A simple but precise model would improve the computation efficiency for planning a large-scale power system. Recent development of a new network reduction algorithm yields such a model with a given grouping of buses. While the flow over the reduced network is estimated precisely, a proper value of the flow limit still remains unresolved. In this study, a method is proposed to group buses based on the congestion profile in the original network and to assign the flow limits of a reduced network. The method is tested on modified IEEE 30-bus and 118-bus systems at various load profiles, and the simulation results are compared.