Pole mount xfmr 3

[Fuselink]

Dear Friend,

I have a 45KVA pole mounted single phase transformer (7300V to 204/120V) which feeds two distribution panel, through 3C/1/0 cables in 2” conduit. I need to calculate short circuit current at the panel.
What I don’t have is the xfmr impedance and impedances on the wire.Could you please refer me to a source that I can get this info. It’s been long since I have dealt with this stuff

 

[dpc]

http://www.geindustrial.com/publibrary/checkout/38652.30055.29438.9182/generic/GET-3550F.pdf

You'll find approximate value in here. Watch out for the 120/240V three-wire systems. The 120 V fault current will be higher than the 240 V fault current, at least close to the transformer.

 

[BJC]

The easy answer is here.

http://www.alabamapower.com/architect/pdf/A E Fault Currents Tables FINAL 8 2003.pdf

 

[Fuselink]

BJc,
The service length shows the secondary wiring right?
Does this mean if I were to use a panel with main breaker at 240V , 10 feet away from transformer , then I need to have a main breaker with SCCR greater than 10423 A and also all single pole breakers need to be rated greater than 17119A?

feet 240V 120V
0 11161 22323
10 10423 17119

Thanks for the reply

 

[jghrist]

The main breaker would have to interrupt 17,119 amps for a single line-to-ground fault in the panel.

 

[BJC]

No, It means a single phase fault is going to be 17,119 amps. The single phase fault will flow throught one pole of the double pole breaker so it will have to be rated greater than 17,119 amps.
Those numbers are for a 37.5 KVA ( 45 KVA are not a standard size for pole mounted xfmrs.
You should use a 50 KVA and be more realalistic about the distance. The transformer is going to be at least 20' up the pole.
That should put you below 10 KA, 22 KAIC panels are pretty common if you'r not sure buy 22 KAIC.

 

[Fuselink]

So, based on this all single pole feeder breakers also need to have same rating as the main breaker. because a single phase to ground will pass through them?
Sorry, transformer was 37.5 KVA.

 

[BJC]

Fuselink - you'll learn more reading the catalog and buying your local SquareD, Siemens, GE or Eaton peddler lunch.

 

[dpc]

The values in the Alabama Power tables appear to be extremely high. Seems to be based on an assumption that the transformer impedance for a 120V fault will be 50% of the impedance across the full winding. This does not agree with anything I've seen before. The estimated current in the GE reference I cited is considerably less.

For an analysis of short circuit currents in single-phase transformer, see Conrad St. Pierre's book "A Practical Guide to Short Circuit Calculations."

 

[cuky2000]

Beware that the length of the cable have significant damping effect on the available shorth circuit at the end of the feeder.

See the enclose link with information from different sources.

http://cuky2000.250free.com/SC_Calc_Dist_Tranf_1.jpg?view_image=1

http://cuky2000.250free.com/SC_Single_Ph_Transf.gif?view_image=1

 

[Zogzog]

Contact your utillity and ask for the fault current ans X/R ratio for that transformer, they will provide you with that info.

 

[kyruby]

Fuselink,
A Kuhlman 37.5 KVA pole mount single phase transformer, 7620 x 120/240 has an impedance of 1.7%. This should be similar for other manufacturers of like transformers.

One note this is an oil filled transformer, not a dry type and the rating is taken at 85 deg C. Also this transformer has core loss of 106 watts and load loss of 410 watts.

This information comes from Kuhlman test data.

Hopes this helps

 

[Fuselink]

Thanks everybody for valuable inputs.
Transformer is from “ Can Tran Co”. Unfortunately, design impedance is not shown. It says” Tested % impedance to be entered “ . SKM shows 3.5% which to me is high for this type of transformer.
Transformer is 8320 GrdY/4800 . I am not sure if I can model this using single phase module in SKM. Because it’s asking for VLL and H2 is grounded in transformer.

 

[Fuselink]

The problem with modeling was that I had to choose YGrd for three phase and then I would get VLN.

 

[Fuselink]

ZogZog,
Utility is not that cooperative in this regard. On the other hand, I can see that level of SC MVA will affect fault currents substantially.
SKM requires three phase MVA and Line-ground utility contribution data. Since, I am dealing with single phase scenario, how three phase contribution can affect the calculation. As I mentioned, H@ is grounded and therefore, I only have MVA contribution from utility on one phase.

 

[waross]

Be aware that available fault current as determined from the transformer impedance is not the actual fault current. The actual fault current or asymetrical current) will be based on the available fault current, the X/R ratio of the transformer and the point in the sign wave that the fault occurs. The actual fault current or asymetrical current may be double the available fault current.
However, the good news is that this is taken into consideration in the ratings and acceptance tests of panels and breakers.
For instance a breaker rated for 10,000 amps RMS will safely interupt the worst case current from a transformer with an available fault current of 10,000 amps or less.
Consideration of the reactance of the feeder cables will often drop the available fault current down to a lower rating class.
respectfully