AIC Rating if Unmarked

[beanland]

Installing a service rated ATS on an old 480Y/277V service. Main disco rated 800A. But, no AIC rating to be found on 40 year old switchgear anywhere. Does the NEC give guidance for determining breaker AIC requirement when no switchgear AIC rating is provided? My gut says "go low" and call if 10kA. Existing service transformer is 500kVA so 10kA is about right for the AFC from the transformer.

 

[charlie b]

I typically use 5.75% as the assumed impedance of a transformer, if I don't have the actual value. That would put the AFC at about 10,500 amps. So if you are looking to specify a minimum rating for the ATS, I would suggest calling for the next higher standard (above 10.5KA) rating for such equipment.

 

[GoldDigger]

UL gives some guidance, which you might not like. If no actual AIC test has been conducted on equipment, the manufacturer is allowed to specify an AIC of 5000 if they meet all of the other listing requirements.

 

[jim dungar]

UL gives some guidance, which you might not like. If no actual AIC test has been conducted on equipment, the manufacturer is allowed to specify an AIC of 5000 if they meet all of the other listing requirements.

The 5kA SCCR is true for 'control' type equipment which was typically built to UL508A standards. So I would use it for end use equipment.
I believe the minimum rating of non-supplemental molded case circuit breakers and other protective devices has been 10kAIC since at least the early 70's. I know 10kA is the rating for unfused disconnects.
A solenoid style ATS could be anything and might not even be as high as 5kA for its withstand rating.

 

[Jraef]

AIC = Amps Interrupting Capacity. For a device to have that, it must be a device designed for INTERRUPTING a fault, i.e. a fuse or circuit breaker. If the ATS is comprised of motorized circuit breakers, then it can have an AIC rating and 10kAIC would be the maximum if there is not other listing. But usually, the ATSs that are built out of CBs use standard off-the-shelf CBs and you can look up the AIC rating of those breakers.. AIC ratings have been standard on CBs for longer than 40 years.

But it is buult from solenoid controlled devices like contactors, it does not and cannot have an AIC rating, but it still must be suitable for use at the available fault current, basically what an SCCR is for. So as Jim said you cannot assume anything more than 5kA, often referred to as the "courtesy" rating of untested/ listed equipment.

 

[electrofelon]

Installing a service rated ATS on an old 480Y/277V service. Main disco rated 800A. But, no AIC rating to be found on 40 year old switchgear anywhere. Does the NEC give guidance for determining breaker AIC requirement when no switchgear AIC rating is provided? My gut says "go low" and call if 10kA. Existing service transformer is 500kVA so 10kA is about right for the AFC from the transformer.

I am not clear on what you are trying to do. Are you wanting to determine if the existing main breaker has sufficient AIC so that you can "bring it up to code" if its not?