In recent years several methods have been proposed to identifying the voltage sag source location. The basis of most of these methods is to determine if the source originated upstream or down stream from the monitoring location. This study proposes a method that is capable of identifying the voltage sag originated bus. A technique of observing the network based coefficients is applied network wide, where any unmonitored bus voltages are estimated prior to making a decision on voltage sag source location. The study concludes with the outcome and limitation of this method and proposes further improvement.