Re: Adding central air load to service
You either have to calculate the load on the entire house (using the link from Infinity is one way to do that) or you may connect a load monitor and measure the peak load over a 30 day period. You can also use one year's worth of utility bills to get the peak load over that period.
In Washington State, there is a formal process for using measured data as part of a load calculation. The basic method is to take the highest reading over the time period, add 25%, then add your proposed new load. You might need to make a "seasonal adjustment," if your example you took your 30 day readings during the fall and the house has all-electric heat. If the final result is within the rating of your equipment, then you will be given a permit to perform the work.