# Industrial AIC calculation



## jdkars (May 20, 2011)

I am trying to calculate the AIC for the installation of a new 480delta 800 amp square d i line panel. We have a standard 35kva transformer main fed to a 480volt GE switchgear 4k amp. Would anyone be able to advise me on what/where i need to pull the information for this calculation? I have several electrical fault current calc forms just need to know if i need to go back to the utility comp to get their fault limits or if i can pull info from the switchgear?


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## HARRY304E (Sep 15, 2010)

jdkars said:


> I am trying to calculate the AIC for the installation of a new 480delta 800 amp square d i line panel. We have a standard 35kva transformer main fed to a 480volt GE switchgear 4k amp. Would anyone be able to advise me on what/where i need to pull the information for this calculation? I have several electrical fault current calc forms just need to know if i need to go back to the utility comp to get their fault limits or if i can pull info from the switchgear?



The Info will be on the switch gear,,,,But you also want to get the info from the POCO as well...



Welcome to the forum...:thumbup::thumbup:


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## McClary’s Electrical (Feb 21, 2009)

HARRY304E said:


> The Info will be on the switch gear,,,,But you also want to get the info from the POCO as well...
> 
> Welcome to the forum...:thumbup::thumbup:


Harry, how could the numbers be on a new switchgear if nobody has calculated it?


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## pjmurph2002 (Sep 18, 2009)

Start with the utility company. What is the AIC rating of the XFMR feeding the service.


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## micromind (Aug 11, 2007)

Something doesn't make sense here; I've connected a lot of transformers, but I've never heard of a 35kva one. If it exists, its full-load current would be 42 amps. Is the transformer possibly fed from a 34.5KV line?

If you have access to the transformers nameplate, you can determine what the absolute maximum current it can deliver into a bolted fault. You'll need to know the full-load current of the transformer and the impedance. Simply divide the current by the impedance (expressed as a decimal).

Example; 2000KVA transformer operating at 480V 3ø with an impedance of 6.4%

2,000,000 ÷ 831 (831 is the VA per amp of a 480V 3ø system) = 2407. This is the full-load current of the transformer. Now; 2407 ÷ .064 = 53,234 amps. This is the current that the transformer would deliver to a bolted fault across its secondary bushings with full voltage applied to the primary. 

The actual current will always be lower because there will be less than full voltage at the primary bushings and there will be some impedance from the secondary bushings to the breaker. 

This is an easy calculation that might result in not having to up-size the AIC of a breaker. In the above example, if the breaker had an AIC rating of 65,000 amps, there is no possible way for the system to exceed the breakers rating. If the breaker was rated at 42,000 amps AIC, the system very likely would exceed it. 

If the system cannot exceed the rating of the breaker, there's no need for any additional study. If it can exceed it, a study is certainly in order. 

Rob

P.S. Around here, the POCO will give the the maximum fault current that their system can deliver to the switchboard terminals free of charge. We just have to ask, and it's even in writing. I always give it to the building department to file along with the permit and inspections. That way if there's ever a problem later on, I can prove that the system was ok as it was originally installed.


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## nitro71 (Sep 17, 2009)

Thanks for that write up micromind. I just learned a lot about calculating AIC. Thanks!


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## NolaTigaBait (Oct 19, 2008)

nitro71 said:


> Thanks for that write up micromind. I just learned a lot about calculating AIC. Thanks!


Mike Holt has a cool calculator for aic rating also...


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## don_resqcapt19 (Jul 18, 2010)

NolaTigaBait said:


> Mike Holt has a cool calculator for aic rating also...


You don't calculate an AIC rating. That is a function of the equipment design and is the maximum amount of current that the device can interrupt. You calculate the available short circuit current.


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## NolaTigaBait (Oct 19, 2008)

don_resqcapt19 said:


> You don't calculate an AIC rating. That is a function of the equipment design and is the maximum amount of current that the device can interrupt. You calculate the available short circuit current.


Sorry thats what I meant...


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