Within the framework of the problem of ensuring the electric power system (EPS) balance reliability with taking into account the balance of not only active, but also reactive power, the article considers the problem of assessing and selecting criteria for constructing the domain confining the permissible variation ranges of operating parameters. The specific features pertinent to calculations of EPS modes in assessing their reliability lies in the need to analyze the combinations of serviceable equipment given that the quantities being analyzed are probabilistic in nature. It is of relevance to select the dominant criteria for overloading the EPS elements, as this makes it possible to reduce the time taken to calculate the modes of EPS individual states. As a consequence, it becomes possible to increase the number of considered states and improve the quality of assessing the EPS balance reliability indicators. The power line is represented by a two-node equivalent circuit with a power consumption node and a power supply node with a constant voltage modulus. It is proposed to consider the ratio of the maximum transmitted power to the short-circuit power at the power line receiving end as an estimated parameter of the limit power transmitted via the line. This indicator allows different link overloading criteria to be harmonized at a qualitative level. It is shown that in the general case, there is no dominant constraint in assessing the transmission capacity of AC power lines. It is proposed to take the quarter of the short-circuit power at the power line end as an initial estimate for the permissible power flow limit. In assessing the EPS balance reliability, it is recommended to take into account the power line efficient operation domain.