Inter-array power cables are used to connect wind turbines to the collector and export cable. In the transition from turbine tower to sea, the cable is installed in a J-tube, which has an unfavourable thermal environment and can thus be the thermal bottleneck of the cable installation. To optimize cable installation and reduce CAPEX, improved transient ampacity calculations can be used to determine the dynamic rating. In this work FEM have been applied to calculate the ampacity of a three-core HV cable situated in a J-tube. It was found that by including the trajectory of solar influx, the maximum temperature increased above the admittable cable core temperature compared to the steady-state case. High cable loads will always coincide with wind and thus increased convective heat transfer. By increasing the heat transfer coefficient to a value corresponding to wind speed of 20 m/s at high power production and thus large current, it was found the highest core cable temperature decreased by 18 °C compared to the steady-state case. These more accurate ampacity calculations can be exploited by either increasing the admissible current in the cable by 17% or decreasing the cable cross section.