Utility Impedance & Isc

[jOmega]

2000 kVA transformer
4160 / 480 5%IZ


What would be a reasonable assumption or WAG ..

for available Short Circuit Amps ... and impedance on the utility side (4160)

Thanks ..

jOmega

 

[gordonl]

WAG Isc=50KA symetrical
As for the utility fault level any WAG would be absolutely that, except from the utility of course.

 

[PwrCntEE]

Call your utility and they will give you their system caculations for the specific data at your location. Be sure to push them, because they will often give you some data but not all. Typically, they do not want to give you an X/R ratio of their system cause it can/will change in the future. However, you need to know what the current value is now to validate your design.

Ignoring the data from the utility and figuring the theoretical worst case:

Isc utility = [VA / (V *sqrt(3))] * [1/%Z]
Isc utility = [2000000/ (480 * 1.73)] * [1/0.05]
Isc utility = 48,112 A (symetrical)

This is the maximum let-thru from the utility transformer. You also need to account for your load. Assuming all motor loads to a maximum loading of 80% of capacity:

Isc laod = [(VA * %load) / (V * sqrt(3))] * 6
where 6 is the average inrush of a motor
Isc load = [(2000000 * 0.80) / (480 * 1.73)] * 6
Isc load = 11,547 A (symetrical)

Isc = 48,112 + 11,547 = 59659 A (symetrical)

This is the worst case disign requirements. Your load will have some non-contributing loads and the utility will be less than this. In-addition, your feeders will limit the current due to resistance and length. See Bussmans website for some literature for additional hand calculations on feeders.

 

[rbulsara]

I have seen, in the USA, the available SC at 13.8 kV from 3000A to 10,000 as quoted by the utiltiy companies. (amounting to 95 mVA to 300MVA). At 4.16 kV it should be somewhat less as some additinal transformation is involved, 4.16 kV is an odd supply voltage for an utiltiy co now a days in the USA.

But in absence of any data assuming anything less than 350 MVA could be a bold approach.

 

[jbartos]

Suggestion: The above calculations are assuming:
No utility system impedance Zsys. The system impedance Zsys will make the transformer secondary short circuit current level somewhat lower.

 

[advidana]

Use worst case design requirements because in the future the utility could built a substation closer to your load. It happens a lot in the boom town I live in.