I've got a 120/240 600A panel with a 65KAIC rating. There's a 65KAIC 3P 150A breaker feeding a 20HP compressor. Can I use a breaker with a 10KAIC for the compressor?
The AIC rating of any breaker is simply the absolute maximum current that it can safely interrupt.
To determine what rating to use, you must know the short-circuit ability of the service. This information is usually available from the POCO. They will tell you the maximum current their system can supply at the meter terminals.
In all likelyhood, the chances of a 120/240 single phase service being capable of more than 10,000 amps is rather slim. Even a big service.
The panel stated 65,000 amps because that is the rating of the bus. It has nothing whatsoever to do with the capacity of the supply.
If this is indeed a single-phase system, and it's driving a 20 HP motor, there must be a phase converter. If it's a 120/240 3 phase system, then it's a delta. There is a much greater possibility of the high leg of a delta system being able to deliver more than 10,000 amps.
A 65,000 AIC breaker is EXPENSIVE!! It's actually pretty rare to need such a high AIC rating even on a 480 volt breaker.
If this is a single-phase panel, it wouldn't bother me a bit to use a 10,000 AIC breaker. If it's a 3 phase delta, I'd need a bit more info to form an opinion.
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