Mains electricity
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Image:F plug.jpg The term "mains" usually refers to the general purpose AC electrical power supply (as in "I've connected the appliance to the mains"). The term is not usually used in the United States and Canada.
In the United States, mains power has a variety of names. It is often called "household power", "household electricity", "domestic power", "wall power", "line power", "AC power", or "grid power".
In Canada, any of the American terms for mains power can be used, but it is usually called "hydro" because much of the Canadian generating infrastructure is hydroelectric.
See also List of countries with mains power plugs, voltages and frequencies.
All European and most African and Asian countries use a supply that is within 10% of 230 V, whereas Japan and most of the Americas use a supply between 100 and 127 V.
Following voltage harmonisation co-ordinated with CENELEC countries, all electricity supply within the EU is now nominally 230 V ± 10% (though some countries have stricter specifications: for example, the UK specifies 230 V +10% −6%). In practice this means that countries such as the UK that previously supplied 240 V continue to do so, and those that previously supplied 220 V continue to do so. However equipment should be designed to accept any voltages within the specified range, and in practice most do so. Similarly, Australia has converted to 230 V as the nominal standard, and like the UK, 240 V is within the allowable tolerance. "240 volt" spoken as "two forty volt" remains a synonym for mains in Australian and British English.
ANSI standard C84.1 and Canadian standard CAN3-C235 specify that the nominal voltage at the output should be 120 V and allow a range of 114 to 126 V. Previously 110, 115 and 117 volts have been used at different times and places in North America
In Japan, the electrical power supply to households is at 100 V. Eastern and northern parts of Honshu (including Tokyo) and Hokkaido have a frequency of 50 Hz, whereas western Honshu (including Nagoya, Osaka, and Hiroshima), Shikoku, Kyushu and Okinawa operate at 60 Hz. To accommodate the difference, appliances marketed in Japan can often be switched between the two frequencies.
History of voltage and frequency
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The system of three-phase alternating current electrical generation and distribution was invented by Nikola Tesla in the 19th century. He considered that 60 Hz was the best frequency for alternating current (AC) power generating (possibly because 60:1 was a convenient gearing ratio for electric clocks employing synchronous motors). He preferred 240 V, which was claimed to be better for long supply lines. Thomas Edison developed direct current (DC) systems at 110 V and this was claimed to be safer. For more information about the early battles between proponents of AC and DC supply systems see War of Currents.
The German company AEG built the first European generating facility to run at 50 Hz, allegedly because the number 60 did not fit into the numerical unit sequence of 1, 2, 5…. At that time, AEG had a virtual monopoly and their standard spread to the rest of the continent. In Britain, differing frequencies (including 25 Hz 40 Hz and DC) proliferated, and the 50 Hz standard was established only after World War II.
Originally much of Europe was 110 V too, just like Japan and the US today. It was deemed necessary to increase voltage to draw more power with reduced loss and voltage drop from the same copper wire diameter.
The choice of supply voltage is governed more by tradition than by optimization of the distribution system. In theory a 240 V distribution system will use less conductor material to deliver a given quantity of power. Incandescent lamps for 120 V systems are more efficient and rugged than 240 V lamps, while large heating appliances can use smaller conductors at 240 V for the same output rating. Practically speaking, few household appliances use anything like the full capacity of the outlet to which they are connected. Minimum wire sizes for hand-held or portable equipment is usually restricted by the mechanical strength of the conductors. One may observe that both 240 V-system countries and 120-volt system countries have extensive penetration of electrical appliances in homes. National electrical codes prescribe wiring methods intended to minimize the risk of electric shock or fire.
Many areas using (approximately) 120V make heavy use of dual voltage systems to reduce the issues in supplying larger appliances. Major appliances, such as dryers and ovens, are connected to the higher voltage supply. Where the step down transformer (from MV wiring) is single 240 V split in two 120 V supplies between neutral and hot wire is fairly typical. 3 phase systems may either be connected in a star (giving 208 V between any two hots and 120V between any hot and neutral) or delta with the midpoint of one winding grounded (giving 240V between any two hots, 120V between two of the hots and neutral and 208V between the remaining hot and neutral. The latter system gives 240V which allows the same equipment to be used as for split single phase supplies. However it also puts all 120V equipment on the same phase of the 3 phase supply.