"Maximum" AIC vs. "Ultimate" AIC
"Maximum" AIC vs. "Ultimate" AIC
(OP)
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.
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.






RE: "Maximum" AIC vs. "Ultimate" AIC
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.
RE: "Maximum" AIC vs. "Ultimate" AIC
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!
RE: "Maximum" AIC vs. "Ultimate" AIC
of 3.6% then the available fault about 57ka at 208 volts.
RE: "Maximum" AIC vs. "Ultimate" AIC
RE: "Maximum" AIC vs. "Ultimate" AIC
RE: "Maximum" AIC vs. "Ultimate" AIC
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.
RE: "Maximum" AIC vs. "Ultimate" AIC
RE: "Maximum" AIC vs. "Ultimate" AIC
Thanks again for everyone's replies!
RE: "Maximum" AIC vs. "Ultimate" AIC
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