Extra high voltage (EHV) lines have been used to interconnect many grids. However, what the proportion of underground cable and overhead line over an EHV line should be has not gained enough investigations. In this paper, a new game-theoretic framework is proposed to understand how customers choose the proportion of underground cable and overhead line over an EHV line, such that both the expenditure of voltage sag and the cost of construction can be minimized. We theoretically prove the existence of Nash equilibrium (NE) for the proposed game, and propose a best-response based learning algorithm to achieve the NE. Moreover, we study how the NE is affected by the distance. The simulation results verify our mathematical analysis and the effectiveness of our model.