Abridgment of the calculation of cable temperatures in subway ducts

Underground transmission and distribution circuits are being continually extended, and large amounts of new capital invested each year in the conduit and cable systems which this extension requires. More and more frequently the cable engineer is called upon for accurate information as to the maximum loads that may safely be impressed on underground cables without exceeding the temperature limits recognized in present day practise. The importance of the study of cable temperatures has long been recognized, and much excellent material has been published concerning its several phases. In particular, the copper temperature rise of loaded cables above the surrounding air has been fully analyzed. Less thorough treatment, however, has been given the determination of the temperatures existing in loaded duct structures, and to the behavior of cables carrying cyclic loads. It is the purpose of this paper to present a comprehensive survey of the whole problem of cable temperatures by consolidating information on cable temperature characteristics with the equally important problem of the characteristics of duct structure when the cable circuits are subjected to the cyclic loads usually found in practise. The information herein presented on the subject of duct bank temperatures is the result of an investigation in the metropolitan district of New York, carried on for several years under widely varying conditions. The constants given for certain types of duct structures may not be universally applicable to other localities, but it is believed that the method of attack will be helpful to other engineers in analyzing test data obtained upon their systems and in separating the various factors involved.