If your AC is accidentally wired to the 15amp breaker for the water heater, and vice versa, then, yes, that could be your problem. Typically the AC units are wired to 20amp breakers, even though they might can run successfully on a 15amp circuit. What might take them over the 'edge', though, is that fact that you are running it during extreme heat - which means that the wire itself is already at an 'above average' ambient temperature, and can easily add to the 'tripping' of the breaker. You might otherwise not even have realized this had the AC been running in lesser temperature.
Another thought: some of us have had individual AC 20amp breakers tripping, even though they are certainly sized correctly. In high heat situations, as above, AND if the wire exiting the breaker is not 'tightened' correctly(or otherwise is loose), the AC may run fine for a time, then suddenly trip the breaker for no other reason. The wire has become HEATED, and the expansion/contraction of the wire can make it lose it's connection, or otherwise 'short circuit' the breaker, causing it to trip.
This happened to us on our first outing, in 99degree temps in central Texas. The front AC unit worked fine, while the rear would trip periodically. The wire exiting the breaker was loose. I simply tightened it...No problems since.
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