Availible Fault Current Calculation

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Gpelectrical

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Canton,Ga
I have a resturant I am wiring the inspector is giving me fits. I know the falt current of the trans former 52127 it is 3 phase the incoming voltage 14400 and the secondary voltage is 120/208. I ran parraell runs of 4 in. rigid with 500 mcm copper to a 800 amp square d i line panel the breaker 240 v with aic rating of 65k he wants me to show him the calucation on the main. I could not find it in any of the books could some help me this is all I need to get my building hot.
 
This link will take you directly to the free software.
http://www.bussmann.com/7/FreeSoftware.html
The Cooper Bussmann Point-to-Point Short-Circuit Calculator is a simple, easy-to-use program that allows you to calculate prospective short-circuit currents with a reasonable degree of accuracy. These values can be calculated on the load side of a transformer, at the end of a run of cable or at the end of a busway. Calculations can be made for single or three phase systems.

Just my $.02
 
Gpelectrical said:
I have a resturant I am wiring the inspector is giving me fits. I know the falt current of the trans former 52127 it is 3 phase the incoming voltage 14400 and the secondary voltage is 120/208. I ran parraell runs of 4 in. rigid with 500 mcm copper to a 800 amp square d i line panel the breaker 240 v with aic rating of 65k he wants me to show him the calucation on the main. I could not find it in any of the books could some help me this is all I need to get my building hot.
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How did you get the 52127 ampere fault?
 
Gpelectrical said:
that is the caluation on the transformers fault current from georgia power engineer
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Then that is all you need. No matter what you do with the rest of the system, even if you used superconductive materials for all conductors and bus bars, the fault current your system can see at any point will never be higher than that number. Since you main bus is rated higher than that value, I don't see any open issues.
 
fault

fault

charlie,

if i were inspeting, i'd ask for documentation that the system, not only the main, be rated to the available fault current---series or fully rated, either
 
Gpelectrical said:
where would I get that info? I have contacted the enigneer but my time and patience are running thin.
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What size is the transformer? I'd bet Ga Power gave you the infinite bus
fault current.
 
fault current

fault current

If there were submitals on the job, they should show the rating of the gear and the fault current to which the system was designed.
If there were no submittals, the gear supplier should be able to give you documentation on the rating of the components and the series rating. Hopefully all the gear is od the same manufacturer.
 
it is all square d. The inspector wants to see the calulations for the main. I am alittle lost on how to do the calulations. Do not want to appear dumb but that is a little out of my scope.
 
Why does he need a calculation for the main..The available fault current per GA Power is less than the Main AIC ratrings. I am missing something or is he?
 
That is exactly what I keep asking myself. This is my first job in this county. I do not know if he is testing me or what. He will not give teporary power on the building untill I give him the calculation:confused:
 
Gpelectrical said:
That is exactly what I keep asking myself. This is my first job in this county. I do not know if he is testing me or what. He will not give teporary power on the building untill I give him the calculation:confused:
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Who knows? You Just need and answer

Gpelectrical said:
... but that is a little out of my scope.
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Yes it is. This question belongs to the engineer of record. Not the Georgia Power engineer

Gpelectrical said:
... where would I get that info?.
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From the nameplate get the FLA or kva and the impedance. Divide the
FLA by the impedance.

For example, a 1000kva, 208V, 5.5%Z xfm has an FLA of 1000,000/208/(sqr(3) = 2775A With an infinite bus, the available SCC = FLA /% Z or 2775/.055 = 50451A

so get us some numbers

carl
 
Gpelectrical said:
here are the numbers
300 kva transformer
24949 grdy
14400 primary voltage
208/120 secondary voltage 3 phase
2.1 impedance;)
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Gp -
For infinite bus, SCCA at the xfm terminals:
FLA = 300,000va/208V/1.732 = 833A

SCCA = 833A/.021 = 39,653A

I highly recommend you get an engineer involved

carl
 
fault

fault

Carl says 39k , POCO says 52k,,,,in either event you have a 65k main.
If the systems is series rated to 65, and you can provide some valid paperwork showing that, it would seem that would be all the inspector should need. It's all we need in this jurusdiction, anyway.
 
As others have said, Georgia Power has provided you with the available fault current. The gear is clearly sized adequately in regards to this fault current. I'm not sure why the inpector is asking for more.

If the inspector insists on this calculation, send an RFI to the engineer of record.
 
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