Mike Holt Short Circuit Calculator

[Vonn Mann]

When using Mike Holt short circuit calculator, City Utility Company tells me to use 10000amps for input into Utility Fault Current at the transformer. This means that all panels beyond this on the electrical circuit will be less than 10000amps. So 10kva Short Circuit Current Rating on electric panels would be adequate. How does City Utility company set the Utility Fault Current to only be 10000amps? So for example: 150kva transformer, 1.6% impedence, 208v, 3phase service. I, Current at transformer is 150kva x 1000 / sqrt3 x 208v = 416 amps. I, Short Circuit at transfomer is 416amps / 0.016 = 26025amps. How does City Utility Company set the Fault Current at 10000amps? Instead of the calculated 26025 amps. Please help me understand this. Thankyou. Vonn

 

[mivey]

They gave you the wrong data or they don't know the correct data.

 

[BAHTAH]

Utility Company Fault-Current Information

Utility Company Fault-Current Information

When using Mike Holt short circuit calculator, City Utility Company tells me to use 10000amps for input into Utility Fault Current at the transformer. This means that all panels beyond this on the electrical circuit will be less than 10000amps. So 10kva Short Circuit Current Rating on electric panels would be adequate. How does City Utility company set the Utility Fault Current to only be 10000amps? So for example: 150kva transformer, 1.6% impedence, 208v, 3phase service. I, Current at transformer is 150kva x 1000 / sqrt3 x 208v = 416 amps. I, Short Circuit at transfomer is 416amps / 0.016 = 26025amps. How does City Utility Company set the Fault Current at 10000amps? Instead of the calculated 26025 amps. Please help me understand this. Thankyou. Vonn

Typically utilities figure fault-current using an infinite primary so that should there be a change in the primary system your fault current value would not increase after installation. My utility provides a list of their transformers with the fault-current values we should use. From your statement I do not know what size transformer the utility is using nor the rating of your service. You give an
Example for a 150Kva transformer, but is that what is being installed? I would look to the utility engineering department directly to see if they provide information for their transformers to verify
what is being installed. When I have had projects where fault-current was an issue, I have gone down and actually looked at the transformer the utility was going to use so I could get an proper
calculation. If your transformer is as you state in your example, than it is obvious someone gave you bad information and you need to get to the right person ( engineering department) for
your answer. Don't be surprised if the size of the transformer is lower than you would figure for your service, as utility companies do not fall under the NEC and apply demand factors based on their historical information to determine just what size transformer they will install.