A general method of calculating the temperature rise in multiconductor rubber insulated electric cables based upon the thermal constants of the cable has been derived. The predictions of the theory have been verified experimentally for a number of cables and values of the thermal resistivity constants have been obtained as a function of size and composition of the cable. The results are used to predict the maximum safe current for different size cables to limit the temperature rise at the center to 25°C above an ambient temperature of 50°C.