avail. short circuit

[Toros]

To calculate new panel AIc based avail. s.c, (The new panel is fed through 150kva, 480-120/208-3ph. xfrmer), can I use xfrmer's impedance and load that is 150kva @ 480v= 181A thank you

 

[augie47]

In it's simplest form all you need is transformer KVA and it's impedance.

 

[jim dungar]

To calculate new panel AIc based avail.

The load on the transformer is not important. As augie said, just use kVA and impedance.

Technically you are calculating the available Short Circuit Amps (SCA), in order to select the proper AIC.
AIC ratings come from the the manufacturers, AIC requirements come from the available SCA

 

[kwired]

To calculate new panel AIc based avail. s.c, (The new panel is fed through 150kva, 480-120/208-3ph. xfrmer), can I use xfrmer's impedance and load that is 150kva @ 480v= 181A thank you

The 181 amps is the rated full load of the transforrmer, place more load on it and it will attempt to deliver that load. A short circuit is a situation where the load is much more than the transformer can deliver, that is when the actual amount of current that will flow is limited by the impedance of the transformer, in combination with the impedance of conductors between the transformer and the short circuit point, and the amount of current the supply circuit to the transformer can deliver will also have limitations. The simplest calculations assume infinite ability of the supply circuit, and basically from there you take voltage and impedance and calculate how much current would flow given those values. You do need to figure out if the transformer has say 2% impedance just exactly what kind of impedance in Ohms that means to do the simple calculation though and then it just becomes E=IxR. Someone else would be better for telling you just exactly how to determine R from the 2% impedance than me.

There used to be and probably still is a fault current Excel calculator on the free stuff at Mike Holt's website, which I would recommend you maybe try to use it for your application. Again it is not 100% accurate and at some point assumes infinite ability from the source, when in reality the source usually will have some limitation that effects total available fault current.

 

[GoldDigger]

Fortunately the real world calculation will be no worse than the infinite bus calculation.
So as long as the calculated result is acceptable, you can go with it.

Tapatalk!

 

[kingpb]

The formula is:

MVA_xfmr/Z_xfmr= MVA_sc


150KVA/0.02 = 7.5MVA (The assumption here is the impedance is 2%; you need to use the actual value)

You can now calculate the I_sc at whatever voltage you want.

@ 480V it would be around 9000A, and at 208V it would be around 21,000A