I would probably inspect all electrical connections. Somewhere you either have an excessive load or a poor connection. A hot day will add to the resistence of the system. Resistence equals amps, which will cause the breaker to trip.

A compressor that is overcharged can cause the breaker to trip on a hot day. A fan motor that has a bad capacitor can cause the breaker to trip. A weak breaker can also trip. I have found poor connectors between the wire from the pole and the house feed wire to cause a voltage drop, which affects everything in the house.

Finally, if the wiring to the condenser is too small, it can cause the breaker to trip.

What else is running when the main breaker trips? Is this a new installation? How large is your electric service? Does it trip when the air is on, dryer is running, four burners and the wall oven and the vacuum and then you turn on the pinball machine?
There is either too large an electrical load or the breaker is bad.

When was the last time you had the system serviced? Chances are its something like a plugged up condenser coil in a low end unit that doesn't have any hi or low pressure controls, or badly corroded or loose terminals. Many times the compressor terminals are overlooked when checking electrical connections and they do get loose.
If its been a while have your system serviced first before you go calling an electrician.

Thanks for your replies...
The HVAC unit is 9 yrs old.

when the breaker tripped the microwave was running once, other time the dryer was running.. all other times it was the A/C and probably TV was running.. no other appliances were running..

I changed the breaker for HVAC (its a 30 amp breaker) and it still trips..The main breaker outside is 100 Amp.

I'm calling a HVAC contractor tommorrow for servicing. what should I ask him to check for ? are checking AMPS or leaks part of the service ?

I finished my basement last fall and also ran a seperate line for microwave(both done by a contractor). but after this I have used my heating, washer, dryer, microwave etc. simultaneously. never tripped.

thanks for your input guys..

How much would it cost on an average to service a heating & cooling system ? I live in Central New Jersey.
Thanks..

I agree- have the a/c checked out first.
The 100 amp breaker may be going bad and the stabs it connects to in the panel may be corroded some. If there is a loose connection or corroded stabs, this will overheat the breaker and sometimes slowly cook it to the point it needs replacing.