Voltage levels around the World

All European and most African and Asian countries use a supply that is within 10% of 230 V, whereas Japan, North America and some parts of South America use a voltage between 100 and 127 V.

A distinction should be made between the voltage at the point of supply (nominal system voltage) and the voltage rating of the equipment (utilization voltage). Typically the utilization voltage is 3% to 5% lower than the nominal system voltage; for example, a nominal 208 V supply system will be connected to motors with "200 V" on their nameplates. This allows for the voltage drop between equipment and supply. Voltages in this article are the nominal supply voltages and equipment used on these systems will carry slightly lower nameplate voltages.

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. 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 utilization voltage is governed more by tradition than by optimization of the distribution system. In theory a 230 V distribution system will use less conductor material to deliver a given quantity of power. Incandescent light bulbs for 120 V systems are more efficient and rugged than 230 V bulbs, while large heating appliances can use smaller conductors at 230 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 are usually restricted by the mechanical strength of the conductors. One may observe that both 230 V system countries and 120 V 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 (nominally) 120 V make use of three-wire, single-phase 240 V systems to supply large appliances. 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.

Following voltage harmonization all electricity supplied within the European Union is 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%. 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 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.

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 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 "one-ten"; however, 120 is the nominal voltage.

As of the year 2000, Australia has 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" spoken as "two forty volt" remains 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. To accommodate the difference, appliances marketed in Japan can often be switched between the two frequencies.

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 2300/4160 V whereas 3300 V is the common standard for 50 Hz systems.