For higher amounts of current, bundle conductors are used for several reasons. Due to the skin effect, for larger conductors, the current capacity does not increase proportional to the cross-sectional area; instead, it is only with the linear dimension. Also, reactance decreases only slowly with size. But the cost and weight do increase with area. Due to this, several conductors in parallel become more economical.
Bundle conductors consist of several parallel cables connected at intervals by spacers, often in a cylindrical configuration. The optimum number of conductors depends on the current rating, but typically higher-voltage lines also have higher current. There is also some advantage due to lower corona loss. American Electric Power is building 765 kV lines using six conductors per phase in a bundle. Spacers must resist the forces due to wind, and magnetic forces during a short-circuit.