The purpose of this guide is to provide a reference for engineers to use to enable switchgear to perform satisfactorily and have a reasonable life expectancy when operated within the temperature limits established; for example, in the following standards: — IEEE Std C37.20.1™ — IEEE Std C37.20.2™ — IEEE Std C37.20.3™ — IEEE Std C37.20.9™ — IEEE Std C37.21™ — IEEE Std C37.23™ — UL 347 — UL 845 — UL 891 — IEEE Std C37.04™ These standards specify the temperature rise limits above a standard (maximum) ambient temperature of 40 °C. This is satisfactory for indoor applications where the temperature rise is due entirely to heat release (internal losses). In outdoor applications, the limiting temperatures result from the net effect of internal losses and external influences, principally the sun, wind and local ambient temperatures. All of these must be considered in determining the current-carrying capability of outdoor switchgear. The magnitude of these factors will vary geographically and from season to season. The time relationship of maximum circuit loads with respect to maximum ambient temperature is important. It is not practical to design switchgear on the basis that all adverse factors reach their maxima coincident with maximum loads. If this does not occur, full current ratings may be realized. Recommendations are made to point out the cumulative effect of these various influences.