"Maximum" AIC vs. "Ultimate" AIC

[birddogger]

In obtaining available fault current for a 750kVA service being provided by a local utility, the utility rep told me, "maximum available fault current is 17,055 AIC, and the ultimate available fault current is 56,845 AIC."

I didn't ask what the difference was, thinking I could research it myself fairly easily, but information has been hard to come by so far. I've seen various definitions for Icu (ulimate breaking capacity) and Icw (short-time withstand current), but nothing has made clear to me which is the value to use in determining if our service-entrance switchgear is adequately braced. To me, it would seem that an "ultimate" fault current of 56.8kA would contradict the definition of a much-lower "maximum" fault current.

Any information would be helpful! Thanks.

 

[busbar]

 
The higher number could be based on the likely future largest transformer that would replace the existing transformer. The quoted 3X “spread” [56kA/17kA] can have a major influence on switchgear specifications [and price!] They may be doing extra ‘CYA’—for instance—later replacing the 750 with a 2,000-2,500kVA transformer.

The utility should define terms in an unambiguous manner.

 

[rbulsara]

not heard of the term..better ask the person who told you this clarify it...

One of the possibilities may be, he meant symmetrical vs. assymmetrical..(but that ratio usually is between 1.6 to 2.6), but why guess..ask!

 

[wareagle]

It sounds like the 17ka is the available fault current with the utiltiys distribution system impedance included in the caculations. The 57ka is the available fault using only the transformers inpedance. If the transformer has an impedance
of 3.6% then the available fault about 57ka at 208 volts.

 

[jghrist]

My first inclination was that of wareagle, but this would mean that the system impedance would be 8.6% on a 750 kVA base. This would mean only a 400A available fault on a 12.5 kV primary; much too low.

 

[RonShap]

I would agree with Busbar. It is a CYA from the utility in the event they upgrade the distribution on their side of the meter. You will have to go with the 56,845A number and determine the fault current at your equipment taking into account any impedances between the transformer and your equipment.

 

[birddogger]

I can't see the utility voluntarily upgrading the service on their side of the meter. This is a 750 kVA dedicated service for a single "big-box" retail store. The only way it would be upgraded is if the store owner (or some future tenant) requested it, at which point you should redo your fault calcs anyway.

I suppose the utility could tap off the transformer primary to install another service in the vicinity, but wouldn't this ultimately lower the fault current?

Thanks for the replies, they've all been interesting so far. I have a call in to the utility to ask him to clarify.

 

[RonShap]

If we assume that it is a 208V service, the theoretical infinite primary calculation for the 750kVA xfmr results in 36,248A. So I can see the possibility of a 17,055A value that they gave, as there is surely lots of impedance on their side of the meter back to the substation. So no matter how they may upgrade the utility side of the distribution in the street, which they constantly do, they couldn't exceed 36,248A on your side of the xfmr.

 

[birddogger]

I just got a callback from the utility. Ultimate fault current is the value if the service transformer is upgraded to the largest size available. Since this will not be happening any time soon, and since doing so would require an entire new service entrance switchboard (currently it is 1600A @ 480V/3ph), I see no reason to use anything other than the quoted max. available fault current to ensure that our gear is properly braced.

Thanks again for everyone's replies!

 

[mc5w]

A 750 KVA transformer is only about 56% of the maximum capacity of a 1600 amp 480 volt switchboard. ( I am assuming that the main breaker is 100% rated instead of 80% rated.) If your main breaker is rated 80% you need to allow for an upgrade to a 1,000 KVA transformer assuming that the sun is NOT beating on it. If the sun is beating on it then an upgrade to a 1.5 MVA transformer is not unreasonable.

Use the higher value of short circuit current. Also, you did not mention if the utility distribution is 2,400Y4,160 volts or some other crummy voltage that could be upgraded.

Mike Cole, mc5w@earthlink.net