Determine the amperage requirements of the building. Detached garages and barns often require little in the way of power capacity and are well suited for low-amperage service panels ranging from 40 to 60 amps. A residential home, however, is usually serviced with 100 to 200 amps of electrical power.
List the electrical devices that will be installed within a building. For an attached garage, for example, items to consider include electrical power tools, lighting and motors. Residential homes have varied electrical needs. However, within a home there are few devices that use large amperages for an extended period of time. For example, a residential dryer or air conditioner may use large-amperage motors but only run for several minutes every hour or two.
Add the total amperages needed within the building. For example, a 50-amp motor, 15 amps worth of lighting and a 30-amp air conditioner within a garage would add up to 95 amps. This is the maximum amperage capacity required for the service panel.
Take the operational duration of devices into account. Residential homes often have 100-to-150-amp service panels installed. This is despite having well over 200 to 300 amps worth of devices installed within a home. If many of the high-amperage devices operate for only a short time per day, they can be given far less weight in the final determination of the service panel capacity.
Choose a service panel capacity that covers, at a minimum, the continuous power load within the house (lighting, water heater and ranges) and can operate one or two of the high-power items simultaneously.