The first stage in any machine design process involves a substantial heuristic exercise to select the machine best suited to a set of specifications. The second stage obtains the dimensions of a selected machine and a design that satisfies specifications. The final stage usually involves optimization and the finite-element method. We present an object-oriented knowledge-based system for distribution transformer design with the first two stages fully automated. We describe the rationale for the system, the architectural design procedure, and the approach taken in constructing the system for practical applications.