What device changes the voltage 2400 V to 120 V before electricity enters a house?

Answer 1

The 240 volts coming in is coming from two separate 120 volt lines (phases). In your home, it's called single phase, although it really should be called two phase or double phase. In the box, there are two "hot" wires coming in, and one "neutral" wire. You add a ground wire to that yourself (check local codes).

Basically (this is very simple description), one of the hot wires gives 120 volts, and the other gives 120 volts, but just a little bit behind the first phase. If you pull the cover off your electric panel, you would see two "hot" wires connected to two separate 'bus bars'. Every other slot on the left side of the breaker box is connected to hot 1, and the alternate slots are connected to hot 2. When you need 240 volts, the breaker you install will take up two slots, adding the 120 (hot 1) plus 120 (hot 2) to give you 240. For your regular plugs, a single slot breaker will deliver 120 volts.

If you have a 240 volt plug, it will normally have 4 prongs. Two of the prongs have 120volts each (relative to ground) and 240 volts relative to each other (due to the lagging/leading phase difference.)

Answer 2

A transformer