<P> 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 V distribution system will use less conductor material than a 120 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 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 . Electrical appliances are used extensively in homes in both 230 V and 120 V system countries . National electrical codes prescribe wiring methods intended to minimize the risk of electric shock and fire . </P> <P> 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 2400 / 4160 V (2300 / 4000 V in the US) whereas 3300 V is the common standard for 50 Hz systems . </P> <P> Following voltage harmonisation, electricity supplies within the European Union are now nominally 230 V ± 10% at 50 Hz . For a transition period (1995--2008), countries that had previously used 220 V changed to a narrower asymmetric tolerance range of 230 V + 6% / − 10% and those (like the UK) that had previously used 240 V changed to 230 V + 10% / − 6% . No change in voltage is required by either system as both 220 V and 240 V fall within the lower 230 V tolerance bands (230 V ± 10%). Some areas of the UK still have 250 volts for legacy reasons, but these also fall within the 10% tolerance band of 230 volts . In practice, this allows countries to continue to supply the same voltage (220 or 240 V), 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 the United States and Canada, 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 . </P> <P> In 2000, Australia converted to 230 V as the nominal standard with a tolerance of + 10% / − 6%, this superseding the old 240 V standard, AS2926 - 1987 . 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 Japan, the electrical power supply to households is at 100 V. Eastern and northern parts of Honshū (including Tokyo) and Hokkaidō have a frequency of 50 Hz, whereas western Honshū (including Nagoya, Osaka, and Hiroshima), Shikoku, Kyūshū 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 . </P>

A standard voltage for homes in the united states is 110 v