Universal cable is a type of wire capable of carrying any form of power available within the Technic modpacks - Industrialcraft EU, Buildcraft MJ, and Universal Electricity U's. The power within the cable also can be outputted in each of these forms, eliminating the need for any power conversion type equipment. Presumably less loss occurs since the conversion is instant.
In Voltz, universal cable can be used to carry electricity stored in Energy Cubes to any of the machines. It also accepts power outputted by any other generation devices such as Solar Panels, Advanced Solar Generators, Wind Turbines, and Reactor Turbines. There is no limitation as to the maximum voltage and current that the cable is able to carry and minimal losses are incurred.