Practically all homes are supplied alternating voltage between 115 and 125 V rms, at a frequency of 60 Hz. This is a sine wave voltage produced by a rotary generator. The electricity is distributed by high voltage power line from the generating station an reduced to the lower voltages used in the home. Here the incoming voltage is wired to all the wall outlets and electrical equipment in parallel. The 120 Volt source of commercial electricity is the 60 Hz power line or the mains, indicating it is the main line for all the parallel branches.
Advantages of 60 Hz AC Power Line:
The incoming electric service to residences is normally given as 120 V rms. With an rms value of 120 V, the ac power is equivalent to 120 V dc power in heating effect. If the value ere higher, there would be more danger of a fatal eclectic shock. Lower voltages would be less efficient in supplying power.
Higher voltage can supply electric power with less I2R loss, since the same power is produced with less I. Note that the I2R power loss increases as the square of the current. For applications where large amounts of power are used such as central air conditioners and clothes dryers, a line voltage of 240 V is often used.
The advantage of ac power line is greater efficiency in distribution from the generating station. Alternating voltages can easily be stepped up by means of a transformer, with very little loss, but a transformer cannot operate on direct current. The reason is that a transformer needs the varying magnetic field produced by an ac voltage.
Using a transformer, the alternating voltage at the generating station can be stepped up to values as high as 500 kV for high voltage distribution lines. These high voltage lines supply large amounts of power with much less current and less I2R loss, compared with a 120 V line. At the home, the lower voltage required is supplied by a step down transformer. The step up and step down characteristics of a transformer refer to the ratio of voltages across the input and output connections.
The frequency of 60 Hz is convenient for commercial ac power. Much lower frequencies would require much bigger transformers because larger windings would be necessary. Also, too low a frequency for alternating current in a lamp could cause the light to flicker. For the opposite case, too high a frequency results in excessive iron core heating in the transformer because of eddy currents and hysteresis losses. Based on these factors, 60 Hz is the frequency of the ac power line in the United States. However, the frequency of the ac power mains in England and most European countries is 50 Hz