It is easier to boost the voltage of alternating current than that of direct current. Alternating current is therefore easier to transmit than direct current. For this reason, electric power plants generate alternating current. The typical power plant generator can produce about 1 million kilowatts of electric power at up to 22,000 volts. Devices called step-up transformers then boost this voltage as high as 765,000 volts for transmission.
Some large industries require high-voltage current and receive it directly from transmission lines.
But high-voltages are unsafe in homes, offices, and most factories. The voltage must therefore be decreased before electricity is distributed to them. High-voltage electricity is carried by the transmission lines to subtransmission substations near the area where the power will be used. These substations have devices called step-down transformers that reduce the voltage to 12,500 to 138,000 volts. The voltage is then further reduced at distribution substations to 2,000 to 34,500 volts.