Why must 2 wires run between a house and the electric power plant?

Answer 1

You must need 2 wires between a house and the electric power plant because one wire cannot hold that much power. If there is 1 wire the wire might burst. The burst can hurt anyone around there.

Answer

It's far more complicated than simply 'two wires between the power plant and a house'! So let's stick to the final part of the connection -i.e. between the distribution transformer and a house!

In Europe, there are indeed two wires between the distribution transformer and a house. These are called the 'line conductor' (the 'hot' conductor) and the 'neutral conductor'. The neutral conductor is connected to the earth (ground), while the line conductor has a potential of 230 V with respect to the neutral conductor; in simple terms, the current then passes along the line conductor and back along the neutral conductor. All appliances in European houses are rated at 230 V.

In North America, there are, in fact, three wires between the distribution transformer and the house. One of these is the earthed (grounded) 'neutral conductor', while the other two are the 'line conductors' (the 'hot' conductors). The potential difference between the two line conductors is 240 V, while the potential difference between either line conductor and the neutral conductor is 120 V. Again, in simple terms, currents pass along the line conductors and back along the neutral conductor. In North America, most appliances are rated at 120 V so are connected between a line conductor and the neutral conductor, but some 'high power' appliances (like stoves, etc.) are rated at 240 V and must be connected between the two line conductors.