It must be aluminum instead of copper. You know what? On one hand, the resistance of copper wire is small, with the same cross-sectional area of copper wire through the flow is larger than aluminum wire; On the other hand, aluminum wire has better transmission characteristics like lighter, lower price, wider supply of goods and so on.

PS: Due to high-voltage wires are overhead; However, copper wires are too heavy to be erected over long distances; As the voltage increases, the current turns smaller so does the line loss; Besides, considering the cost of erection and security. Aluminum is absolutely the best consideration for high voltage lines.