Abstract: A computer-based method and apparatus are provided for designing a building's electrical power distribution system. An electrical circuit design module accepts electrical requirements for individual circuits and automatically determines values for a set of variables that completely specify the design of the circuits and the physical attributes of the components needed to construct those circuits. The circuit design module determines the set of circuit variables with the aid of electrical standards data, such as the National Electrical Code. Further, multiple circuit design modules may be linked so as to share the data between them thereby providing an hierarchical design process where the design data for higher level electrical circuits is automatically determined from the plurality of individually specified, lower-level circuit data.