Yes, it's probably too large. It doesn't run long enough to dehumidify.

We have 3 tons for 1800 sf. Our temp is 78, humidity is 47-49% and our power bill for May was $105. We're 15 miles from the Gulf.

Someone sized your system by the "rule of thumb" method instead of doing a proper heat load.
Have the system checked out anyway, a slightly warm coil will make matters worse.
Depending on the system and conditions, the blower speed may be reduced to help.

don b 1 -
You say your system is sized properly at 3 tons for 1800 sq ft. That works out to be 600 sq ft/ton. But then you say that her 5 ton AC is too big for her 2900 sq foot house. Guess what that works out to - 600 sq ft/ton. So how is her's too big, but your's is right size?

Jessica -
Yes, a proper load calc for your specific house is needed. But, depending on the age and construction of the house - and being in the south - 600 sq ft/ton isn't all that much out of line. Again, no one on here can say if your system is too big or right sized without knowing more about age/construction/location. How long are your system's runtimes? (along with the corresponding outside temperature). Where is your ductwork located? What type of house do you have?

My feeling on your situation is this. I think your main problem is infiltration. Either from leaks in the ductwork itself, or though leaky windows/doors/walls.

Another thing to check is if the the fan speed on the airhandler is too high - which will hurt moisture removal.

Michael

Good point Michael but square foot rules don't mean squat. The only way to know is do a proper Manual J analysis. Thumb rules and generic sizers say I need 4 tons. Man J says I need 3. Regardless, she has some serious problems. Even though her electric bill indicates she's cooling the entire neighborhood, I would think if her infiltration is bad enough to maintain 73% humidity, a properly sized system wouldn't be able to pull it down to 70° no matter how long it runs. Who knows?

I would also check for infiltration. Your crawlspace is no doubt vented. Warm air into a cool crawlspace = high RH. Make sure the water from the roof is getting AWAY from the house. My next house will have insulated/sprayed non-vented crawlspace.

I would never say you have too much AC from size alone. Whwere 300 sqft /ton is ok for desert regions, we have a house in Dallas that is 1100 sqft /ton and cools nicely. A buddy built a 2200 sqft well insulated house in 1984, and he has a 4 ton blower with a 3-ton coil. Hhe had to increase the blower cfm to achieve adequate air flow. The AC runs about 10 minutes /hour during hot weather.