Rules of thumb are riddled with qualifiers.
Here in Dallas,Texas residential a/c is going to be 400- 500 ft² per ton.
How well insulated,how much glass, the exposures, the outside design temp. all figure in.
The link given above offers a heat/cool load calculation software package for homeowners at a very reasonable price and Don supports it well.
If in doubt, run the numbers and see what comes up. Too much a/c can be worse than too little.
[This message has been edited by wolfdog (edited 01-01-2003).]
