Editing Mains electricity (section)

=== Choice of voltage ===
{{Main|Mains electricity by country}}

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&nbsp;V distribution system will use less conductor material than a 120&nbsp;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&nbsp;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.

Many areas, such as the US, which use (nominally) 120&nbsp;V, make use of [[Split-phase electric power|three-wire, split-phase]] 240&nbsp;V systems to supply large appliances. In this system a 240&nbsp;V supply has a centre-tapped neutral to give two 120&nbsp;V supplies which can also supply 240&nbsp;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&nbsp;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&nbsp;volts, are occasionally used for special-purpose systems such as oil well pumps. Large industrial motors (say, more than 250&nbsp;hp or 150&nbsp;kW) may operate on medium voltage. On 60&nbsp;Hz systems a standard for medium voltage equipment is 2,400/4,160&nbsp;V whereas 3,300&nbsp;V is the common standard for 50&nbsp;Hz systems.