A minimum of 4-gauge wire and 1 1/4-inch conduit is required for 100-amp service. Buildings that have gas heat may not need any more than 100-amp service.
The service size of a building is the capacity of the electrical system to power electrical items. Smaller items such as televisions and hair dryers do not need a lot of service, but water heaters, electric dryers, hot tubs and central heating and cooling units require a lot of electricity. The main circuit breaker's amp rating is equal to the service size; the circuit breaker shuts off power whenever there is a draw greater than its capacity to prevent overload and possible fire.