The installations of weather-dependent renewable energy sources, such as wind turbines and solar plants, increased significantly in the last two decades. They are often built in remote areas without appropriate power grid connections. The construction of a new power transmission line or improving the old one requires significant financial and time costs. The grid operators prefer to limit the power production in order to keep the load of the power line under its rating. Instead of broadly-used static rating, the existing lines can be rated in real time using a dynamic thermal rating (DTR) system. DTR of power transmission lines can usually provide a significant increase of transmission capacity compared to the static rating. The main inputs to DTR systems are measured or forecast meteorological data. The relation of DTR to the renewable resources is obvious when we consider input parameters to the calculation scheme - wind speed, ambient temperature and shortwave radiation. Exactly same variables are determining production of wind and solar energy. A case study of virtual wind farm and corresponding power transmission line shows limits of renewable energy production at given site. The results demonstrate that the optimal size of wind farm is approximately triple when using DTR comparing to the static rating.