DC transmission circuits are economical when overhead lines extend over how many miles?

DC transmission circuits are generally considered economical for overhead lines that extend over approximately 400 miles. This is primarily due to the characteristics of direct current (DC) compared to alternating current (AC) transmission.

DC transmission allows for lower line losses over long distances, particularly because it does not suffer from reactance and can transmit power more efficiently without the need for reactive power management. Furthermore, the infrastructure needed for DC transmission can be less expensive over long distances since it requires fewer substations to regulate voltage levels and can accommodate higher voltage levels without causing stability issues associated with AC systems.

When calculating economic viability, factors such as the cost of line installation, power loss calculations, and the energy market's demands come into play. Over the 400-mile threshold, the advantages of using DC transmission typically outweigh the costs, making it a preferable choice for long-distance power transport.