The justification for investing in the transmission network in order to meet the regulatory framework of the European Union, i.e. Article 16, Paragraph 8 of Regulation (EU) 2019/943 has been analysed. The introductory part provides an overview of the legislative framework of the European Union and a description of the internal electricity market. Special attention is paid to the comparison of wholesale electricity prices in different markets of the European Union, with an emphasis on the markets of Croatia, Slovenia and Hungary. It has been shown that the analysis of the electricity market needs to be carried out by comparing hourly wholesale prices. Applying the adopted methodology, a high level of price convergence in the trading zones of Croatia and Slovenia was determined. At the same time, the calculations show that certain elements of the transmission system on that border did not meet the criterion of a minimum 70% of the transmission capacity that should be made available to market participants according to before mentioned provision of Regulation. For this reason, an evaluation of the profitability of the new transmission line construction to fulfil the 70% criterion was undertaken. By analysing the costs and benefits of the new transmission line construction, the profitability of that network reinforcement was assessed. Indicators of project profitability are net present value and internal rate of return. The results showed that the considered project is profitable for the used input parameters.