From the receiving stations, power travels through 69,000-volt sub-transmission lines to distribution substations. These substations usually serve a four-square-mile-area and several thousand customers. At the substations, power is transformed from 69,000 volts to 12,000 volts and is split among several circuits. Each circuit serves as many as 600 residential customers. In newer neighborhoods, 12,000-volt circuits are underground. In older neighborhoods, overhead lines still are in use.