Electric central heating is typically the largest-load item in the household, thereby demanding 240-volt wiring. In addition, the heating system must be supplied by its own branch circuit, from the main service panel. This is required by code. The idea behind the separate heating circuit is to make sure your heating could still operate, should power go out in another system. However, related equipment such as humidifiers or electrostatic air cleaners can be included on the heating system circuit. Room air conditioners, also called 'window units,' are available in either 120-volt or 240-volt ratings. Both types must be grounded, and meet certain code stipulations. A central air-conditioning system will require 240-volt wiring from the panel. Heating and air-conditioning systems also need special circuit breakers or fuse disconnects, as extra protection against overload. Because of the complexity involved with wiring for electric heating and air conditioning, this type of work should be left to a professional. For more details, contact an electrical expert in your area.