This paper presents a new solution to the problem of transmission cost allocation to its users. The proposed technique utilizes modified Z-bus theory

equivalent current injection and impedance of the generators and loads

and is developed by the equal-sharing as well as the orthogonal projection principle. The procedure is performed in three steps. First

the modified Z-bus theory is used to calculate the contribution of the users into the network bus voltages as well as the branch currents. Then

the equal sharing principle is confirmed by the game theory solutions and is subsequently applied to identify the users' contributions into the branch power flows. After that

the orthogonal projections of the contributions are calculated and the concept of effective contributions is suggested. The proposed methodology provides the percentage shares of the users on the network complex variables

which help to better assess the contributions. A 2-bus and the IEEE 30-bus test system are used to validate the proposed technique. Finally

the proposed methodology is applied to the polish 2383-bus system to emphasize its applicability to large practical systems.