In city, identification for 3 phase-4 wires power distribution lines is difficult due to compound installation of overhead and underground lines, transposition, bad view caused by trees or big buildings. It is necessary that the correct and fast identification method is required for load balancing among distribution lines. Tracing off-line method with high power impulse signal injection has been used recently. Our proposed method uses to identify live lines with very small power high frequency signal injection based on data communication technology. Two end communication terminals are required to be synchronized between them for determination on electrically same phases. Challenging issue is to achieve synchronization without GPS providing synchronizing time. A novel power transformer and phase identification system is designed and implemented. The system consists of a transmitter and a receiver with power-line communication module. Some experiments are conducted to verify the theoretical concepts in a big commercial building.