I am adding a breaker for a 24000 BTU AC/heat pump. What size breaker would that be?
The smartest choice would be what's recommended by the manufacture; which will be the "only" right answer.
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Check the amperage rating on the nameplate of the unit your installing.
You also need a breaker the 'trip curve' of which handles compressor [motor] loads, otherwise you might need a 'soft start' kit.
On the side of every unit is a "Minimum circuit ampacity" and a "Maximum fuse or breaker size".
You size the wire to the former and the breaker to the latter.
DO NOT guess at this. BTU ratings are irrelevant.