The use of distributed generators (DGs) has increased in the past decade and projections indicate that penetration will further increase. In this scenario, with the improvements in power-electronics-based converters, the DG units have the potential to effectively resolve voltage/current control issues in low-voltage distribution systems. In this paper, a new probabilistic index is defined to measure the controllability of voltages and currents in the buses and lines of distribution systems. Using this index greatly facilitates the analysis and measurement of voltage/current controllability levels of distribution systems. Different types of DGs are allocated in the system to optimize the voltage/current controllability by using the defined index. The results are compared with those obtained through the conventional approach of DG allocation, which is based on the minimization of energy losses. A new combined index is defined to include both V-I controllability and energy losses. Several sensitivity studies are then performed to show the effect of the penetration level of different types of DGs on the proposed indices.