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This study is to evaluate the existing methods to determine the design capacity of the multi-branched water transmission systems. Three types of methods are generally used to determine the design capacity of water transmission systems.The first method is that total average day demand and peak factor of whole area is considered to estimate total maximum day demand, and then it is assigned to each pipe in the system. The second method is that total demand for each pipe in the system is calculated by adding up maximum day demand of each area. Because the calculation of design flow for each pipe is simple and convenient, this method is generally used for the design currently. The third method is that the each proper peak factor for each pipe in the system are estimated to predict maximum day demand of each pipe.In this study, two real water transmission systems were considered as study areas, and three methods were applied. The first method tends to underestimates the design capacities especially for branch pipes. On the other hand, the second method tends to overestimates the design capacities for most pipes.As a result, it is found that the third method generates most reasonable design capacities for each pipe. However, mass balance for the analysis of water transmission systems can not be satisfied automatically in the third method. Therefor, dummy nodal flows method are suggested to satisfy the Mass Balance Equations in this study.