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 Post subject: Short Circuit Study
PostPosted: Thu Apr 26, 2018 3:33 pm 

Joined: Thu Apr 26, 2018 3:15 pm
Posts: 3
I am doing a short circuit study for a new system where don't know what type of loads are finally connected to panels. In one particular panel I am getting 32.33 KAIC, and I believe this is due to the feeder cable size and the breaker upstream, 400A. Is it save to assume that the interrupted current won't be more than 32.33 KAIC, and thus buy a panel with a 35 KAIC RATING?
Thanks.


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 Post subject: Re: Short Circuit Study
PostPosted: Thu Apr 26, 2018 4:49 pm 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 514
Location: Wisconsin
JulioR wrote:
Is it save to assume that the interrupted current won't be more than 32.33 KAIC, and thus buy a panel with a 35 KAIC RATING?
Thanks.


Do you want the salesperson's probable response, or the power systems engineer's response?

When dealing with molded case breakers, the upstream protective device does not impact the downstream available fault current, unless you are looking for series ratings.

What assumptions have been made in the determination of the available fault current?
Was your source fault current an infinite bus value?
Have you made adjustments for the X/R ratio in your calculation versus the value of the breaker's 'listing' test?


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 Post subject: Re: Short Circuit Study
PostPosted: Thu Apr 26, 2018 5:13 pm 

Joined: Thu Apr 26, 2018 3:15 pm
Posts: 3
Thanks JBD for your response.
It was assumed to have infinite bus and the main transformer was size up as per owner requirements.
The power engineer would be better.
In three panels I am using series breakers.
Thanks.


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 Post subject: Re: Short Circuit Study
PostPosted: Mon Apr 30, 2018 6:15 am 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 498
Location: New England
Are there motor loads that would add to the fault current?
If no, and you used an infinite bus, you would be ok to go with the 35KAIC panel and breakers.
As an approximation, you would probably drop about 10% on the fault current if you used a finite bus.
Your other consideration is that AIC is RMS value, not asymetrical peak. But the pf used in the RMS value is quite low and most real world conditions would not exceed it. Remember also, the panel is probably rated to 65 or 200K, as the manufacturer usually makes one type for its highest rated breakers, and only the breakers change. Label the panel with min AIC permitted. I don't use the calculated value, I use the breaker value. So if you calc 32KA, and the next higher breaker range is 35K, then label it as 35K.

Note also you have to be confident that you won't be increasing fault current later with site modifications. But if not, then there is quite a cost difference in breaker price as you move up the AIC value.


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 Post subject: Re: Short Circuit Study
PostPosted: Mon Apr 30, 2018 8:22 am 

Joined: Fri Aug 26, 2011 7:38 am
Posts: 32
Location: Baltimore, MD
Your figure is very close to 35 kA. It's not that hard to image changes to the electrical system that would cause the available fault current to increase. If I were you (based on the information previously posted) I would go one level up on the AIC rating, just out of engineering conservatism.


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 Post subject: Re: Short Circuit Study
PostPosted: Mon Apr 30, 2018 11:56 am 

Joined: Thu Apr 26, 2018 3:15 pm
Posts: 3
Thanks Hurwitz, JBD and Haze10 to help me clarify this issue.


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 Post subject: Re: Short Circuit Study
PostPosted: Wed May 02, 2018 7:10 am 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 514
Location: Wisconsin
Hurwitz wrote:
Your figure is very close to 35 kA. It's not that hard to image changes to the electrical system that would cause the available fault current to increase.


I am curious as to what changes you imagine are likely to cause an increase in the fault current?
According to the OP: the source was assumed to be an inifinite bus (it is hard to go higher than that).The source transformer in the study was also assumed to be the next larger size so likely future growth has been included. this is a 400A panel, it would be hard to load it with more than 200HP of motor. So it is difficult to envision a fault current increase greater than 2000A due to motor contribution. We were not told but I an assuming this is a 480V service.


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 Post subject: Re: Short Circuit Study
PostPosted: Wed May 09, 2018 8:24 am 

Joined: Fri Aug 26, 2011 7:38 am
Posts: 32
Location: Baltimore, MD
JBD wrote:
Hurwitz wrote:
Your figure is very close to 35 kA. It's not that hard to image changes to the electrical system that would cause the available fault current to increase.


I am curious as to what changes you imagine are likely to cause an increase in the fault current?
According to the OP: the source was assumed to be an inifinite bus (it is hard to go higher than that).The source transformer in the study was also assumed to be the next larger size so likely future growth has been included. this is a 400A panel, it would be hard to load it with more than 200HP of motor. So it is difficult to envision a fault current increase greater than 2000A due to motor contribution. We were not told but I an assuming this is a 480V service.


It is a new system. The designer is making assumptions about the length of the feeder and conductor size. During construction, the contractor could (unlikely, but still) use larger wire than the design. More likely is that the locations of the panel, and the upstream distribution panel, change so the feeder is shorter. It is also possible that the contractor installs the feeder in a straight line, literally "cutting corners".


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