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Connection of the Dinorwic pumped storage station in Snowdonia to the 400 kV supergrid is to be made by two 400 kV cable circuits rather than by overhead lines because of the outstanding natural beauty of the area. The cables have 2500 mm2 copper conductors, oil-filled paper insulation and aluminium sheaths and will be cooled by circulating water in high density polyethylene pipes laid with the cables. Part of the cable route chosen runs along a narrow strip of land adjacent to the lower lake, and the system has been designed to transmit the peak demand of 2340 MVA through two circuits occupying a width of 2.1 m. The paper describes the design of the cables and of the water-cooled accessories and the tests to which they were subjected. The cooling-station designs are also described and the special methods of laying and jointing the cables in the restricted space available.