Calculating Fault Current?

[fly.explore]

Hey guys!

I have a background in electrical engineering but am new to the world of electrical design. Would someone be able to explain how the ISC and AIC values are obtained?

Also, in the example I was given, I tried calculating the ISC for the main distribution board. I got ISC=(3000kVA/(480V*1.732))/(5.75%)=62,755 just using the values from the secondary side of the transformer. However, this doesn't seem to match the 49,503A shown on the drawing. Could someone please shed some light on what I am doing wrong?

I've been doing research online but getting really confused. I would be really thankful if you can use my drawing to show some example calculations. Any help is greatly appreciated!

 

[Ingenieur]

Your calculation assumes an infinite utility bus and isc at xfmr lugs
2 things may reduce the isc
-bus is not infinite, has limited fault mva (either given or determined from x/r)
-the isc is not at the xfmr lugs but at the end of the service conductors/feeder which includes additional Z

 

[GoldDigger]

The diagram shows an assumed or specified 10,700A ISC on the primary side from the utility.
Your calculation assumes an "infinite bus" on the POCO side.
I have not run the numbers to see whether that calculates to the secondary number shown

Sent from my XT1585 using Tapatalk

 

[Ingenieur]

Can't read the print
ehat is the primary voltage?

 

[GoldDigger]

Can't read the print
ehat is the primary voltage?

The print has plenty of resolution if you open it in Google Docs or any good rendering program instead of your browser.

4.16KV (3000KVA Xformer to 480Y/277)

Sent from my XT1585 using Tapatalk

 

[Ingenieur]

edit 4160
Ssc 77.1 MVA >> 3 MVA xfmr
sec isc based on Ssc 92.7 kA alone (no pu Z)
so util Ssc is not the limiting factor

must be bus bar Z ~ 2.05 mOhm (xfmr Z 7.65 mOhm)

 

[don_resqcapt19]

The diagram shows an assumed or specified 10,700A ISC on the primary side from the utility.
Your calculation assumes an "infinite bus" on the POCO side.
I have not run the numbers to see whether that calculates to the secondary number shown

Sent from my XT1585 using Tapatalk

Using the Bussmann app, I get 38,800 at the secondary of the transformer secondary based on the information given in this thread. That assumes no motor contribution.

 

[Ingenieur]

I did this
assume ph-ph fault
calc util Z based on 4160/10700 = 0.3888 Ohm
reflected to sec 0.0017254 (assume n = 4160/277 delta wye)
calc xfmr Z = 0.007649 Ohm
Z total 9.3744 mOhm
480 / Z total = 51.2 kA

a diff of 0.322 mOhm from the dwg
480/49503 = 9.6964 mOhm
busbar?

 

[topgone]

Using the Bussmann app, I get 38,800 at the secondary of the transformer secondary based on the information given in this thread. That assumes no motor contribution.

I get the same around 37,584A with no motor contribution. I was played with numbers and arrived at the nearest Isc of 49kA when a motor load of 50% of the transformer capacity was considered. Isc from transformer was 37,584A while the motor contribution was 11,624.1; totalling 49,202A!

 

[Ingenieur]

I get the same around 37,584A with no motor contribution. I was played with numbers and arrived at the nearest Isc of 49kA when a motor load of 50% of the transformer capacity was considered. Isc from transformer was 37,584A while the motor contribution was 11,624.1; totalling 49,202A!

the dwgs 50 kA
the calcs 38 kA
delta 12 kA or 10 MVA of motor contribution to the fault, close to your 11.6 kA
rule of thumb motor contributes 4:1 x its kva to the fault, so 2.5 MVA in this case (2500 HP give or take)
or 80% of the xfmer capacity
that seems high from looking at the single line, the cable lengths, and the loads