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. Distribution lines may carry this medium-voltage current directly to commercial, industrial, or institutional users. Distribution lines also carry electric power to distribution transformers on poles, on the ground, or in underground vaults.
Distribution transformers reduce the voltage to the levels needed by most users.
Wires from the transformers run to homes, stores, offices, and other users. Nearly all such consumers in the United States and Canada receive electric power at about 110 or 220 volts. Providing reliable service. Equipment failures or damage caused by storms or accidents can interrupt local service of electric power. Such interruptions are known as power blackouts.