# Maximum Interrupting Rating (AIR) Main



## paulgarett (May 8, 2012)

What is the standard Maximum Interrupting Rating for Main Breakers. The electrical distributor sold me a 10,000 AIR main breaker. But the sub panels main breakers are 22,000 AIR. What is the standard interrupting rating for utility transformers. Should the main breaker AIR be sized larger than the sub panels breakers AIR? A properly selected system seems just as important as load calculations.


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## HARRY304E (Sep 15, 2010)

paulgarett said:


> What is the standard Maximum Interrupting Rating for Main Breakers. The electrical distributor sold me a 10,000 AIR main breaker. But the sub panels main breakers are 22,000 AIR. What is the standard interrupting rating for utility transformers. Should the main breaker AIR be sized larger than the sub panels breakers AIR? A properly selected system seems just as important as load calculations.


That depends on you POCO but it usually 10,000 But you must verified with them.:thumbsup:


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## BBQ (Nov 16, 2010)

It depends, as Harry mentioned 10k is the norm for small services but don't count on it. 


Now more details are needed, are thes subs the same voltage as the main or are they supplied via transformers?


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## McClary’s Electrical (Feb 21, 2009)

Around here, Dominion guarantees a SFD to be under 10,000. But that means nothing in commercial.


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## LBC Jesse (Apr 26, 2012)

Would NEC110.10 & NEC110.9 help point you in right direction? (once the POCO's AIC/R is determined.. )... 
I'd also like to tag the OP's question with another.. does the distance between the transformer and the first main breaker in line make a difference of the proper AIC/R rating of that main?


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## HARRY304E (Sep 15, 2010)

LBC Jesse said:


> Would NEC110.10 & NEC110.9 help point you in right direction? (once the POCO's AIC/R is determined.. )...
> I'd also like to tag the OP's question with another.. does the distance between the transformer and the first main breaker in line make a difference of the proper AIC/R rating of that main?


I do not think it would make that much difference ,But it should always be checked out with the local POCO.:thumbsup:


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## don_resqcapt19 (Jul 18, 2010)

LBC Jesse said:


> ...
> I'd also like to tag the OP's question with another.. does the distance between the transformer and the first main breaker in line make a difference of the proper AIC/R rating of that main?...


 The impedance of the conductors between the transformer and the service disconnect can reduce the available fault current significantly in some cases.


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## mbednarik (Oct 10, 2011)

there are several calculators available online and for your phone. I usually put infinite for the utility system primary and go from there. 90% of the time 10 kaic is fine. Utilities here require 22,000 for anything larger than a 100 amp service anyhow. The phone app is nice because you have to label anything other than a resi service anyhow.


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## thoenew (Jan 17, 2012)

don_resqcapt19 said:


> The impedance of the conductors between the transformer and the service disconnect can reduce the available fault current significantly in some cases.


Metal or pvc conduit also affects the AIC.


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