The paper describes a sophisticated visually interactive computer-aided design(c.a.d.) system for electricity distribution networks. A version of this has been made available for the personal use of planning engineers at the Merseyside and North Wales Electricity Board (MANWEB). An original spare-capacity model and an automatic network reinforcement algorithm are presented. These have been incorporated in the c.a.d. system, thereby providing a comprehenisve network performance assessment of the simulation.