AIC rating of service switchboard

[mshields]

Did I see somewhere in the NEC that you now have to assume a fault current rating based on infinite available on the primary of a utility transformer? i.e. you can no longer, base fault current on the secondary based on a worst case available on the primary as provided by the utility?

Thanks,

Mike

 

[Jraef]

Unless it's changed, you only used the infinite bus method when you DON'T know the actual values on the utility side. I've read several documents recently outlining the 2017 changes to the NEC with regard to SCCR, all of them still read the same way regarding determining the Available Short Circuit Current (they did apparently change from calling it "Available Fault Current"). They still say that you CAN, conservatively, use the infinite primary bus method along with transformer data plate information to calculate it, OR you can use a more detailed calculated value if you want to possibly save money on equipment if it comes out lower. Nothing I read says you MUST use the infinite bus method.

 

[publicgood]

Agree with Jraef. 2017 is requiring you to label the incoming service with this information and it can be actual or infinite. We should all be conscious of using infinite for the sake of equipment cost and more importantly assumptions being made in arc flash calculations.

 

[augie47]

I've not been involved with "how the number originates" for a few years, but our largest utility routinely used infinite primary and based the transformer impedance on the "worse case scenario" from replacement transformers the had on the yard.
They stated they did so to remove any liability as the grid changed or they had to replace a transformer.

 

[texie]

Unless it's changed, you only used the infinite bus method when you DON'T know the actual values on the utility side. I've read several documents recently outlining the 2017 changes to the NEC with regard to SCCR, all of them still read the same way regarding determining the Available Short Circuit Current (they did apparently change from calling it "Available Fault Current"). They still say that you CAN, conservatively, use the infinite primary bus method along with transformer data plate information to calculate it, OR you can use a more detailed calculated value if you want to possibly save money on equipment if it comes out lower. Nothing I read says you MUST use the infinite bus method.

I would agree with this for equipment selection. You can't go wrong over specifying the SCCR. While the OP is concerned with equipment ratings and this is perfectly reasonable for equipment selection, the problem arises with incident energy calculations. Also the new NEC requirement for posting breaker clearing time. If you use inflated values rather than exact calculated values for incident energy, in many cases this can lead to a dangerously wrong incident energy value.