Utility AIC exceeds MSB Rating

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Bussmann has a free app to do the short circuit calculations for the length, number of conductors and type of raceway.
 
woods95662 said:
I have a situation on a project. The engineer missed the AIC from the utility is 68200 and the MSB is rated for 65k. The gear can't be fixed without being rebuilt. A few questions. Does the utility use the length of the secondaries for their calculation? If so then couldn't we increase the length of the secondaries to reduce the AIC?
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What type of breaker is your main?
 
As someone posted earlier that fewer than 50% plans call to "verify". As another point of talk I have been told by one supply house when I specified the fault current and series rated , Ya know you are one of very few who asks for this.

Was told many of his customers don't even specify equipment greater than 10k.

I also had a replacement of a residential panel where the whole panel flashed over inside. I thought it was a bit excessive for a 100 amp residential panel. I contacted the utility for fault current prior to installing a new panel as I thought the Fault current was higher than 10k as the conductors were very short. I got the answer " all Residential is no higher than 10k.
I ran my own calcs from the calculator tool and I found it to be 14k. I installed a 22k rated panel. If I recall it just had a 22k main and series rated branch at 10k.
 
I recently had a similar problem; 48kA AFC (with motor contributions added), 42kA rated MCCs. We were going forward with adding air core reactors ahead of the mains, when someone suggested reviewing the details of the available fault current determination. Sure enough, it was done by using an infinite bus calc, not the actual impedance. When we had it redone with the actual impedance, it came out to 41.2kA on one, 39.8kA on the other. Word to the wise...
 
I just had the engineer run a short circuit analysis based on our secondary feeder length from the utility transformers to the MSB and it came in at 64,787. He stamped and signed the calc so we should be good to go.

Lesson learned I will never release gear again without reviewing the utility commitment drawing even when it is not my responsibility. I always check it when I'm spec'd to do so or design build but I was not in that loop on this go around.
 
woods95662 said:
I just had the engineer run a short circuit analysis based on our secondary feeder length from the utility transformers to the MSB and it came in at 64,787. He stamped and signed the calc so we should be good to go.
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Are there any motor loads, and, if so, did the engineer consider the motor contribution to the total available fault current?
 
don_resqcapt19 said:
Are there any motor loads, and, if so, did the engineer consider the motor contribution to the total available fault current?
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I'll have to ask. All I'm trying to do is get the city inspector to sign off right now. He wouldn't because the utility AIC at the utility transformer was higher than the MSB. Now that the engineer has provided his fault current taking the secondaries into account i don't see any reason why he wouldn't.
 
jim dungar said:
But, don't forget all devices (NEC 110.9) and equipment (NEC 110.10) must be rated for the available fault current, not just the main device.
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Yes but that gear should be designed to accept that breaker so it should have sufficient fault current ratings for it. Moot point now.
 
I don't agree, 64,787A is essentially 65kA, and accounting for error in the calc, would never accept it. With a 4000A amp service, it is hard to believe there isn't some sort of motor contribution from the system feeding back onto the main, thus putting you over.

The utility is going to give you the maximum available on the system, based on future (long-term) planning says it will be. This number can't be argued with; it is what it is. Reducing the size of service will reduce the short circuit current, IF the transformer ahead of the main is also downsized.

Calcs can be done to verify the thru fault current of the devices. It is a fallacy that every device has to be rated the same; they only need to be capable of handling the KA through them; assuming calcs are done, but with that said typically only done on MV breakers/buses.

Check with the factory and find out what the gear is actually rated for, not what is on nameplate. Often times, to save on production costs, equipment has nameplate with lower rating than what they could physically handle. If so, you can pay a fee for the factory to come out and re-nameplate.
 
Out of curiosity, have you looked at the downstream equipment ?
If the 65k was not taken into account on the gear it is probable that you have some equipment connected that has a withstand rating of 5K or 10K.
Happens here way too often.
 
The utility actually re did their calc. Originally they didn't have nameplate info and the transformer locations had changed several times. Once they re did the calc with nameplate information and adjusting a few other factors it came down to 44k.

The gear bracing was maxed at 65k so other than rebuilding or replacing it there was no other option.
 
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