The functionality of sewer networks is strongly affected by the correct operation of their appurtenances; the dendritic structure of urban drainage systems implies that junction manholes represent a crucial hydraulic structure, allowing two conduits merging into one. Hydraulic features of combining flows become quite complex when supercritical flows are involved, as in the case of steep urban context, with consequent formation of shockwaves and surging phenomena. Former studies conducted by Gisonni and Hager resulted in an optimized layout of sewer junctions operated under supercritical approach flow conditions. Recently, an extensive experimental campaign was performed on a physical model with generalized geometrical conditions, including various conduit diameters. Furthermore, physical model tests have been used to implement and validate a numerical model, aiming to explore a wider range of junction angles, which were limited to 45° and 90° for the physical model. In particular, the numerical model focused on the flow condition where both approach flows are supercritical. Based on the dataset constituted from both physical and numerical model results, comprehensive equations are proposed for the prediction of energy losses at junction manholes with different upstream and lateral conduit diameters, with particular reference to supercritical combining flows.