Load Ratings or Cable-II

Since the author's 1939 paper,¹ further data have been obtained regarding the loading of underground power systems having impregnated-paper-insulated lead-covered underground cable. The results of the studies may be summarized as follows: 1. The factors affecting the setting of maximum safe conductor temperatures are so numerous that no fixed value (or values) can be assumed as applicable to all installations of a given design and size of cable in this country. 2. For some cables, the present temperature limits for normal day-in and day-out operation may be safely exceeded, especially for wartime conditions. 3. For emergency operation safe temperatures may be even higher than listed in the previous paper, especially for wartime operation. 4. Operation in wartimes at special temperatures will mean in some cases substantial shortening of the life of underground circuits and an accompanying increase in service interruptions. 5. Wartime increases in usual maximum daily loading and in load factors may cause large increases in duct and copper temperatures, even if past current ratings are maintained. These temperatures will sometimes exceed those in the present standards. 6. The safe temperature for emergency operation, particularly for extra-high-voltage cables, may be limited by the joints. Also, for all kinds of cable sufficient room must be provided to avoid mechanical damage of cable or joints in manholes with cable movements incidental to emergency loads. 7.