Powerline can be used as a propagation channel to transfer the information of some communication systems. In order to employ all three-phase powerlines, a novel communication channel model is proposed in this paper. One of the advantages of the proposed model is that both source inner admittance and load admittance matrices are considered and this can use the proposed model to simulate a practical powerline network. Based on the multiconductor transmission lines theory, a complete solution of the voltages and currents on all three-phase powerlines is derived in detailed. The communication signal strength of powerline networks can be predicted. The comparison between theoretical results and measurements is obtained and a good agreement between the analytical model results and measurements is presented, which means the proposed channel model and the derived formula are correct.