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 US and Canada.

See also List of countries with mains power plugs, voltages & frequencies.

European and most other countries in the world use a supply that is within 10 % of 230 volts, whereas Japan and most of the Americas use between 100 and 127 volts.

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 230V +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.

A close synonym to "mains" in Canadian provinces that use hydroelectric power would be "hydro".

ANSI standard C84.1 and Canadian CAN3-C235 specify that the nominal voltage at the output should be 120 V and allows a range of 114 to 126 V. California deliberately runs in the voltage range 114 to 120 V to reduce power consumption, however the reasoning behind such a move is debatable.

In Japan, the electrical power supply to households is at 100 V. Eastern and northern parts of Honshu (including Tokyo) as well as 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 for the difference, appliances marketed in Japan can often be switched between the higher and lower frequencies.

History of Voltage and Frequency

The system of three-phase alternating current electrical generation and distribution was invented by Nikola Tesla in the nineteenth century. He considered that 60 Hz was the best frequency for alternating current (AC) power generating. 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 proliferated, and the 50 Hz standard was only established after World War II.

Not only is 50 Hz 20% less effective in generation, it is 10–15% less efficient in transmission. It requires up to 30% larger windings and magnetic core materials in transformer construction. Electric motors are much less efficient at the lower frequency, and must also be made more robust to handle the electrical losses and the extra heat generated. Nevertheless, today only a handful of countries (Peru, Ecuador, Guyana, the Philippines and South Korea) follow Tesla’s advice and use the 60 Hz frequency together with a voltage of 220–240 V.

Originally Europe was 110 V too, just like Japan and the US today. It has been deemed necessary to increase voltage to get more power with reduced loss and voltage drop from the same copper wire diameter. At the time the US also wanted to change, but it was deemed too costly to change all of the existing infrastructure.

Americans are still often stuck with the problems of the lower voltage. A device at 120 V draws twice as much current as a device with the same power draw at 240 V. A 3000 W domestic clothes dryer requires 12.5 A at 240 V or 25 A at 120 V. The end result is that wiring must be larger, and each outlet supplies less power. This may have been a factor in the use of circuit breakers in America long before they became common in Europe.

Smaller North American buildings get 240 V split in two 120 V supplies between neutral and hot wire. Larger buildings often have 3 phase with 208V between any two hots. Major appliances, such as dryers and ovens, are now connected to 240 V or 208 V supply. Americans who have European equipment can connect it to these outlets, as long as it can accept the U.S. frequency of 60 Hz rather than 50 Hz.

See also: Power connector, Three-phase electric power, Electricity, Potential difference, Domestic AC power plugs & sockets.