Short Circuit Rating

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BMacky

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Foster City, CA
Can anyone please help me understand about or guide me to a simple explanation of short circuit ratings for equipment?

I had an inspector ask me about the short circuit rating of a main panel and then that of a sub-panel, and something about the sub-panel not having the correct rating. He mentioned increased ratings for the sub-panel breakers and a decreased rating for the main breaker feeding the sub-panel. Also mentioned a calculation based on length and size of the feeders from the main to the sub.

You can probably tell by my last paragraph that I have no idea what he is talking about. Regardless, I'd like to figure this all out before I reschedule another inspection.
 
There is not enough information provided to address your concern.
Off of the cuff though with the minimun of information thatwas provided I would state that the subpanels KAIC rating has nothing to do with the main panels rating as noblody advise the available fault current at the main panel in the first place.
If the available fault current at the main panel is 30ka and the main panel is rated 65kaic and the subpanel is rate 30kaic what's the problem? There is none.
The AHJ may be of a false opinion that may be based upon the rating of the main panel and not the actual available fault current available.
you may have to request the value of the fault current available from the POCO. This value is bases upon the KVA and impedance of the POCO transformer at the service entrance and the fault current available at the primary of that transformers.
If you knew the KVA and impedance of that transformer you could get a fairly good idea what to expect.
 
It's my understanding that you are required to put a SCCR on your panel and the facility where the panel is being installed is responsible for sending to that panel a feed that is at or less than your SCCR. You tell them what your panel can handle and they're required to give you no more than that.
A easy way to increase the SCCR of your panel would be to put a 1:1 isolation transformer ahead of your main disconnect. This might be necessary if the facility has a problem sending you a feed to match your SCCR.
Not sure if this helps. Good luck.
 
Fault Current

Fault Current

As mentioned you should check with the utility to see what the available fault current value is for your service panel location. The utility will typically use a calculation based on an infinate primary value so that any alterations to the primary system will not increase the value to a level higher than they provide at the present time. As far as the ratings of the equipment, that information is available from the equipment manufacture and those ratings follow a certain progression (10K, 22K,18K,30k etc). As an example if your service panel required a 30K rated main breaker, that does not mean the sub-panel also needs to be 30K. The impedance of the feeder conductors will lower the fault value depending on the distance. If you go to the Bussman web site you can down load free software that will allow you to make basic fault current calculations. You can do a calculation on your feeder to determine what the available fault current will be at the sub panel. In addition there are other steps that can be taken such as "Series Rating" between the main and sub panel breakers. There are certain restrictions on using that method that are detailed in the NEC.
 
BM, here's a simple explanation: under a high-current short circuit (called a 'bolted fault'), all of the current the utility transformer can supply for an instant will try to flow, limited only by the impedances of everything between the transformer and the short.

This includes the transformer's windings themselves, the wires between it and the main disconnect, the feeders, the panel buses, the breakers, sub-panel feeders, sub-panel buses, breakers, branch circuit wires, etc. (did I miss anything? :grin:)

At each point along the supply system, the available short-circuit current is lower than those points ahead of them, due to the impedances mentioned above. The goal is that equipment can withstand and/or interrup this current without being destroyed.

The main disconnect must be able to handle the current available to that point, which is limited by the transformer's internal impedance and the service conductors. The panel's requirement takes into consideration any feeders' impedances; a sub-panel has its feeders, etc.
 
Larry,

Thanks! Your explanation was right on the money as far as more clarification is concerned. I've discussed the issue further with the inspector and he has guided me through the various articles associated with this.

He asked the client for a letter from the utility with the scci value, which turned out to be 18kv (here's the missing data for the others that replied). I'm meeting the inspector today and he is going to run me through the process of engineering the circuit based on the value cited.

The whole issue began when a non-permitted remodel at a commercial job got red-tagged and every minor detail was looked into in follow-up inspections. Seems a sub-panel was added and the inspector wanted to be sure it was properly rated from the main panelboard down to the new sub-panel. I may be replacing the main breaker or whatever necessary to get into compliance.

Thanks to everyone who replied.

Bob
 
BMacky said:
He asked the client for a letter from the utility with the scci value, which turned out to be 18kv. . . .
Click to expand...
I think you mean ?18ka,? as in ?18,000 amps of fault current.?

You might not need to replace anything. If the sub-panel has a fault current rating of at least 18ks, and if the sub-panel?s main breaker is rated to interrupt a fault current of 18ka, then the existing configuration is acceptable as-is. Those, by the way, are two different considerations, the panel?s rating and the breaker?s rating. The panel?s rating has to do with its ability to sustain that high a current, without having its bus bars melt or fly apart or break their support bolts or stuff like that. The breaker?s rating has to do with its ability to sustain that high a current, without having its contacts melt and fuse together, which would prevent the breaker from ever opening and clearing the fault.

But some panels and breakers have lower ratings than 18ka, but that does not mean that you have to replace them. If the utility can supply 18ka to the house, then the wiring within the house will cause the short circuit current available at the sub-panel to be lower than 18ka. A short circuit calculation would tell you what amount of fault current is available at the location of the sub-panel. The only things that matter are that the ratings of the panel and of the breaker are at least as high as the amount of fault current available at that location.
 
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