Over how many miles do DC transmission circuits become economical for overhead lines?

The economic viability of using direct current (DC) transmission circuits for overhead lines significantly increases when the transmission distance exceeds approximately 400 miles. This threshold is primarily due to the inherent characteristics of DC transmission.

DC systems have lower transmission losses compared to alternating current (AC) systems, especially over long distances. As the distance becomes longer, the advantages of reduced line losses become more pronounced, making the overall cost of transmission more favorable for DC.

At distances less than 400 miles, AC transmission may still be more cost-effective due to established infrastructure and existing technology. However, when distances surpass 400 miles, the efficiencies gained from the DC method outweigh these initial advantages, thus making it the preferable choice for overhead lines in those scenarios.

Therefore, the correct answer reflects a key understanding of the economic and technical factors at play in long-distance power transmission.