I see this all the time. The doorbell transformer is shoved into a 2 gang box installed high up in the first floor hallway, with the chime doing double duty as a face plate. Im not saying it's the right way to do it, but it seems to be getting passed in new construction around atlanta. It might just be one of those things the inspectors are so used to seeing they don't even think about it.
For what its worth, i don't often come across one's with signs of heat damage to the wiring in that box, unless its got a lighted push button. In the case of the nest that transformer will be providing a continuous load (just like for the lighted push buttons)...its not going to have a chance to cool down.
Another nest note. A lot of the AHJs around me, there's tons in the atlanta area, are letting us use the nest smoke/ carbon detectors in leu of hardwired, interconnected smokes. The direct wire nests only have a hot and neutral in the harness, and they will talk with the "10 year" battery ones. Its saved me alot if trouble on service upgrade/ replacements
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