A disconnecting means or "disconnect switch" must be provided in the service entrance to disconnect all the wiring in the building from the electrical power supply. This may be accomplished with a separate switch called a service switch or main disconnect. Many years ago it was common to install a separate switch next to the fuse box to shut the power off. In modern residences, the service switch is actually the main breaker housed inside the breaker box which shuts off power to all the other breakers in the box.
The size of the service switch or main breaker depends on the electrical load served. Service switches come in standard sizes of 30, 60, 100, 150, 200, 400 and larger. The National Electrical Code requires a minimum 100-amp service size for all residences, although many newer houses typically use a 150 amp or larger service.