University Physics with Modern Physics (14th Edition)

Household wiring (from the power line to our house, and the wiring within the walls) is typically quite long and therefore of relatively high resistance. Too low of a voltage would mean that the currents would have to be higher, which would result in unacceptable heating losses. (The heat loss in the wiring can be expressed as $P=I^2R$. For a given wire size, using lower currents makes the process much more efficient.) Too high of a voltage would be dangerous. The wires in a car are shorter, and of relatively low resistance. A lower voltage supplies sufficient power for the car’s electrical needs, without being dangerous.