Wire-driven parallel architecture is a new robot mechanism which is suitable especially for high-speed transfer of lightweight objects. In this paper, the optimal wire configurations are derived within a class of mechanisms redundantly driven by 8 wires from the viewpoint of working space. The working space is determined by conditions that the traveling plate must be able to receive arbitrary force and moment and that the wires must not contact. Heuristics on wire configurations were obtained through optimizing and analyzing isotropic configurations of actuator units. Classification and comparison of all the symmetric configurations derived the 2 optimal configurations: Rod-type and T-type. Simulation results proved the validity of these designs and showed their merits and demerits.