The rating of cables for intermittent or fluctuating loads

The extent to which the rating of a cable is affected by an intermittent or fluctuating load as compared with continuous running conditions is investigated theoretically, and formulÿ are given by means of which the effect of any type of loading can be calculated. It is shown by experimental determinations on various sizes and types of cables that the thermal time-constant can be calculated either from a heating or cooling curve of a particular cable, or from the specific heat and mass of the various components of the cable. Tables are given showing the rating for the cables for which load tables are given in the I.E.E. Wiring Rules, on the same basis as for motors, i.e. ¿-hour and 1-hour ratings.