This paper presents a new method for transmission loss allocation based on complex power flow tracing. Building on previous works of the author, the paper presents a generalized model for the transmission line, which is used as the building block for the complex power flow tracing algorithm. The developed line model is similar to the famous generalized circuit constants, ABCD, model. While the ABCD line model relates voltage and current at one end of the line to those at the other end, the developed line model relates the partial flows caused by a load/generator at the receiving/sending end of the line to the partial flows caused by that load/generator through all lines following that line in the upstream/downst ream direction. The proposed model has properties regarding series and parallel equivalence similar to those of the ABCD model. The only difference is that the proposed model parameters are determined based on the line flows whereas the ABCD parameters are functions of line constants. Although the power flow tracing algorithm employed is similar to the previously proposed by the author, the modification introduced in this paper enables more insight to the loss allocation process as the cross effects of active and reactive power flows on both active and reactive losses can be quantified more clearly. The paper also describes a method for loss allocation in the presence of loop flows based on signal flow graph manipulation techniques.