Many European governments have defined ambitious energy policies that are newly promoting the installation of small distributed generators and encouraging the customers' aggregation through energy communities. This paper assesses the impact of Renewable Energy Communities (REC) on the electricity distribution grid, with a focus on the case study of Terni (Italy). In order to assess the impact of RECs, power flows on the grid were calculated varying the composition and the behavior of energy communities. Assuming the full development of a scenario based on the European renewable energy growth program REPowerEU, the study examines the integration of renewable energy sources within local communities and their effects on the distribution grid's performance and their economic viability. Key technical aspects analyzed include the grid's capacity to accommodate RES integration, reverse power flow and self-consumption. In addition, the economic analysis evaluates the financial feasibility of REC implementation, considering potential revenue streams, within the Italian regulatory framework.