How to Size a Building's Electrical Service

Classify the structure based on its intended use. There are four common classifications that are used. Sub-residential structures are those separate from a main home, such as a detached garage or barn. Residential structures are homes intended for human habitation. Commercial structures are medium to large places of business, such as restaurants or stores. Finally, there are industrial installations that require large electrical installations, often to power heavy machinery.

Assign the base capacity based on the class of the structure. For example, residential buildings are, as of 2011, required by the National Electical Code 230.79(c) to have a minimum of 100 amps incoming electrical power. Homes are often installed with capacities between 100 and 150 amps. Sub-residential structures have no minimum, but installed devices such as heavy motors and appliances must be accounted for in their planning. Common installation sizes for sub-residential buildings range from 40 to 100 amps.

Adjust the minimum capacities as required to suit any specialized electrical loads. For example, a heated pool or spa requires significant electrical power, often necessitating a separate sub-panel, and must be considered in the home's minimum electrical requirements. The addition of one or more large loads requires an increase in the electrical capacity.