Load center and circuirt breaker questions

I am adding an A/C to my garage and need to add a 240V circuit. The loadcenter is conveniently in my garage and has plenty of open spaces. However, I want to make sure I am not overloading it.
What are the rules for adding circuit breakers? The label on the loadcenter says 120Amp max. but it already has 12 20Amp circuit breakers in it. 12X20 = 240Amps which is twice the rating. So, I am a bit confused.
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It is more about what all you will have running at the *same* time. So you may have a 20 amp circuit with a 5 amp appliance running and a 15 amp circuit with a 1 amp something running, etc.
You may only be using a maximum of 20 amps say at *your* peak use times when you have several things running at the same time. Just figure out what the worst situation will be for running everything at the same time and if that plus the A/C will be more than the loadcenter can provide max.
I like to use the Thanksgiving Day example; family and friends visiting: Oven on, several burners on range going, all lights in house on, microwave going, several TV's going, stereo in kids room going, video games in another kids room going, hot water heater going, etc. The max use situation.
The total amperage of all breakers typically will exceed the total amperage of the panel. This is because you are not using the full amperage of every single circuit at the same time. The breakers protect the wiring and prevent the wiring from being overloaded. The main breaker to the panel prevents the panel from being overloaded. People do not typically have *everything* on at the same time.
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The max rating of the panel is nonsense unless the panel was fed with 125 amp wire. Check the wire feeding the panel and the breaker protecting the wire...
Do a load calculation on the feeder to the panel including the new load. Ya also may want to do a voltage drop calculation for the panel with the new load connected.
http://www.electrician.com/vd_calculator.html
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