AVAILABLE FAULT CURRENT - NO MOTOR LOADS OR AC UNITS

[AWinston]

I got a request from an inspector that is looking for available fault current on a 600A switchboard. The switchboard has been installed in empty building with only lights, outlets and 2 small water heaters. Am I missing something here. Does this mean he wants fault current for all branch circuits? There aren't even any transformers. The switchboard is rated for 42kAIC.

 

[GoldDigger]

Available fault current here refers to how much current the utility transformer and service conductors could supply to a bolted fault at the switchboard.
It has nothing to do with load current or the current drawn by a fault downstream of the switchboard.

This calculated current (or just ask POCO for a value) must be less than or equal to the 42kAIC of the switchboard.

 

[AWinston]

Available fault current here refers to how much current the utility transformer and service conductors could supply to a bolted fault at the switchboard.
It has nothing to do with load current or the current drawn by a fault downstream of the switchboard.

This calculated current (or just ask POCO for a value) must be less than or equal to the 42kAIC of the switchboard.

That's exactly what I told the electrician but he seemed to think it had to do with the branch circuits!.

 

[texie]

I got a request from an inspector that is looking for available fault current on a 600A switchboard. The switchboard has been installed in empty building with only lights, outlets and 2 small water heaters. Am I missing something here. Does this mean he wants fault current for all branch circuits? There aren't even any transformers. The switchboard is rated for 42kAIC.

Sounds to me like he is doing his job. Assuming you are referring to service equipment, 110.24 requires field marking of the available fault current at that point. In addition, all equipment (including equipment downstream) must be comply with 110.9 and 110.10. POCO info and some calculations are in order to comply with these code requirements.

 

[Jraef]

That's exactly what I told the electrician but he seemed to think it had to do with the branch circuits!.

He is wrong.

Here's what he might be thinking of though. If there were a lot of motor loads, the potential fault current of the system might be a tad higher than what is available from the utility because of the "motor contribution" from any motor that is spinning at the moment of the fault. Potentially, any rotating AC motor can become a generator when a fault occurs, feeding current into the fault. So when determining the ratings of equipment, it's best to know of any potential contributions other than the utility power.

But in the absence of that information, the AFC is the Utility Fault Current at their transformer terminals, minus whatever impedance there is in the cables and distance between there and your Switchboard. basically, the inspector just wants to see that whomever selected the Switchboard and devices in it understand that it has to be rated for AT LEAST the Available Fault Current.

 

[Julius Right]

In my opinion, you may follow the IEEE Std 141/1993 [ Red Book] chapter 4 Short-circuit current calculations way.
Of course, as Jraef already said ,induction motors will contribute current in a short-circuit case. However, in my opinion, this contribution could be not more then 10%.

 

[iwire]

That's exactly what I told the electrician but he seemed to think it had to do with the branch circuits!.

He is wrong.

Let's be careful here, it does have to do with branch circuits in the fact that branch circuit breakers must have a rating that exceeds the available fault current avaible at that breakers location.