Main breaker replacement

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peter d

peter d

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New England
I have to replace a main breaker in a 1997 Eaton/CH BR panel. It has a 10K AIC rating. I've been looking online for prices and noticed that 25K AIC breakers are available in addition to the 10k's. I've also noticed that new Square D panels come equipped with a 22 K main. So, would there be any reason I couldn't use a 25K breaker? It's a standard 120/240 single phase service, not residential. I certainly understand fault current and coordination, but it doesn't seem like it would be an issue in this non-critical situation.
 
I agree with the others in that it would not be a problem to use a 22K main in the 1997 CH/BR panel. The S/C rating of the existing branch circuit breakers should be rated at 10K. I think you would have a problem if you installed a lower S/C rated breaker ahead of higher rated breakers.
 
iwire said:
It's fine to go bigger if you want.

Do you have reason to believe the fault current is over 10,000?
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Not necessarily but this building is served by a single 25 kva transformer that is fairly close to the service entrance. Granted, it's overhead and the smaller poco conductors will limit the fault current.
 
The available fault current at the panel will be less than 10,000 unless the transformer has an impedance of less than 1%.
 
peter d said:
I have to replace a main breaker in a 1997 Eaton/CH BR panel. It has a 10K AIC rating. I've been looking online for prices and noticed that 25K AIC breakers are available in addition to the 10k's. I've also noticed that new Square D panels come equipped with a 22 K main. So, would there be any reason I couldn't use a 25K breaker? It's a standard 120/240 single phase service, not residential. I certainly understand fault current and coordination, but it doesn't seem like it would be an issue in this non-critical situation.
Click to expand...
It the breaker in question a factory installed main breaker? If so it likely would have been a 22 or 25 kA unit. If it were a field installed back fed main the chance of it being a 10kA breaker is pretty high.

goldstar said:
I agree with the others in that it would not be a problem to use a 22K main in the 1997 CH/BR panel. The S/C rating of the existing branch circuit breakers should be rated at 10K. I think you would have a problem if you installed a lower S/C rated breaker ahead of higher rated breakers.
Click to expand...
Take a look at your typical loadcenter from any of the 4 major manufacturers. They usually have 22 or 25 kA main breakers, and the typical 10kA branch breakers that are used are series rated with that main, so you can have up to 22 or 25 kA available at the main but still use the 10kA breakers as branch breakers.

Unless it has very low impedance a 25 kVA transformer won't even have 10kA available at it's output terminals.

I used the same fault current calculator that is on Mike's site and put in an impedance of 2.0% which is probably close to what most of what you will find on pole mount utility transformers of that size and it comes up with 5208 amps line to line and 7813 line to neutral at the secondary terminals. I then added 10 feet of 1/0 aluminum conductor, which is a very short run for most applications and it dropped the available fault current at the end of that run to 4848 line to line 6141 line to neutral.
 
kwired said:
It the breaker in question a factory installed main breaker? If so it likely would have been a 22 or 25 kA unit. If it were a field installed back fed main the chance of it being a 10kA breaker is pretty high.
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Yes, it's the factory original, service has not been touched since it was installed other than some circuits that I added.

Take a look at your typical loadcenter from any of the 4 major manufacturers. They usually have 22 or 25 kA main breakers, and the typical 10kA branch breakers that are used are series rated with that main, so you can have up to 22 or 25 kA available at the main but still use the 10kA breakers as branch breakers.

Unless it has very low impedance a 25 kVA transformer won't even have 10kA available at it's output terminals.

I used the same fault current calculator that is on Mike's site and put in an impedance of 2.0% which is probably close to what most of what you will find on pole mount utility transformers of that size and it comes up with 5208 amps line to line and 7813 line to neutral at the secondary terminals. I then added 10 feet of 1/0 aluminum conductor, which is a very short run for most applications and it dropped the available fault current at the end of that run to 4848 line to line 6141 line to neutral.
Click to expand...

Thanks. :cool: I wish I hadn't thrown out my American Electrician's Handbook because it had a nice table in there with available fault current from poco transformers of varying imedances. Of course all that stuff is online now anyway.
 
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