A constant challenge related to Brazilian electrical energy distribution systems is the improvement of the calculation and management of losses occurring in energy transport. Energy losses are divided into technical losses, inherent to energy distribution, and non-technical losses, caused, among other factors, by activities such as theft and fraud. Non-technical losses can cause losses for concessionaires due to the values ​​recognized by the National Electric Energy Agency, in addition to possible network maintenance costs, as unplanned loads are connected improperly and can overload the network. Given this scenario, this article proposes a methodology to calculate non-technical losses in electrical energy distribution systems. This approach considers the energy balance, treating feeders that have undergone reconfiguration in the network differently, demonstrating the relevance of identifying these maneuvers for calculating the indicator. The results demonstrate that, under the conditions evaluated, reconfiguration can have a relevant impact on non-technical losses.