Commercial electricity is generated and transmitted as three-phase electricity. Instead of a single emf, three separate wires carry currents for the emfs E1=E0cos\omega t
, E2=E0cos(\omega t+120∘)
, and E3=E0cos(\omega t−120∘)
over three parallel wires, each of which supplies one-third of the power. This is why the long-distance transmission lines you see in the countryside have three wires. Suppose the transmission lines into a city supply a total of 450 MW
of electric power, a realistic value.