In order to design distribution systems rationally, distribution engineers consider several requirements such as regional characteristics, feeder characteristics, load characteristics and economic constraints. Since distribution systems are complex in nature, the selection of the conductor size and type, the transformer location and capacity, and the switch location and type are very difficult problems to be decided. Therefore, the problems described above are solved by heuristic knowledges based on past design experience. In this paper, the expert system using interactive inference procedures that are based on expert's heuristic knowledges is developed. In particular, the link problem between the existing distribution design system and this expert system is explained. The needed knowledges are obtained from the distribution design experts.