Abstract: This paper analyzes the thermal behavior‌ and loss characteristics of single-core cables under harmonic currents. An electromagnetic-thermal coupled model was established to analyze the mechanisms of ‌conductor skin effect, proximity effect‌, and ‌frequency-dependent dielectric losses‌ in insulation. Experimental validation elucidates the governing principles of how ‌harmonic currents‌ affect cable losses and ‌temperature distribution‌. Results demonstrate that ‌harmonic components‌ significantly elevate ‌AC resistance‌ and induce ‌temperature fluctuations‌ in conductors. This research provides a theoretical foundation for securing cable operation in harmonic-polluted environments.