A new sensitivity factor which is called ZBUS distribution factor (ZBDF) to calculate active and reactive three-phase line flows for distribution automation is proposed in this paper. This method is associated with both the changes of individual phase-active and-reactive power injection at each bus into the three-phase real and reactive power line flows. Hence, the proposed ZBDF reflects load changes to the line flows, while the loading conditions change from the base case loads. Accordingly, the existing implicit ZBUS Gauss method is applied to solve for the base case three-phase power flow (TPPF). Based on the power flow solutions, the ZBUS matrix, the primitive line impedance, and the changes of the individual phase-active and-reactive power at each bus, the three-phase line flows for distribution automation are determined accurately and rapidly from the presented method without any iteration. Finally, a sample system is used to verify the developed approach. The numerical simulation results demonstrate that the proposed method not only provides high-speed computation, but also can be applied to distribution automation.