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#18030 - 12/05/02 11:55 AM A.I.C.
Wirenuttt Offline

Registered: 11/10/02
Posts: 267
Loc: Massachusetts
Lets say you're wiring a new school. Main electric room calls for a certain aic rating on all the main switch gear. Does this aic rating have to follow suit throughout the building, to all the sub panels and breakers?
Is there a code?

[This message has been edited by Wirenuttt (edited 12-05-2002).]

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#18031 - 12/05/02 01:30 PM Re: A.I.C.
Redsy Offline

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
Wouldn't this be a case where "series-rated" system design would come into play?
I'm sure an informative thread could be generated on the subject.

#18032 - 12/05/02 01:41 PM Re: A.I.C.
Gwz Offline

Registered: 04/29/02
Posts: 199
Check 110.9, 110.10, 110.22 ( 2nd paragrpah ), 240.2 Current-Limiting OCP device, 240.83(C), 240.86.

Suggest you try the Bussmann site and look for their SPD download and their many other free software and bulletins.

#18033 - 12/05/02 01:49 PM Re: A.I.C.
Tom Offline

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
The AIC rating of breakers in downstream subpanels and the bracing of the busbars in these panels can probably be lower than what is required at the main. Inductive reactance will lower the available fault current.

Hopefully, the engineer did the analysis ahead of time.

If you want to play around with some short circuit calculations, visit the following site for free software downloads
Few things are harder to put up with than the annoyance of a good example.

#18034 - 12/05/02 02:05 PM Re: A.I.C.
electric-ed Offline

Registered: 07/08/02
Posts: 184
Loc: Canada
The use of current-limiting fuses in the main service disconnecting means can also limit the available fault current downstream.


#18035 - 12/05/02 04:38 PM Re: A.I.C.
Ron Offline

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
Current limiting fuses do not reduce the short circuit available at down stream equipment. They will limit energy for flash protection calculations, but not fault current for AIC or withstand ratings. The up-over-down method of determining let-through from the fuse curve as been found to be inaccurate due to "dynamic impedance" with downstream overcurrent protection. Similar reasons with a listed series combination between fuse and downstream breaker must be tested before it can be considered series rated.

#18036 - 12/05/02 04:44 PM Re: A.I.C.
Ron Offline

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
From the website:
Up-Over-Down Method:
With series rated combinations, the up-over-down method is not able to be used to determine the protection of newer
style circuit breakers with blow-apart contacts (which exhibit dynamic impedance). For these applications, listed
equipment which has been labeled for use with tested combinations must be installed. Despite the inability to predict
protection for circuit breakers using the up-over-down method, the method is still (as it always has been) valid for
static components (components passive within first half-cycle). Some applications which can utilize the up-overdown
method include:
1. Bus-Bracing Requirements
2. Wire and Cable Protection

Poster's note: It appears that a portion of equipment withstand ratings can use this chart method.


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