When selecting breakers I sometimes see 40° C stamped on the face. What does that mean? Is it related to 60 or 75° ratings for conductors and other components?
While wires, cables, service panels, and other parts are often rated for 60° or 75° C, this describes their maximum intended temperatures while in use. It indicates the capability of the insulation, for example.
The 40° C indication on a circuit breaker, however, refers to its ambient air temperature test conditions, or the maximum room temperature wherein this breaker should be used.
When testing trip temperature curves, this maximum ambient temperature is provided local to the breaker. Because the cooling effect of the local air has an impact on current capacity and trip behavior for a given breaker, operating in higher ambient temperatures can result in nuisance trips.
In short, it means don't use this breaker if the air in the room will regularly be over 40° C. 40° C is about 104° F, which is adequate for most residential and commercial situations. Industrial applications or those in extremely hot climates may need higher ratings.