Central air conditioning systems are designed to suite the size of your home and the amount of air it needs to cool or heat. The size of the specific AC unit, the condenser, is chosen to be efficient for 98% of the typical high temperatures in your area (climate). So for that 2% extreme, like we are having this summer, your AC system may actually be undersized. But this is by design. Otherwise, if you went with a larger system that had no troubles pumping the volumes of cool air needed during these extremely hot days, your system would be quite inefficient 98% of the time. And that’s really almost all the time – not a good thing.

The larger system costs more to install and it consumes more energy to run, all costing you more money. Because it pumps more air volume it cools the house much faster. You might think this was a good thing but instead it means the system is running through off and on cycles at a much higher rate. The shorter cooling cycles means it is not quite running long enough to be pulling humidity out of your home. Removing humidity is part of how an air conditioner works. The condensing coil condenses water out of the air and the heat exchange process during that activity is what creates the cooler air. Warmer air and moisture is then exited from your home. Dryer air, even when warmer, is more comfortable for the human body as it allows our internal air conditioning, evaporation of sweat, to be more efficient. Not removing sufficient humidity from your home could also lead to mold problems.

These are some of the reasons an over-sized AC system is just an all around bad idea. But you can still make the system you have work for you in these 100+ degree conditions.

Taken from http://www.cleartheairac.com/comfort-blog/comfort/running-your-ac-when-its-100-degrees-outside/