It is difficult to consider factors such as wind speed, water flow velocity, and solar radiation when using the IEC 60287 standard to calculate the current-carrying capacity of shore power cables in port microgrids. Therefore, based on the equivalent thermal circuit model and heat balance equation, this research takes solar radiation as the heat source of the cable used in port microgrids and proposes a mathematical calculation method for the current-carrying capacity of shore power cables based on the Newton–Raphson method. The influence of wind and water speed, environmental temperature, and solar radiation on current-carrying capacity is compared and analyzed using this mathematical calculation method and simulation calculation method. Shore power cables exhibit higher ampacity in water than air due to water’s superior thermal conductivity. Maximum ampacity difference occurs at 0.17 m/s flow (26.8 A analytically) and 0.066 m/s flow (64.4 A simulation). Air-laid cables show amplified ambient temperature effects from solar radiation, while water-laid cables demonstrate near-linear ampacity variations (Δ40 °C: 0–40 °C temperature range). This research can provide a reference for the revision of the standard for calculating the current-carrying capacity of shore power cables and optimizing renewable-energy-integrated port power systems.