This paper presents an automated tool for optimizing the routing of primary cables in underground residential distribution systems (URD). Although initially developed as one of the modules in Puget Sound Power and Light Co.'s automated electrical plat design program, Automated Primary Router (APR) can also be used in standalone mode. APR, implemented in a geographic information system (GIS)-based environment, is capable of efficiently accessing and manipulating geographically referenced data from a facilities management system database. It also provides a full-fledged graphic user interface, along with on-line visualization and accurate cost estimation to facilitate the design process. APR employs a heuristic search algorithm to find the best primary cable routes either for new residential developments, or for existing URD systems as part of the ongoing cable replacement program. Based on the test results, APR shows significant stability and efficiency in finding the optimal solution for primary cable routing. With this performance, APR can help distribution engineers improve the quality of URD design, producing standardized and economically justifiable primary cable routes. Because geographically referenced design data are digitally stored in the GIS database, APR further increases the reusability and accessibility of the URD design information.<<ETX>>