The voltage and frequency of AC electricity varies from country to country throughout the world. Most use 220V and 50Hz. About 20% of the countries use 110V and/or 60Hz to power their homes. 220V and 60Hz are the most efficient values, but only a few countries use that combination. The United States uses 110V and 60Hz AC electricity.
The choice of frequency for a.c. power represented a compromise among the needs of different types of equipment. During the early years of a.c. systems, numerous different frequencies ranging from 25 to 133 1/3 cycles were used. For generators, lower frequencies tend to be preferable because this requires fewer magnetic poles inside the rotor, though this constraint became less significant as high-speed steam turbines supplemented and replaced slow-moving hydroturbines and reciprocating steam engines. For transmission, lower frequencies are especially desirable because a line’s reactance increases with frequency and constrains the amount of power that can be transmitted on a given line. For loads, on the other hand, higher frequencies are often preferable. This is particularly true for incandescent lamps, whose flickering becomes more and more noticeable to the human eye at lower frequencies. After due consideration of the different types of equipment already in use and the prospects for adapting new designs, efforts to standardize power frequency finally resulted in convergence to a 60 cycle standard in the United States and 50 cycles in Europe.