kAIC Calculation

[BatmanisWatching1987]

I have a transformer with the following nameplate
750kVA, 480V-208/120V, 3P, Z=5%
I got a rating of 41.6kAIC and rounded to the nearest standard I get 42kAIC.

But from the Engineering drawing, the MDP after the transformer is rated with the following data
1600A, 208/120,3P,4W and a 14kAIC.

How did they get a kAIC of 14kAIC.

The 750kVA transformer is coming off a 1500kVA-13.2kV-480Y/277V Delta Primary-Wye Secondary,3P, 4W, 5% Impedance)

 

[David Castor]

How long is the feeder from the transformer to the MDP? Fault current is reduced significantly by the feeder impedance at 208 V. That said, they could be wrong.

 

[BatmanisWatching1987]

About 100ft

The conductor is 4 set of 600kcmil

 

[petersonra]

I have a transformer with the following nameplate
750kVA, 480V-208/120V, 3P, Z=5%
I got a rating of 41.6kAIC and rounded to the nearest standard I get 42kAIC.

But from the Engineering drawing, the MDP after the transformer is rated with the following data
1600A, 208/120,3P,4W and a 14kAIC.

How did they get a kAIC of 14kAIC.

The 750kVA transformer is coming off a 1500kVA-13.2kV-480Y/277V Delta Primary-Wye Secondary,3P, 4W, 5% Impedance)

They probably did the calculations and came up with a SCC less than 14 kAIC.

 

[BatmanisWatching1987]

I got 29.3kAIC when I added into the Fault Current Calculator . Can someone please verify if I used the correct inputs

I still don't get how they calculated it for 14kAIC

 

[petersonra]

I got 29.3kAIC when I added into the Fault Current Calculator . Can someone please verify if I used the correct inputs

I still don't get how they calculated it for 14kAIC

They may have had better data on the available SCC and not had to use infinite source numbers. Is this all AL or CU wire?

I think the calculator you used only does DC resistance.

 

[wbdvt]

I think they may have had the actual available fault current from the utility but that would have to be confirmed from the engineering firm. They could have modeled the system using that data and field data to determine the fault current. Is the drawing stamped by a PE?

 

[david luchini]

I got 29.3kAIC when I added into the Fault Current Calculator . Can someone please verify if I used the correct inputs

I still don't get how they calculated it for 14kAIC

You modeled the 1600A, 208V feeder on the primary side of the 750kVA transformer.

I don't think you've indicated what is between 1500kVA and 750kVA transformers.

 

[BatmanisWatching1987]

You modeled the 1600A, 208V feeder on the primary side of the 750kVA transformer.

I don't think you've indicated what is between 1500kVA and 750kVA transformers.

There is only conductor in-between the 1500kVA and the 750kVA transformer

I thought your suppose to use the secondary voltage after the 750kva transformer for SCC

 

[BatmanisWatching1987]

They may have had better data on the available SCC and not had to use infinite source numbers. Is this all AL or CU wire?

I think the calculator you used only does DC resistance.

All CU wire

 

[david luchini]

There is only conductor in-between the 1500kVA and the 750kVA transformer

I thought your suppose to use the secondary voltage after the 750kva transformer for SCC

You mentioned that there was 100' of 4 sets of 600mcm from the 750kVA secondary to the MDP.

But in the fault calculator you showed tjat between the 2 transformers.

The conductor sizes and lengths between the 2 transformers will also affect the available fault current at the mdp.

 

[topgone]

I got 29.3kAIC when I added into the Fault Current Calculator . Can someone please verify if I used the correct inputs

I still don't get how they calculated it for 14kAIC

My numbers do not agree with yours. There must be an impedance in between the 1.5 MVA, 13.2kV/480V transformer and the 750 kVA, 480/208V transformer.

 

[Tulsa Electrician]

By chance the engineer us the 5 %.
Maybe he used a different value.

 

[BatmanisWatching1987]

You mentioned that there was 100' of 4 sets of 600mcm from the 750kVA secondary to the MDP.

But in the fault calculator you showed tjat between the 2 transformers.

The conductor sizes and lengths between the 2 transformers will also affect the available fault current at the mdp.

There's a 100' of 4 sets of 600mcm from the the 1.5MVA to the 750kVA transformer

By chance the engineer us the 5 %.
Maybe he used a different value.

Yes, I used a 5% for the 1.5MVA and 750kVA transformer.

My numbers do not agree with yours. There must be an impedance in between the 1.5 MVA, 13.2kV/480V transformer and the 750 kVA, 480/208V transformer.

Any chance I can see how you come up with your calculations

 

[topgone]

There's a 100' of 4 sets of 600mcm from the the 1.5MVA to the 750kVA transformer


Yes, I used a 5% for the 1.5MVA and 750kVA transformer.

Any chance I can see how you come up with your calculations

It was unclear to see, you know? First I though that 4 X 600MCM of around 100 feet is the run from the 208V transformer to your MDP!
Just to give a quick calculation, assuming there are no motor loads connected and using the MVA method:

Source MVA = 1.5MVA/ 0.05 = 30 MVA​

Conductor impedance from the 1.5MVA to the 750 kVA: 4 x 600MCM for 100 feet --> r = 0.025 ohm/1000 feet; x= 0.048 ohms/1000 feet -->Total impedance = 0.001353 ohms​

SC MVA for the conductor = 0.48^2/0.001353 =170.29​

SC MVA of the 750 kVA = 0.75/0.05 = 15 MVA​

Then:

1/total SC MVA= 1/30 +1/170.29+1/15​

total SC MVA = 9.44 MVA​

Fault = 9.44X 10^6/ (1.732X208) =26.2 kA
If ever, there is some distance from your 750 kVA to your MDP (my original thought), the 26kA could drop some more!

 

[BatmanisWatching1987]

It was unclear to see, you know? First I though that 4 X 600MCM of around 100 feet is the run from the 208V transformer to your MDP!
Just to give a quick calculation, assuming there are no motor loads connected and using the MVA method:

Source MVA = 1.5MVA/ 0.05 = 30 MVA​

Conductor impedance from the 1.5MVA to the 750 kVA: 4 x 600MCM for 100 feet --> r = 0.025 ohm/1000 feet; x= 0.048 ohms/1000 feet -->Total impedance = 0.001353 ohms​

SC MVA for the conductor = 0.48^2/0.001353 =170.29​

SC MVA of the 750 kVA = 0.75/0.05 = 15 MVA​

Then:

1/total SC MVA= 1/30 +1/170.29+1/15​

total SC MVA = 9.44 MVA​

Fault = 9.44X 10^6/ (1.732X208) =26.2 kA
If ever, there is some distance from your 750 kVA to your MDP (my original thought), the 26kA could drop some more!

Thank you so much.

The distance from the transformer to the MDP is probably 3ft . Here is a picture for your reference.

 

[BatmanisWatching1987]

I was able to have access to the other electrical room. Which shows the main disconnect for the MDP. But I didn't see the KAIC rating on the breaker but I looked it up on my the model number and it said it was 100kaic.
Siemens Breaker with a model number of WLL2F316

See photo for your reference.