Generally, negative line loss occurs in transmission and distribution networks, which directly affects the economic benefits of both the power supplier and the electricity user. It also harms the power equipment and endangers human’s safety. In this paper, the equivalent model of a single-phase transmission line is used to derive the numerical relationship between the negative line loss and the comprehensive error of the measurement device. The critical value of the line loss rate is obtained. Considering the line transposition, the inductance and capacitance matrices of the double-circuit line on the same tower are derived, according to the law of electromagnetic induction. In the matrices, only the lines with the same phase in the double circuit are contained. The numerical examples show that the line loss is approximately proportional to the square of the current at the sending end. Thus, if the measuring error is considered, the negative line loss is more likely to occur on the light-load operation mode. When the coupling effect is taken into account, the line loss rate decreases and then increases with the current at the sending end. And they are all positive values. However, if the measuring error is taken into account, the negative line losses may occur.