Answer for UK, Europe and countries running a 50 Hz supply service.
Standard voltages are used so that equipment can be produced cheaply in quantity. It is not economic to use a lot of different voltages.
The low-voltage standards for electricity in Europe is 230 v single phase, 400 v 3-phase.
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Answer for USA, Canada and countries running a 60 Hz supply service.
Low voltage in North America 120/240 single phase. Main three phase voltages are 120/208, 277/480 and 347/600. Any leg to ground on the three phase or leg to leg voltages can also be classed as single phase.
How do you calculate mf with this following ct ratio 1. 75/5A, VA 5, Class 5.0, 660v. 2. 250/5A, VA 10, Class 1.0, 660V 3. 3200/5A, VA15, Class 1, 660v the meter ratio is 5/5A.
Three-phase is transmitted at many different voltages but a 440 v 3-ph supply has 440 v between the phase wires by convention and is used in supplying power to groups of houses or small businesses. The voltage between any of the phases and neutral is 440/sqrt(3) which equals 254 v. A typical 440 v 3-ph transformer might be able to supply 700 amps, which gives 180 kVA, enough for about 90 houses.