Accurately assessing the rated current carrying capacity of transmission lines is crucial for the lines. This paper compares the results of rated current carrying capacity calculations under IEEE standards and national standards, and provides a detailed analysis of the effects of wind speed, wind direction, ambient temperature, and solar radiation on the results. Additionally, the study compares the calculated solar radiation power in the IEEE standard with actual sensor values to explore whether the formulas in the standards accurately represent real-world conditions. The research results indicate that the rated current carrying capacity calculated under the national standard still has a significant margin, and meteorological parameters significantly impact the calculation results. The current carrying capacity calculation method proposed in this study can fully represent the transmission capacity of transmission lines under current meteorological conditions, providing theoretical support for dynamic capacity enhancement and power inspection of transmission lines, which helps improve the safety and economic efficiency of the power system.