AC or DC?

During the initial years of electricity distribution, DC was the standard for the United States. Direct current worked well with motors, and incandescent lamps. DC could be used to charge batteries to be used to level the load and act as a backup in case of power failure. Direct current generators could be easily paralleled, allowing economical operation by using smaller machines during periods of light load and improving reliability, and the only meters available for measuring consumption were designed for DC.

There are significant disadvantages however. DC systems do not allow voltages to be transformed. The voltage throughout the system must be at a level that will not be a hazard – less than a couple of hundred volts. This results in significant power losses, sinceLow voltage power lines mean high currents and high power losses in the transmission cables.

In the alternating current system transformers are used to transform the voltage up for transmission over long distances and down for transmission into people's homes. is used between the (relatively) high voltage distribution system and domestic supply.

Lamps and small motors can still be operated at some convenient and safe low voltage. However, the transformer would allow power to be transmitted at much higher voltages, say, ten times that of the domestic supply voltage. For a given quantity of power transmitted, the wire diameter would be inversely proportional to the voltage used. Alternatively, the allowable length of a circuit, given a wire size and allowable voltage drop, would increase approximately as the square of the distribution voltage. This had the practical significance that fewer, larger generating plants could serve the load in a given area. Large loads, such as industrial motors or converters for electric railway power, could be served by the same distribution network that fed lighting, by using a transformer with a suitable secondary voltage.

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