Application of apparatus and conductors under various ambient-temperature conditions

Various electrical standards are reviewed from the viewpoint of the selection and interpretation of ambient temperature values. Equivalent aging temperatures for insulation have been calculated from recorded ambient temperature data for several typical outdoor and indoor locations. From these the suitability of ambient values now used for the establishment of temperature rises for rating purposes is discussed. These values seem to be well chosen, but some clarification in meaning and in methods of using them for rating and application purposes is desirable. For apparatus rated on a 40 degrees centigrade ambient basis, appreciable margins in permissible temperature rise exist under many conditions and for many places of application. Permissible increases in loading for certain motors without exceeding conservative hot-spot temperatures are suggested, subject to the limitations imposed by other operating considerations.