# Isc Current for Panel Boards / SwitchBoards / Urgent Final request??



## EDISON_NY (Oct 24, 2011)

Could you please help me to solve these 2 questions; I did try different ways and got different answers, just want to find where is my mistake:

1) Service panel served from 100 KVA, 1ph, 2% impedance with 1/0 AWG conductor, 50' away, what is the max fault current at panel?

2) 1000A switchboard rated 480V, 3ph, 60 Hz, served from 1000KVA, 3ph, 5% padmounted xfrm, using 3 sets of 400 kcmil copper in non-metallic conduit, 100' away. What is the max fault current at Panel?

Thank you in advance.


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## EDISON_NY (Oct 24, 2011)

*Any comments!!!!*


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## Dolphin P.E. (Oct 24, 2011)

1) You haven't provide the voltage or the conduit type. Assuming the line voltage is 480V, and the wire runs in steel conduit.

Short Circuit KVA for transformer = 100 / 0.02 = 5000 KVA

Short Circuit for feeder = (480 * 480) / (1000 * 0.00275) = 83780 KVA (note: 0.00275 is the value of the 50ft feeder reactance from NEC table 9. )

Resultant KVA short circuit = (5000*83780)/(5000+83780) = 4718 KVA

Max Short Circuit Current = 4718 / 480 = 9.8 KA

Next KAIC panel rating available is 10 KAIC

2) Same method as above.

Short Circuit KVA for transformer = 1000 / 0.05 = 20000 KVA =20 MVA

Short Circuit for Feeder = (0.48 * 0. 48) / (0.00163)= 141 MVA (Note: 0.00163 is the value of the 100 ft , 3 set of feeder from NEC table 9)

Resultant MVA Short Circuit = (20 * 141) / (2 + 141) = 17.52 MVA

Short Circuit Current = (17.52 * 1000) / (480 * 1.732 ) = 21KA

Next KAIC panel rating available is 22 KAIC.

I hope it will help.


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## vdubEE (Oct 24, 2011)

I was hoping someone would answer this as I was interested and have not run across problems like this. While I don't think we will have to size panels for the PE, the calculations that could pop up. Thanks, Dolphin.


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## EDISON_NY (Oct 25, 2011)

Thanks Dolphin P.E. ,


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## Mahmoud (Oct 25, 2011)

where did you find the next Panel size (KA Rating)? which standard you are looking at ?


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## Dolphin P.E. (Oct 25, 2011)

KAIC standard depends on the manufacturer, for example 'Square D' has10K, 14K, 18K, 22K, 25K, 35K, 42K, 65K, 100K, and 200K just for 120/208V panelboards.


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