The ones powering 120-volt circuits will be standard sizes depending on the location of the outlets, the expected use of the outlets and the electrical equipment they will power. The 240-volt circuits are used to power individual appliances and will be sized to the amperage requirement for the appliance to which it’s dedicated.
Minimum Size
Circuits designed to power living areas of a home will likely be protected by 15-amp circuit breakers. None of the lights, radios, televisions, computers or other electrical equipment used in such areas normally have a large amperage demand. Even when several of these types of equipment are powered on simultaneously they won’t overload a 15-amp circuit breaker.
Large Size
The National Electric Code requires circuits powering kitchens, laundry rooms, garages and other specified areas to be protected by 20-amp (or larger) circuit breakers. You are more apt to be using appliances such as toaster ovens, power tools or clothes irons in these areas that draw higher amperage from the electrical system.
Mega-Size
Using anything over a 20-amp breaker in a home installation is unusual but if you are building, remodeling or rewiring and know in advance you'll have an area requiring a circuit with a high amperage circuit, it’s possible to upgrade from the usual. Drop in a 30-amp circuit with the required wire size and set up the biggest Christmas display on the block. Otherwise, go with multiple 15 or 20-amp circuits.
220 Circuits
Heaters, stoves, water heaters, air conditioners, water pumps and other electrically powered equipment that use high amounts of power will be individually protected by circuit breakers from 20 to 60 amps depending on their size. If you have unattached garages, sheds or workshops at your home powered through your main load center, these buildings may be protected by 60 to 100-amp circuit breakers which feed a load center which breaks that electricity into discreet circuits in the remote building.