Choosing a breaker based on its trip setting? Careful. You might need to make some adjustments . . . .
When specifying a new breaker, pay attention to its temperature ratings, both its termination ratings and the ambient temperature it’s calibrated to.
If a breaker is operating in an environment with a higher ambient temperature than it was calibrated for, then its trip setting will likely be lower than what it’s marked at. A 15-amp breaker calibrated for a 40ºC ambient temperature might trip at 13 amps in a 50ºC environment. The opposite is also true: it might trip at 17 amps in a 25ºC environment.
These are real-life examples I pulled from a Siemens datasheet. You can take a look for yourself here:
The breaker’s terminations may be rated less than a selected cable’s insulation, resulting in the need to derate your cable ampacity.
Also, remember this: if the equipment the cables terminate to isn’t specifically listed as higher than 60ºC then you most likely, in the absence of better information, must limit your cable’s ampacity to its 60ºC values [see NEC 2014, Article 110.14(C)(1)(a)(3)].
Engineers get snagged here. Just because a piece of equipment is rated for 60ºC (or whatever) doesn’t mean that you must limit yourself to starting with the 60ºC ampacity ratings for a cable that is rated for 90ºC. You can start your derate using the 90ºC ampacity.