Standardization
Following voltage harmonization, electricity supplies within the European Union are now nominally 230 V ± 10% at 50 Hz [1]. 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%[2]. Note that 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 ±6%). In practice, this allows countries to continue to supply the same voltage (220 or 240 V), at least until existing generating plants are replaced. Equipment used in these countries is designed to accept any voltage within the specified range.
In the United States[3] and Canada[4], national standards specify that the nominal voltage at the source should be 120 V and allow a range of 114 to 126 V (-5% to +5%). Historically 110, 115 and 117 volts have been used at different times and places in North America. Main power is sometimes spoken of as 110; however, 120 is the nominal voltage.
In 2000, Australia converted to 230 V as the nominal standard with a tolerance of +10% -6%.[5], this superseding the old 240 V standard, AS2926-1987.[6] 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 HVDC substations which convert the frequency; these are Shin Shinano, Sakuma Dam, Minami-Fukumitsu, and the Higashi-Shimuzu Frequency Converter. To accommodate the difference, frequency-sensitive appliances marketed in Japan can often be switched between the two frequencies.