Central AC tripping breaker

I have a rather old central ac unit. It wasn't working at all and it looked like the contactor was bad. I replaced the contactor and the unit started right up. It will run for about 45 minutes and cools fine. Then it trips the breaker. It cools fine while it is running. The compressor already has a hard start kit on it. I have a couple of questions
How can I determine weather the condenser fan or the compressor is causing the breaker to trip?
Will a dirty condenser /debris or refrigerant charge contribute to the breaker tripping?
What should I check from here?
Thanks for any help
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komobu wrote:

A similar thing happened to me. My son's neighbor, a midnight A/C technician, diagnosed the problem in a nonce.
CAREFULLY put a finger on the breaker (side of the breaker is best). If the breaker's warm, replace it.
$15 at the box store and everything is cool.
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Measure the current they are drawing. Also make sure the breaker is sized correctly.

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When ever I'm working on mine I also clean out the debris.
I'd start by replacing the breaker. They get weak with age and multiple trips. Then they trip at less than the rated current. I'd also check and tighten ALL the connections between the breaker and the compressor.
The fan motor is typically a small fraction of the current draw. The compressor is the bulk of it.
A low charge can cause problems. If it cools ok when running then it's probably fine. I assume you don't have gauges. The old school technique is the big copper pipe, which is the return, should feel like a beer can just out the fridge where it goes into the outside unit after it's been running for a little while.
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komobu wrote:

Capacitor checked?
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Many good suggestions already made. Also make sure that the condenser fan is always running when the compressor is. If the fan stops on internal overload, the head pressure will soar and the compressor will also go out on overload.
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You already got some suggestions on possible problems, but I have another route you may want to consider.
If that AC is very old, you may be able to replace it and save enough on the electric bill to pay for most or all of it. Start thinking about it while the AC is still running and you can take the time to get what you really need at a good price. Maybe a fall or winter job rather than the first hot day of next summer when prices will be higher.
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On Wed, 10 Aug 2011 04:52:33 -0700, komobu wrote:

Need a set of gages and an Amp Clamp. Or call someone who knows what they are doing.
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Baum is too negative, the earlier suggestions were all worthwhile following/investigating.
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