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Cable circuits installed in unfilled troughs must often support high current ratings. To achieve higher ratings in unfilled troughs in the U.K., trough lids can be replaced by ventilated grilles, provided that the trough is within a substation site. While several methods exist for rating the traditional covered trough design, no standard method exists for naturally ventilated installations. To examine the possible uprating available, a coupled numerical model has been created for cable trough installations. Following successful benchmarking tests where the covered trough was modeled, the method has been extended to troughs with full natural ventilation. The results have been compared to commonly used engineering assumptions in order to validate simpler analytical methods. It was found that by allowing full natural ventilation of existing covered troughs, the continuous rating could be increased by as much as 28%.