The total supply capability（TSC） is an important new index for distribution systems. However， such fundamental problems as whether the TSC calculating algorithms are accurate or not and how to evaluate the accuracy of TSC results are yet to be solved. This paper performs a comparative research between TSC algorithms and traditional N-1 security verification. Firstly， both TSC calculating methods and the transformer N-1 security verification are treated. Secondly， the indices and scheme of the comparative study are designed， in which a hybrid approximation method of calculating TSC is presented. Finally， case studies show that the current TSC algorithms are not free from errors. By analyzing the error forming process， it is found that there are two main causes for their formation. One is the assumption about the transferring load being continuous and divisible; the other is considering the transformer load instead of feeder load.