Final answer:
DC transmission is economical for overhead lines exceeding 200 miles and for underground circuits over 40 miles because higher voltages used in long-distance transmission reduce power losses.
Step-by-step explanation:
DC transmission circuits become more economical compared to their AC counterparts for long-distance electricity transmission. For overhead lines, DC transmission is more economical when they extend over large distances, typically when they are longer than 200 miles. In the case of underground (UG) circuits, DC transmission becomes economical over distances exceeding 40 miles. Long-distance transmission lines use very high voltages, often in the hundreds of kilovolts, to limit energy losses. This is due to Ohm's law, where the power loss in the transmission lines is proportional to the square of the current. Therefore, by keeping the transmitted voltage high and the current correspondingly lower, power loss due to the resistance of the transmission lines is minimized.