NEC 110.24 Available Fault Current

[Dan6779]

This is my first posting here. I work as an electrical inspector.

NEC 110. 24 requires service equipment for other than dwelling units to be marked with the maximum available fault current. In my state, we consider apartment buildings with more than two apartments to be an "other than dwelling unit." I have been requiring this to be posted at service mains.

Some apartment buildings are set up with main breakers located at the individual apartments, while others have meter/mains set up at a central location. In designs where the mains are located in individual apartments it is easy to determine the available fault current when you know the size and length of the feeders. Where am a bit perplexed, is the times when the mains are located close to the service main.

Say we have 40 unit apartment building, and the service has an available fault current of 30K. The service has a 65K rated main, so we are fine there. The electrician installs meter/mains rated for 10K.

My question is, do all these feeders have a bleeding affect on the available fault current? Can the 10K meter/mains stay?

 

[jraef]

No. Available fault current does not divide or “bleed”. It’s available at all points in the circuit.

It is possible however that the downstream panels are using components that are “series listed” with the upstream breaker. Those listings are extremely specific and can ONLY be verified by the manufacturers of the equipment, plus they do not cross over from one mfr to another. But it is possible that in a series rated combination, the downstream systems that are labeled as 10kA can be used in series with other upstream devices at a total listing that is higher. As the inspector, you can insist on seeing this documentation proving it, but absent that, it would be a violation.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[davidbeach]

The impedance of the conductors reduces the fault current, but it takes calculations to know the effect. Getting from 30kA to 10kA takes a fair amount of impedance.

A series rating, as mentioned, will allow what seems to be under rated equipment to be used within their listing.

 

[Dan6779]

Thanks for the info.

I sort of know these things already. Some days I wonder if I am cut out to be an inspector. I hate giving people bad news.

So, if the fault current turns out to be 20K, an engineer gave his blessing on it, and they are using series listed breakers, it is probably an OK job?

 

[wbd]

Some apartment buildings are set up with main breakers located at the individual apartments, while others have meter/mains set up at a central location. In designs where the mains are located in individual apartments it is easy to determine the available fault current when you know the size and length of the feeders. Where am a bit perplexed, is the times when the mains are located close to the service main.

Say we have 40 unit apartment building, and the service has an available fault current of 30K. The service has a 65K rated main, so we are fine there. The electrician installs meter/mains rated for 10K.

Please note that there is a difference between available short circuit current and infinite bus fault current. The infinite bus fault current is easy to determine and is used for sizing equipment AIC and SCCR ratings. It is basically the transformer FLA divided by impedance.

Available fault current can only be obtained from the utility and is usually given at the riser fuses or transformer primary terminals. The fault current for 110.24 is then calculated based on transformer impedance and secondary cable impedances (and riser cable if fault current given at riser fuses).

So just make sure you using the right fault current for the task.

 

[jraef]

So, if the fault current turns out to be 20K, an engineer gave his blessing on it, and they are using series listed breakers, it is probably an OK job?

If all of those points are true, then yes.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden