 2021}}
{{use British English|date=April 2014}}
[[File:Access to Electricity.svg|thumb|upright=1.4|World map showing the percentage of the population in each country with access to mains electricity (as of 2017), a measure of the extent of [[electrification]].<ref name="el-access">{{cite web |title=Access to electricity (% of population) |url=https://data.worldbank.org/indicator/eg.Elc.Accs.Zs |website=Data |publisher=The World Bank |access-date=5 October 2019}}</ref>
{{legend|#005ce6|80–100%}}
{{legend|#1ba87c|60–80%}}
{{legend|#7bed00|40–60%}}
{{legend|#f0b411|20–40%}}
{{legend|#c2523c|0–20%}}]]
'''Mains electricity'''  or '''utility power''', '''power grid''', '''domestic power''', and '''wall power''', or, in some parts of Canada, '''hydro''', is a general-purpose [[Alternating current|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 [[utility frequency|frequency]] of electric power differs between regions. In much of the world, a voltage (nominally) of 230 volts and frequency of 50&nbsp;Hz is used. In North America, the most common combination is 120&nbsp;V and a frequency of 60&nbsp;Hz. Other combinations exist, for example, 230&nbsp;V at 60&nbsp;Hz. Travellers' portable appliances may be inoperative or damaged by foreign electrical supplies. Non-interchangeable [[AC power plugs and sockets|plugs and sockets]] in different regions provide some protection from accidental use of appliances with incompatible voltage and frequency requirements.

== Terminology ==
[[File:Mains powered electric Lamp.JPG|thumb|A table lamp connected to a wall socket (the mains)]]
In the US, mains electric power is referred to by several names including "utility power", "household power", "household electricity", "house current", "powerline", "domestic power", "wall power", "line power", "wall current", "AC power", "city power", "street power", and "120 (one twenty)".

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]], and [[Hydro One]] (one of the entities resulting from the breakup of [[Ontario Hydro]]). Although there is an entity named [[Newfoundland and Labrador Hydro]], that company is primarily a hydroelectricity generator, though it also directly serves customers in Labrador and some other parts of [[Newfoundland and Labrador|the province]]. The majority of customers on the [[Newfoundland (island)|island of Newfoundland]] deal with [[Newfoundland Power]] as their service provider, and thus electricity is more commonly called simply "power" in that province.<ref>{{cite web|url=https://www.gov.nl.ca/iet/energy/electricity/|title=Industry, Energy and Technology: Electricity|author=[[Government of Newfoundland and Labrador]]|access-date=October 3, 2022}}</ref>

== Power systems ==
{{for|a list of voltages, frequencies, and wall plugs by country|Mains electricity by country}}

Worldwide, many different [[Mains electricity by country|mains power systems]] are found for the operation of household and light commercial electrical appliances and lighting. The different systems are primarily characterized by:
* [[Voltage]]
* [[Utility frequency|Frequency]]
* [[AC power plugs and sockets|Plugs and sockets]] (receptacles or outlets)
* [[Earthing system]] (grounding)
* Protection against [[overcurrent]] damage (e.g., due to short circuit), [[Electrical injury|electric shock]], and fire hazards
* Parameter tolerances.

All these parameters vary among regions. The voltages are generally in the range 100–240 [[Volt|V]] (always expressed as [[root-mean-square]] voltage). The two commonly used frequencies are 50&nbsp;[[Hertz|Hz]] and 60&nbsp;Hz. [[Single-phase electric power|Single-phase]] or [[Three-phase electric power|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&nbsp;Hz and 133&nbsp;Hz and voltages from 100&nbsp;V to 250&nbsp;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&nbsp;V/50&nbsp;Hz and 120&nbsp;V/60&nbsp;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 electric power|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.

== Common uses of electricity ==
Electricity is used for lighting, heating, cooling, electric motors and electronic equipment.  The [[US Energy Information Administration]] (EIA) has published:

'''U.S. residential sector electricity consumption by major end uses in 2021'''<ref>[https://www.eia.gov/tools/faqs/faq.php?id=96&t=3], How is electricity used in U.S. homes?, [[US Energy Information Administration]], 3 March 2022, (retrieved 11 November 2022)</ref>
{| class="wikitable sortable"
|-
! End use
! Petajoules<br>(Terawatt-hours)
! Share of<br>total
|-
| Space cooling
| {{convert|235|TW.h|abbr=values|order=flip}}
| 15%
|-
| Space heating
| {{convert|207|TW.h|abbr=values|order=flip}}
| 14%
|-
| Water heating
| {{convert|176|TW.h|abbr=values|order=flip}}
| 12%
|-
| Refrigeration
| {{convert|87|TW.h|abbr=values|order=flip}}
| 6%
|-
| Clothes dryers
| {{convert|64|TW.h|abbr=values|order=flip}}
| 4%
|-
| Lighting
| {{convert|59|TW.h|abbr=values|order=flip}}
| 4%
|-
| Televisions and related equipment<sup>1</sup>
| {{convert|56|TW.h|abbr=values|order=flip}}
| 4%
|-
| Computers and related equipment<sup>2</sup>
| {{convert|36|TW.h|abbr=values|order=flip}}
| 2%
|-
| Furnace fans and boiler circulation pumps
| {{convert|24|TW.h|abbr=values|order=flip}}
| 2%
|-
| Freezers
| {{convert|20|TW.h|abbr=values|order=flip}}
| 1%
|-
| Cooking
| {{convert|16|TW.h|abbr=values|order=flip}}
| 1%
|-
| Clothes washers<sup>3</sup>
| {{convert|11|TW.h|abbr=values|order=flip}}
| 1%
|-
| Dishwashers<sup>3</sup>
| {{convert|8|TW.h|abbr=values|order=flip}}
| 1%
|-
| Other uses<sup>4</sup>
| {{convert|520|TW.h|abbr=values|order=flip}}
| 34%
|-
| Total consumption
| {{convert|1519|TW.h|abbr=values|order=flip}}
| 100%
|}
:<sup>1</sup> Includes televisions, set-top boxes, home theatre systems, DVD players, and video game consoles.
:<sup>2</sup> Includes desktop and laptop computers, monitors, and networking equipment.
:<sup>3</sup> Does not include water heating.
:<sup>4</sup> Includes small electric devices, heating elements, exterior lights, outdoor grills, pool and spa heaters, backup electricity generators, and motors not listed above. Does not include electric vehicle charging.

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.

== Building wiring ==
{{see also|Electrical wiring}}
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.<ref>[https://books.google.com/books?id=83v7hptwljkC&dq=live+parts+energized+conductive+components&pg=PA29]''Electrical Inspection Manual, 2011 Edition''], Noel Williams & Jeffrey S Sargent, Jones & Bartlett Publishers, 2012, p. 249 (retrieved 3 March 2013 from Google Books)</ref><ref>[https://books.google.com/books?id=4kugySmaTeAC&dq=%22live+part%22&pg=PA44]''17th Edition IEE Wiring Regulations: Explained and Illustrated''], Brian Scaddan, Routledge, 2011, p. 18 (retrieved 6 March 2013 from Google Books)</ref> 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.
{{further|Ground and neutral}}

In northern and central Europe, residential electrical supply is commonly 400&nbsp;V three-phase electric power, which gives 230&nbsp;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 [[kitchen stove]]s, water heaters and household power heavy tools like [[log splitter]]s may be supplied from the 400&nbsp;V three-phase power supply.

Small portable electrical equipment is connected to the power supply through flexible cables terminated in a [[Domestic AC power plugs and sockets|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|air conditioner]] unit would be connected to a wall plug, whereas the central air conditioning for a whole home would be permanently wired. Larger [[Industrial and multiphase power plugs and sockets|plug and socket combinations]] are used for industrial equipment carrying larger currents, higher voltages, or three phase electric power.

[[Circuit breaker]]s and [[fuse (electrical)|fuse]]s are used to detect [[short circuit]]s between the line and neutral or ground wires or the drawing of more current than the wires are rated to handle (overload protection) to prevent overheating and possible fire. These protective devices are usually mounted in a central panel—most commonly a distribution board or consumer unit—in a building, but some wiring systems also provide a protection device at the socket or within the plug. [[Residual-current device]]s, also known as ground-fault circuit interrupters and appliance leakage current interrupters, are used to detect ''ground faults''—flow of current in other than the neutral and line wires (like the ground wire or a person). When a ground fault is detected, the device quickly cuts off the circuit.

== Voltage levels ==
[[File:World Map of Mains Voltages and Frequencies, Detailed.svg|thumb|World map of mains voltages and frequencies, simplified to country level]]
Most of the world population ([[Europe]], [[Africa]], [[Asia]], [[Australia]], [[New Zealand]], and much of [[South America]]) use a supply that is within 6% of 230&nbsp;V. In the [[United Kingdom]] <ref>{{cite web|url=http://www.powerlogic.com.au/Attachments/Voltage%20and%20Equipment%20Standard%20Misalignment%20PaperV.pdf|title=Voltage and Equipment Standard Misalignment|first1=Chris|last1=Halliday|first2=Dave|last2=Urquhart|website=powerlogic.com|access-date=14 March 2014|archive-url=https://web.archive.org/web/20180311234438/http://powerlogic.com.au/Attachments/Voltage%20and%20Equipment%20Standard%20Misalignment%20PaperV.pdf|archive-date=11 March 2018|url-status=dead}}</ref> the nominal supply voltage is 230&nbsp;V +10%/−6% to accommodate the fact that most transformers are in fact still set to 240&nbsp;V. The 230&nbsp;V standard has become widespread so that 230&nbsp;V equipment can be used in most parts of the world with the aid of an [[adapter]] or a change to the equipment's plug to the standard for the specific country.
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&nbsp;V and 127&nbsp;V. However, most of the households in Japan equip [[split-phase electric power]] like the United States, which can supply 200&nbsp;V by using reversed phase at the same time. [[Brazil]] is unusual in having both 127&nbsp;V and 220&nbsp;V systems at 60&nbsp;Hz and also permitting interchangeable plugs and sockets.<ref>{{Cite web|title=Power plug, socket & mains voltage in Brazil|url=https://www.worldstandards.eu/electricity/plug-voltage-by-country/brazil/|access-date=2020-11-27|website=WorldStandards|language=en-GB}}</ref> [[Saudi Arabia]] and [[Mexico]] have mixed voltage systems; in residential and light commercial buildings both countries use 127 volts, with 220 volts at 60&nbsp;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&nbsp;Hz system,<ref>{{Cite web|title=Voltage in Saudi Arabia - Electricity Supply and Power Quality Overview|url=https://www.sinalda.com/world-voltages/middle-east/voltage-saudi-arabia/|access-date=2020-11-27|website=Sinalda|language=en-GB}}</ref> but Mexico has no plans to transition.

=== Measuring voltage ===
A distinction should be made between the voltage at the point of supply (nominal voltage at the point of interconnection between the electrical utility and the user) and the voltage rating of the equipment (utilization or load voltage). Typically the utilization voltage is 3% to 5% lower than the nominal system voltage; for example, a nominal 208&nbsp;V supply system will be connected to motors with "200&nbsp;V" on their nameplates. This allows for the [[voltage drop]] between equipment and supply.{{citation needed|date=December 2019}} Voltages in this article are the nominal supply voltages and equipment used on these systems will carry slightly lower nameplate voltages. Power distribution system voltage is nearly sinusoidal in nature. Voltages are expressed as [[root mean square]] (RMS) voltage. Voltage tolerances are for steady-state operation. Momentary heavy loads, or switching operations in the power distribution network, may cause short-term deviations out of the tolerance band and storms and other unusual conditions may cause even larger transient variations. In general, power supplies derived from large networks with many sources are more stable than those supplied to an isolated community with perhaps only a single generator.

=== Choice of voltage ===
{{main|Mains electricity by country}}
The choice of supply voltage is due more to historical reasons than optimization of the electric power distribution system—once a voltage is in use and equipment using this voltage is widespread, changing voltage is a drastic and expensive measure. A 230&nbsp;V distribution system will use less conductor material than a 120&nbsp;V system to deliver a given amount of power because the current, and consequently the resistive loss, is lower. While large heating appliances can use smaller conductors at 230&nbsp;V for the same output rating, few household appliances use anything like the full capacity of the outlet to which they are connected. Minimum wire size for hand-held or portable equipment is usually restricted by the mechanical strength of the conductors.

Many areas, such as the US, which use (nominally) 120&nbsp;V, make use of [[Split-phase electric power|three-wire, split-phase]] 240&nbsp;V systems to supply large appliances. In this system a 240&nbsp;V supply has a centre-tapped neutral to give two 120&nbsp;V supplies which can also supply 240&nbsp;V to loads connected between the two line wires. [[Three-phase electric power|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&nbsp;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&nbsp;hp or 150&nbsp;kW) may operate on medium voltage. On 60&nbsp;Hz systems a standard for medium voltage equipment is 2,400/4,160&nbsp;V whereas 3,300&nbsp;V is the common standard for 50&nbsp;Hz systems.

=== Standardization ===
Until 1987, mains voltage in large parts of Europe, including Germany, Austria and Switzerland, was {{val|220|22|u=V}} while the UK used {{val|240|14.4|u=V}}. Standard ISO [[IEC 60038]]:1983 defined the new standard European voltage to be {{val|230|23|u=V}}. From 1987 onwards, a step-wise shift towards {{val|230|+13.8|-23|u=V}} was implemented. From 2009 on, the voltage is permitted to be {{val|230|23|u=V}}.<!-- translation from the German article, see there for additional sources to carry over --><ref>CENELEC Harmonisation Document HD 472 S1:1988</ref><ref>British Standard BS 7697: Nominal voltages for low voltage public electricity supply systems – (Implementation of HD 472 S1)</ref> No change in voltage was required by either the Central European or the UK system, as both 220&thinsp;V and 240&thinsp;V fall within the lower 230&nbsp;V tolerance bands (230&nbsp;V ±6%). Usually the voltage of 230V ±3% is maintained. Some areas of the UK still have 250&nbsp;volts for legacy reasons, but these also fall within the 10% tolerance band of 230&nbsp;volts. In practice, this allowed countries to have supplied the same voltage (220 or 240&thinsp;V), at least until existing supply transformers are replaced. Equipment (with the exception of [[Incandescent light bulb|filament bulbs]]) used in these countries is designed to accept any voltage within the specified range.

In 2000, [[Australia]] converted to 230&nbsp;V as the nominal standard with a tolerance of +10%/−6%,<ref>{{cite book |author1=Hossain, J. |url=https://books.google.com/books?id=tI_EBAAAQBAJ&q=as60038&pg=PA71 |title=Renewable Energy Integration: Challenges and Solutions |author2=Mahmud, A. |date=29 January 2014 |publisher=Springer |isbn=978-9814585279 |page=71 |access-date=13 January 2018}}</ref> this superseding the old 240&nbsp;V standard, AS&nbsp;2926-1987. The tolerance was increased in 2022 to ±&nbsp;10% with the release of AS&nbsp;IEC&nbsp;60038:2022<ref>{{Cite web |title=Electrical Contractor Update - Issue 04 - February 2023 |url=https://us5.campaign-archive.com/?u=69932807d57ffb9c816b93a84&id=866a5eb399 |access-date=2024-03-04 |website=us5.campaign-archive.com}}</ref>. 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&nbsp;V is within the allowable limits and "240 volt" is a synonym for mains in [[Australian English|Australian]] and [[British English]].

In the United States<ref>[[ANSI]] C84.1: [http://www.nema.org/stds/c84-1.cfm American National Standard for Electric Power Systems and Equipment – Voltage Ratings (60 Hertz)] {{webarchive|url=https://web.archive.org/web/20070727234316/http://www.nema.org/stds/c84-1.cfm |date=27 July 2007 }}, NEMA (costs $95 for access)</ref><ref name="PG&E_1999">{{cite web |url=https://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_tolerance.pdf |title=Voltage Tolerance Boundary |publisher=PG&E |date=1999-01-01 |url-status=live |archive-url=https://web.archive.org/web/20191110020717/https://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_tolerance.pdf |archive-date=2019-11-10 |access-date=2019-11-22 }}</ref> and Canada,<ref>[[Canadian Standards Association|CSA]] CAN3-C235-83: Preferred Voltage Levels for AC Systems, 0 to 50&thinsp;000&nbsp;V</ref> national standards specify that the nominal voltage at the source should be 120&nbsp;V and allow a range of 114&nbsp;V to 126&nbsp;V ([[Root mean square|RMS]]) (−5% to +5%). Historically, 110&nbsp;V, 115&nbsp;V and 117&nbsp;V have been used at different times and places in North America.{{citation needed|date=November 2022}} Mains power is sometimes spoken of as 110&nbsp;V; however, 120&nbsp;V is the nominal voltage.

In [[Japan]], the electrical power supply to households is at 100 and 200&nbsp;V. Eastern and northern parts of [[Honshu|Honshū]] (including [[Tokyo]]) and [[Hokkaido|Hokkaidō]] have a frequency of 50&nbsp;Hz, whereas western Honshū (including Nagoya, Osaka, and Hiroshima), [[Shikoku]], [[Kyushu|Kyūshū]] and [[Okinawa (city)|Okinawa]] operate at 60&nbsp;Hz. The boundary between the two regions contains four back-to-back [[High-voltage direct current|high-voltage direct-current]] (HVDC) substations which interconnect the power between the two grid systems; these are [[Shin-Shinano Frequency Converter|Shin Shinano]], [[Sakuma Dam#HVDC frequency converter|Sakuma Dam]], [[Minami-Fukumitsu Frequency Converter|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.

== History ==
{{main|Electrification}}
[[File:Czestosciomierz-49.9Hz.jpg|thumb|right|310px|A 50&nbsp;Hz ±5&nbsp;Hz vibrating-reed mains frequency meter for 220&nbsp;V]]

The world's first public electricity supply was a [[water wheel]] driven system constructed in the small English town of [[Godalming]] in 1881.  It was an [[alternating current]] (AC) system using a Siemens alternator supplying power for both street lights and consumers at two voltages, 250&nbsp;V for arc lamps, and 40&nbsp;V for incandescent lamps.<ref>{{cite web |url=http://www.exploringsurreyspast.org.uk/themes/places/surrey/waverley/godalming/godalming_electricity/ |title=Godalming: Electricity |website= Exploring Surrey's Past |publisher=Surrey County Council |access-date= 6 December 2017}}</ref>

The world's first large scale central plant—[[Thomas Edison]]'s [[Holborn Viaduct power station|steam powered station at Holborn Viaduct]] in London—started operation in January 1882, providing [[direct current]] (DC) at 110&nbsp;V.<ref>{{citation |url=https://ntmm.org/~nt/elecpow/history/electricity_supply_in_the_uk__a_chronology_ocrnopic.pdf |publisher=The [[Electricity Council]] |title=Electricity Supply in the United Kingdom |date=1987 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20170401145847/https://ntmm.org/~nt/elecpow/history/electricity_supply_in_the_uk__a_chronology_ocrnopic.pdf |archive-date=1 April 2017 }}</ref> The Holborn Viaduct station was used as a [[proof of concept]] for the construction of the much larger [[Pearl Street Station]] in [[New York City|New York]], the world's first permanent commercial central power plant. The Pearl Street Station also provided DC at 110&nbsp;V, considered to be a "safe" voltage for consumers, beginning 4 September 1882.<ref>{{cite web |url=http://ethw.org/Milestones:Pearl_Street_Station,_1882 |title=Milestones:Pearl Street Station, 1882 |website= Engineering and Technology History Wiki |publisher=United Engineering Foundation  |access-date= 6 December 2017}}</ref>

AC systems started appearing in the US in the mid-1880s, using higher distribution voltage stepped down via [[transformer]]s to the same 110&nbsp;V customer utilization voltage that Edison used. In 1883 Edison patented a [[Split-phase electric power|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&nbsp;V lamps in series more load could be served by the same size conductors run with 220&nbsp;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&nbsp;V and major appliances to be connected to 220&nbsp;V.  Nominal voltages gradually crept upward to 112&nbsp;V and 115&nbsp;V, or even 117&nbsp;V.{{Citation needed|date=December 2017}} After [[World War II]] the standard voltage in the U.S. became 117&nbsp;V, but many areas lagged behind even into the 1960s.{{Citation needed|date=December 2017}} 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.<ref>{{cite web |url= https://powerquality.blog/2021/01/15/voltage-tolerance-boundary/ |title= Voltage Tolerance Boundary|website= Power Quality Blog |publisher= Pacific Gas and Electric Company (PG&E)|date= January 1999|access-date= 2022-08-07}}</ref> Today, virtually all American homes and businesses have access to 120 and 240&nbsp;V at 60&nbsp;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&nbsp;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&nbsp;V system became common. North American practice remained with voltages near 110&nbsp;V for lamps.<ref>[[Thomas P. Hughes]], ''Networks of Power: Electrification in Western Society 1880–1930'', The Johns Hopkins University Press, Baltimore 1983 {{ISBN|0-8018-2873-2}} p. 193</ref>

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 133⅓&nbsp;Hz.{{Citation needed|date=December 2017}} 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 Corporation|Westinghouse Electric Company]] in the United States and [[AEG (German company)|AEG]] in Germany decided to standardize their generation equipment on 60&nbsp;Hz and 50&nbsp;Hz respectively, eventually leading to most of the world being supplied at one of these two frequencies. Today most 60&nbsp;Hz systems deliver nominal 120/240&nbsp;V, and most 50&nbsp;Hz nominally 230&nbsp;V. The significant exceptions are in Brazil, which has a synchronized 60&nbsp;Hz grid with both 127&nbsp;V and 220&nbsp;V as standard voltages in different regions,<ref>{{Cite web|title=Power plug, socket & mains voltage in Brazil|url=https://www.worldstandards.eu/electricity/plug-voltage-by-country/brazil/|access-date=2020-11-27|website=WorldStandards|language=en-GB}}</ref> and [[Electricity sector in Japan#Transmission|Japan, which has two frequencies]]: 50&nbsp;Hz for East Japan and 60&nbsp;Hz for West Japan.
{{see also|Utility frequency}}

== Voltage regulation ==
To maintain the voltage at the customer's service within the acceptable range, electrical distribution utilities use [[Voltage regulator|regulating equipment]] at [[electrical substation]]s or along the distribution line. At a substation, the step-down [[transformer]] will have an automatic on-load tap changer, allowing the ratio between transmission voltage and distribution voltage to be adjusted in steps. For long (several kilometres) rural distribution circuits, automatic voltage regulators may be mounted on poles of the distribution line. These are [[autotransformer]]s, again, with on-load tap changers to adjust the ratio depending on the observed voltage changes. At each customer's service, the step-down transformer has up to five taps to allow some range of adjustment, usually ±5% of the nominal voltage. Since these taps are not automatically controlled, they are used only to adjust the long-term average voltage at the service and not to regulate the voltage seen by the utility customer.

== Power quality ==
{{main|Power quality}}
The stability of the voltage and frequency supplied to customers varies among countries and regions. "Power quality" is a term describing the degree of deviation from the nominal supply voltage and frequency. Short-term surges and drop-outs affect sensitive electronic equipment such as computers and [[flat-panel display]]s. Longer-term power outages, [[brownout (electricity)|brownout]]s and [[power outage|blackout]]s and low reliability of supply generally increase costs to customers, who may have to invest in [[uninterruptible power supply]] or stand-by [[engine-generator|generator]] sets to provide power when the utility supply is unavailable or unusable. Erratic power supply may be a severe economic handicap to businesses and public services which rely on electrical machinery, illumination, climate control and computers. Even the best quality power system may have breakdowns or require servicing. As such, companies, governments and other organizations sometimes have backup generators at sensitive facilities, to ensure that power will be available even in the event of a power outage or black out.

Power quality can also be affected by distortions of the current or voltage waveform in the form of [[harmonic]]s of the fundamental (supply) frequency, or non-harmonic [[intermodulation|(inter)modulation distortion]] 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.

== See also ==
{{portal|Energy}}
* [[Electricity meter]]
* [[Maximum demand indicator]]

== References ==
{{reflist}}

[[Category:Electric power]]