Switchgear will perform satisfactorily and have a reasonable life when operated within the temperature limits established in American National Standard C37.20-1969. 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 capacity of outdoor metalclad switchgear. American National Standard C37.20-1969 specifies the temperature rise above a standard ambient temperature of 40°C. This is satisfactory for indoor applications where the temperature rise is due entirely to heat release. However, when the gear is located outdoors, several additional factors require consideration. The effect of sun, wind, ventilation, ambient temperature, etc., on internal temperatures will be considered in this Guide. The magnitude of these factors will vary geographically and from season to season. The time relationship of maximum circuit-breaker 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 will be made to point out the cumulative effect of these various influences.