I have a situation where we have an existing 480/277V 3000A switchboard that we are adding two buckets to. Except for the 3000A main all over current devices are fused switches. We have the option of adding fused switch buckets or circuit breakers. The existing gear does not have a label telling us what AIC it is rated. The main is rated 100KAIC and the fused switch buckets are rated 200KAIC. The existing plans call for 100KAIC bus but state the available fault current at 42KAIC. I have not received the real available fault current from the utility yet.
My question has to do with the rating of the board. It appears to be rated 100KAIC. The available fault current will probably come in around 20KAIC. If we install 42KAIC rated breakers, are we violating anything in a remodel condition like this?

110-9 & 10 state that you must provide for the fault available at the terminals. The answer to your question is no, putting in 42k AIC breakers or fuses if the available fault current is 42 or less, would not violate the code.

The only question here is: Have the 42k AIC units been examined by UL or some other NRTL for use with the existing gear? The questions arise over mounting methods, positioning, etc.. If they have, you should have a compliant installation.

It is a question you may want to forward to the original designer. With luck, they did some sort of short circuit analysis at the time. Don't forget, in a large installation incoming AIC is only part of the story, in a fault, surge, etc., you do have the starting current of motors, inrush of transformers, and a large list of other items to consider.

Thanks for the response George. The breakers we are going to propose Will be reconditioned ITE breakers.(The gear is ITE Gould) One thing I didn't mention is both added breakers are required to have kirk key interlocks with the main. This is one of the reasons for using breakers as opposed to fused switches. The other reason is the physical size of the 800A switch. The largest existing bucket is 400A. The fact that we are proposing to install breakers by the same manufacturer as the gear should maintain any listing issues and if we have to get a third party certification, that is not a problem either. I think after I get the available AIC from the utility I will propose this to the engineer of recored and see how it goes.

BTW-There is not a huge motor contribution to fault current here. There will be some but the building is mostly office and wearhouse space.

Nick,
Didn't mean to throw a whole lot of "stuff" in the game. What you are proposing is quite sound, I was trying to cover all the bases though.

It's pretty rare when you throw off a circuit analysis, but you are adding enough breaker size for it to throw up a red flag with me.
How much A/C load is on this? Compressors are as bad/worse than reqular motors on occasion.

I wouldn't lose a lot of sleep over it, like I said, what you propose is very good and shows a lot of fore thought on your part.

Geeeez, did I recommend showing this to an engineer?????????? How many that know me BELIEVE I said that? And I've WORKED as an engineer.

The shame, the shame, sorry, I'm getting old, minds slipping.

If the available fault current is 42K, then you need to go to the next higher AIC value. The IEEE short circuit calculation method allows some simplifcations, and the results should always be used with a margin of error.
Although, not foolproof, a reasonable guess at the AIC rating of the gear, is to find the smallest rated OCPD, and use that value. If the main is 100kAIC, then it is fair to assume the board is too. If the board was not required to have a 100kAIC rating, than surely the gear manufacturer would have provided a smaller AIC rated main to save money. This method is not foolproof, but it is a fair assumption.
In addition, the engineer of record might have wanted to plan for future distribution modifications. Maybe parralled generators, redundant utility services with closed transition, etc.