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.


"User Example" wrote in message
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.