Type of lower-voltage electricity most commonly provided by utilities
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.
80–100%
60–80%
40–60%
20–40%
0–20%
Mains electricity or utility power, grid power, domestic power, and wall power, 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 230volts and frequency of 50Hz is used. In North America, the most common combination is 120V and a frequency of 60Hz. Other combinations exist, for example, 230V at 60Hz. 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.
Terminology
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, Hydro One (Ontario), and Newfoundland and Labrador Hydro.
Worldwide, many different mains power systems are found for the operation of household and light commercial electrical appliances and lighting. The different systems are primarily characterized by:
Protection against overcurrent damage (e.g., due to short circuit), electric shock, and fire hazards
Parameter tolerances.
All these parameters vary among regions. The voltages are generally in the range 100–240V (always expressed as root-mean-square voltage). The two commonly used frequencies are 50Hz and 60Hz. 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 25Hz and 133Hz and voltages from 100V to 250V. 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 230V/50Hz and 120V/60Hz, 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.
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[2]
End use
Petajoules (Terawatt-hours)
Share of total
Space cooling
850 (235)
15%
Space heating
750 (207)
14%
Water heating
630 (176)
12%
Refrigeration
310 (87)
6%
Clothes dryers
230 (64)
4%
Lighting
210 (59)
4%
Televisions and related equipment1
200 (56)
4%
Computers and related equipment2
130 (36)
2%
Furnace fans and boiler circulation pumps
86 (24)
2%
Freezers
72 (20)
1%
Cooking
58 (16)
1%
Clothes washers3
40 (11)
1%
Dishwashers3
29 (8)
1%
Other uses4
1,900 (520)
34%
Total consumption
5,470 (1,519)
100%
1 Includes televisions, set-top boxes, home theatre systems, DVD players, and video game consoles.
2 Includes desktop and laptop computers, monitors, and networking equipment.
3 Does not include water heating.
4 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.
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.[3][4] 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 400V three-phase electric power, which gives 230V 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 stoves, water heaters and household power heavy tools like log splitters may be supplied from the 400V 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 conditioner 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.
Circuit breakers and fuses are used to detect short circuits 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 devices, 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
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 230V. In the United Kingdom[5] the nominal supply voltage is 230V +10%/−6% to accommodate the fact that most transformers are in fact still set to 240V. The 230V standard has become widespread so that 230V 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 120volts ± 6%. Japan, Taiwan, Saudi Arabia, North America, Central America and some parts of northern South America use a voltage between 100V and 127V. However, most of the households in Japan equip split-phase electric power like the United States, which can supply 200V by using reversed phase at the same time. Brazil is unusual in having both 127V and 220V systems at 60Hz and also permitting interchangeable plugs and sockets.[6]Saudi Arabia and Mexico have mixed voltage systems; in residential and light commercial buildings both countries use 127volts, with 220volts at 60Hz in commercial and industrial applications. The Saudi government approved plans in August 2010 to transition the country to a totally 230/400-volt 60Hz system.[7]
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 208V supply system will be connected to motors with "200V" on their nameplates. This allows for the voltage drop between equipment and supply.[citation needed] 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.
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 230V distribution system will use less conductor material than a 120V 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 230V 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) 120V, make use of three-wire, split-phase 240V systems to supply large appliances. In this system a 240V supply has a centre-tapped neutral to give two 120V supplies which can also supply 240V 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 230/400V, 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 830volts, are occasionally used for special-purpose systems such as oil well pumps. Large industrial motors (say, more than 250hp or 150kW) may operate on medium voltage. On 60Hz systems a standard for medium voltage equipment is 2,400/4,160V whereas 3,300V is the common standard for 50Hz systems.
Standardization
Until 1987, mains voltage in large parts of Europe, including Germany, Austria and Switzerland, was 220±22V while the UK used 240±14.4V. Standard ISOIEC60038:1983 defined the new standard European voltage to be 230±23V. From 1987 onwards, a step-wise shift towards 230+13.8 −23V was implemented. From 2009 on, the voltage is permitted to be 230±23V.[8][9] No change in voltage was required by either the Central European or the UK system, as both 220V and 240V fall within the lower 230V tolerance bands (230V ±6%). Usually the voltage of 230V ±3% is maintained. Some areas of the UK still have 250volts for legacy reasons, but these also fall within the 10% tolerance band of 230volts. In practice, this allowed countries to have supplied the same voltage (220 or 240V), at least until existing supply transformers are replaced. Equipment (with the exception of filament bulbs) used in these countries is designed to accept any voltage within the specified range.
In 2000, Australia converted to 230V as the nominal standard with a tolerance of +10%/−6%,[10] this superseding the old 240V standard, AS2926-1987. The tolerance was increased in 2022 to ±10% with the release of ASIEC60038:2022.[11] The utilization voltage available at an appliance may be below this range, due to voltage drops within the customer installation. As in the UK, 240V is within the allowable limits and "240volt" is a synonym for mains in Australian and British English.
In the United States[12][13] and Canada,[14] national standards specify that the nominal voltage at the source should be 120V and allow a range of 114V to 126V (RMS) (−5% to +5%). Historically, 110V, 115V and 117V have been used at different times and places in North America.[citation needed] Mains power is sometimes spoken of as 110V; however, 120V is the nominal voltage.
In Japan, the electrical power supply to households is at 100 and 200V. Eastern and northern parts of Honshū (including Tokyo) and Hokkaidō have a frequency of 50Hz, whereas western Honshū (including Nagoya, Osaka, and Hiroshima), Shikoku, Kyūshū and Okinawa operate at 60Hz. 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.
A 50Hz ±5Hz vibrating-reed mains frequency meter for 220V
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, 250V for arc lamps, and 40V for incandescent lamps.[15]
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 110V.[16] 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 110V, considered to be a "safe" voltage for consumers, beginning 4 September 1882.[17]
AC systems started appearing in the US in the mid-1880s, using higher distribution voltage stepped down via transformers to the same 110V 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 110V lamps in series more load could be served by the same size conductors run with 220V 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 110V and major appliances to be connected to 220V. Nominal voltages gradually crept upward to 112V and 115V, or even 117V.[citation needed] After World War II the standard voltage in the U.S. became 117V, but many areas lagged behind even into the 1960s.[citation needed] 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 120volt nominal system and two ranges for service voltage and utilization voltage variations.[18] Today, virtually all American homes and businesses have access to 120 and 240V at 60Hz. 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 220V 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 220V system became common. North American practice remained with voltages near 110V for lamps.[19]
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 133+1⁄3Hz.[citation needed] 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 60Hz and 50Hz respectively, eventually leading to most of the world being supplied at one of these two frequencies. Today most 60Hz systems deliver nominal 120/240V, and most 50Hz nominally 230V. The significant exceptions are in Brazil, which has a synchronized 60Hz grid with both 127V and 220V as standard voltages in different regions,[20] and Japan, which has two frequencies: 50Hz for East Japan and 60Hz for West Japan.
To maintain the voltage at the customer's service within the acceptable range, electrical distribution utilities use regulating equipment at electrical substations 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 autotransformers, 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.
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 displays. Longer-term power outages, brownouts and blackouts and low reliability of supply generally increase costs to customers, who may have to invest in uninterruptible power supply or stand-by 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 harmonics of the fundamental (supply) frequency, or non-harmonic (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.
Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. The abbreviations AC and DC are often used to mean simply alternating and direct, respectively, as when they modify current or voltage.
Electric power distribution is the final stage in the delivery of electricity. Electricity is carried from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 33 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment and household appliances. Often several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a result, power supplies are sometimes referred to as electric power converters. Some power supplies are separate standalone pieces of equipment, while others are built into the load appliances that they power. Examples of the latter include power supplies found in desktop computers and consumer electronics devices. Other functions that power supplies may perform include limiting the current drawn by the load to safe levels, shutting off the current in the event of an electrical fault, power conditioning to prevent electronic noise or voltage surges on the input from reaching the load, power-factor correction, and storing energy so it can continue to power the load in the event of a temporary interruption in the source power.
In electrical engineering, single-phase electric power is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current electric motor does not produce a rotating magnetic field; single-phase motors need additional circuits for starting, and such motors are uncommon above 10 kW in rating.
A power cord, line cord, or mains cable is an electrical cable that temporarily connects an appliance to the mains electricity supply via a wall socket or extension cord. The terms are generally used for cables using a power plug to connect to a single-phase alternating current power source at the local line voltage. The terms power cable, mains lead, flex or kettle lead are also used. A lamp cord is a light-weight, ungrounded, single-insulated two-wire cord used for small loads such as a table or floor lamp.
In electrical engineering, power conversion is the process of converting electric energy from one form to another.
An adapter or adaptor is a device that converts attributes of one electrical device or system to those of an otherwise incompatible device or system. Some modify power or signal attributes, while others merely adapt the physical form of one connector to another.
The Electricity Supply Board is a state owned electricity company based in Ireland with operations worldwide. While historically a monopoly, the ESB now operates as a commercial semi-state concern in a "liberalised" and competitive market. It is a statutory corporation whose members are appointed by the government of Ireland.
AC power plugs and sockets connect devices to mains electricity to supply them with electrical power. A plug is the connector attached to an electrically-operated device, often via a cable. A socket is fixed in place, often on the internal walls of buildings, and is connected to an AC electrical circuit. Inserting the plug into the socket allows the device to draw power from this circuit.
IEC 60309 is a series of international standards from the International Electrotechnical Commission (IEC) for "plugs, socket-outlets and couplers for industrial purposes". They are also referred to as "pin & sleeve" connectors in North America or as "CeeForm" connectors in the entertainment industry. The maximum voltage allowed by the standard is 1000 V DC or AC; the maximum current, 800 A; and the maximum frequency, 500 Hz. The ambient temperature range is −25 °C to 40 °C.
Mains electricity by country includes a list of countries and territories, with the plugs, voltages and frequencies they commonly use for providing electrical power to low voltage appliances, equipment, and lighting typically found in homes and offices. Some countries have more than one voltage available. For example, in North America, a unique split-phase system is used to supply to most premises that works by center tapping a 240 volt transformer. This system is able to concurrently provide 240 volts and 120 volts. Consequently, this allows homeowners to wire up both 240 V and 120 V circuits as they wish. Most sockets are connected to 120 V for the use of small appliances and electronic devices, while larger appliances such as dryers, electric ovens, ranges and EV chargers use dedicated 240 V sockets. Different sockets are mandated for different voltage or maximum current levels.
Industrial and multiphase plugs and sockets provide a connection to the electrical mains rated at higher voltages and currents than household plugs and sockets. They are generally used in polyphase systems, with high currents, or when protection from environmental hazards is required. Industrial outlets may have weatherproof covers, waterproofing sleeves, or may be interlocked with a switch to prevent accidental disconnection of an energized plug. Some types of connectors are approved for hazardous areas such as coal mines or petrochemical plants, where flammable gas may be present.
A split-phase or single-phase three-wire system is a type of single-phase electric power distribution. It is the alternating current (AC) equivalent of the original Edison Machine Works three-wire direct-current system. Its primary advantage is that, for a given capacity of a distribution system, it saves conductor material over a single-ended single-phase system.
In electrical engineering, ground and neutral are circuit conductors used in alternating current (AC) electrical systems. The neutral conductor receives and returns alternating current to the supply during normal operation of the circuit; to limit the effects of leakage current from higher-voltage systems, the neutral conductor is often connected to earth ground at the point of supply. By contrast, a ground conductor is not intended to carry current for normal operation, but instead connects exposed metallic components to earth ground. A ground conductor only carries significant current if there is a circuit fault that would otherwise energize exposed conductive parts and present a shock hazard. In that case, circuit protection devices may detect a fault to a grounded metal enclosure and automatically de-energize the circuit, or may provide a warning of a ground fault.
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 or distribution of electricity to them.
An AC adapter or AC/DC adapter is a type of external power supply, often enclosed in a case similar to an AC plug. AC adapters deliver electric power to devices that lack internal components to draw voltage and power from mains power themselves. The internal circuitry of an external power supply is often very similar to the design that would be used for a built-in or internal supply.
AS/NZS 3112 is the harmonised/joint Australian and New Zealand Standard for AC power plugs (male) and sockets (female). The standard is used in Australia, New Zealand, Fiji, Tonga, Solomon Islands, Papua New Guinea and several other Pacific island countries. In Argentina and China an almost-identical design is also used, but these are separate standards, despite their appearance. The International Electrotechnical Commission (IEC) "world plugs" website defines this design as Plug Type I'.
A voltage converter is an electric power converter which changes the voltage of an electrical power source. It may be combined with other components to create a power supply.
NEMA connectors are power plugs and sockets used for AC mains electricity in North America and other countries that use the standards set by the US National Electrical Manufacturers Association. NEMA wiring devices are made in current ratings from 15 to 60 amperes (A), with voltage ratings from 125 to 600 volts (V). Different combinations of contact blade widths, shapes, orientations, and dimensions create non-interchangeable connectors that are unique for each combination of voltage, electric current carrying capacity, and grounding system.
In electrical safety testing, portable appliance testing is a process by which electrical appliances are routinely checked for safety, commonly used in the United Kingdom, Ireland, New Zealand and Australia. The formal term for the process is "in-service inspection & testing of electrical equipment". Testing involves a visual inspection of the equipment and verification that power cables are in good condition. Additionally, other tests may be done when required, such as a verification of earthing (grounding) continuity, a test of the soundness of insulation between the current-carrying parts, and a check for any exposed metal that could be touched. The formal limits for a pass/fail of these electrical tests vary somewhat depending on the category of equipment being tested.
↑ Electrical Inspection Manual, 2011 Edition], Noel Williams & Jeffrey S Sargent, Jones & Bartlett Publishers, 2012, p. 249 (retrieved 3 March 2013 from Google Books)
↑ 17th Edition IEE Wiring Regulations: Explained and Illustrated], Brian Scaddan, Routledge, 2011, p. 18 (retrieved 6 March 2013 from Google Books)
↑ "Voltage Tolerance Boundary". Power Quality Blog. Pacific Gas and Electric Company (PG&E). January 1999. Retrieved 7 August 2022.
↑ Thomas P. Hughes, Networks of Power: Electrification in Western Society 1880–1930, The Johns Hopkins University Press, Baltimore 1983 ISBN0-8018-2873-2 p. 193
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