By R. (Dick) Hardie Pr Eng
Technical Marketing Manager
Aberdare Cables (Pty) Ltd
Good engineering design has always incorporated factors of safety. For mechanical design, the maximum design load would typically range from 25% of the ultimate tensile strength (U.T.S.) of the material, up to perhaps 70% of the U.T.S. In civil engineering, the Factors of Safety are sometimes even more conservative, for example in the design of concrete structures. A similar philosophy is required when selecting electrical components and equipment in order to design reliably, thus preventing overheating and eventual failure. In so doing, reliable engineering design implies that the equipment will never be called upon to perform beyond its rated capacity.
CURRENT CARRYING CAPACITY OF ELECTRIC CABLE
The ACTUAL current rating of an electric cable is based on the thermal environment in which the cable is installed. In particular, the ambient temperature, depth of burial in the ground, presence and spacing in relation to other cables or other heat sources, and type of soil will have a profound effect on the actual current rating of an electric cable. After the application of applicable derating factors (see tables below) the cable’s actual current rating will usually be lower than the standard (un-derated) value quoted by manufacturer’s brochures.
To assist in determining the rated capacity of motors, transformers and cables, there are a number of formulae, charts and tables available from the equipment manufacturer. These allow compensation for factors which may enhance or detract from the rated capacity of the equipment. In the case of electric cables, manufacturers quote and publish tables of current ratings which are based on standard conditions of installation, not taking any of the derating factors, mentioned above, into account (ie: un-derated; based on standard conditions of installation).