High fault current(> 200 kA) from utility

[MI_Engineer]

Hello all,

I received the following from the electric utility. The system consists of multiple 250 kVA to 500 kVA transformer banks tied together with 208 V, 3ph secondaries.

"The % X/R ratio for the power line is 2.87. {Electric Utility's} contribution to a fault at the customer’s service point is 58,173 Amps at 208V or 19.65MVA three phase symmetrical. The ultimate fault could be as high as 80MVA in the future."

While I don't get the exact same results 19,650 kVA / (208 * sqrt(3) = 54,583 kA, it is similar to what was received in the letter.

At 80 MVA it would be 222.22 kA = 80,000 kVA / (208 * sqrt(3))

Is there service equipment sized to handle >200 kA? Or is the better path to install equipment rated 200 kA and inform the Owner of the decision. I've found Class L fuses rated up to 300 kA but am unable to find enclosures to put them in.

 

[don_resqcapt19]

Not sure you have much choice. The note on of of Eaton's breakers says "Successfully tested at 300 kAIC, although UL recognizes maximum of 200 kAIC at 240 Vac." Based on that I don't think you will find anything rated above 200 kA.

Maybe plan for space to add a future 1 to 1 transformer to reduce the fault current when the utility upgrades to 80MVA.

 

[D. Castor]

I agree with Don's comment. I'll just add that fault current at 208 V drops off pretty quickly as the conductor length increases. I would make sure the utility calculation factors in all of the cable from the transformer secondaries to your fuses.

 

[MI_Engineer]

Thank you for the replies. The transformer option is a good one to keep in mind. I'll follow up with the utility planner to clarify the connection point.