I have dual a/c units to control the upstairs and downstairs of my residence. I have isolated my problem of no cold air to the breaker panel. I need help identifying what the possible reasons may be for the breaker tripping. There are two breakers (one coming directly from the a/c units with a breaker for each of them, they leave that box and go to the main house breaker panel with a single breaker controlling a/c to both units). When the breaker is reset for the unit upstairs on both panels the upstairs unit works properly and cools. When I reset the breaker for the downstairs unit (and the upstairs a/c compresser is running) it immediately trips the breaker on the house main panel AND the breaker for the downstairs unit. Then both condenser units stop working. I can then reset just the upstairs again and it works again. So, the downstairs unit is causing the problem. What can I check for to find the problem of this? I see no signs melting or blackening at the panels.

It is possible the downstairs unit is drawing excess power because it is failing. Best way would be to use a clamp on ammeter to check the power the unit is drawing. If it is higher than the nameplate says, chances are, it is going bad. Otherwise, the breaker, wire size, and nameplate load all need to be rated the same. It is OK to have oversized wire. If the breaker is too small for the load, but the wires are big enough, you can replace the breaker with a bigger one. If the breaker is tripping at less than its rated load, it is bad, and you can replace it with a new one of the same rating.

I got an electrician to check it out and found the capacitor went bad. The capacitor was causing a short and making the breaker trip. $16 later and it's fixed. :-)