The practical advantages of graphical methods for determining current-carrying capacities of cables are indicated, and charts are described by which the ratings of 3-core and single-core cables transmitting balanced 3-phase loads can be rapidly obtained. The charts are applicable to cables laid direct in the ground or in air, and take into account, where necessary, the mutual heating effect of feeders laid in the same trench.In order to exhibit the degree of accuracy obtainable, calculated and graphically determined values of rating are compared For a range of typical cases.