AIC & SCC Relationship

[Rock86]

I had to recreate this post with the correct wording.

I calculate short-circuit current of electrical system using a method from an older Bussman Electrical Handbook. The Short-Circuit Current calculated from this method (from my understanding) should tell me what the fault current would be on the supply side of main electrical panel.

If I have a short-circuit calculations of around 12,000A, and the panelboard SCCR is 200,000A, what AIC would you most likely use? 22,000AIC, 65,000AIC? Is there a standard rate above the calculated SCC which you would select the AIC rating of the main breaker, assuming no equipment requires an AIC above 12,000A?

 

[don_resqcapt19]

As far as I know the SCCR of a panel is always based on the lowest AIC of any breaker installed in that panel. In you application you will need breakers with an AIC that exceeds 12kA. I don't think there is anything between 10 and 22kA so you will need 22kA breakers.
Equipment never requires an AIC.

 

[Rock86]

As far as I know the SCCR of a panel is always based on the lowest AIC of any breaker installed in that panel. In you application you will need breakers with an AIC that exceeds 12kA. I don't think there is anything between 10 and 22kA so you will need 22kA breakers.
Equipment never requires an AIC.

So you would just go next size up? I know that the SCCR rating is based on the lowest breaker, but the Main in the panel if it is a fully rated panel, it could be sized larger than 22kA (In this examples). I would say that a 65kA would be significant overkill in this example, but it could be used.

 

[don_resqcapt19]

So you would just go next size up? I know that the SCCR rating is based on the lowest breaker, but the Main in the panel if it is a fully rated panel, it could be sized larger than 22kA (In this examples). I would say that a 65kA would be significant overkill in this example, but it could be used.

You can use any breaker as long as the AIC is equal to or greater than the available fault current, however as the AIC goes up, so does the price.

 

[Rock86]

You can use any breaker as long as the AIC is equal to or greater than the available fault current, however as the AIC goes up, so does the price.

Facts. But would you have a limit? I have seen spec's call for a 65kA when the SCC was low enough to have a 22kA or even a 10kA. There is no benefit going higher, by my understanding.

 

[electrofelon]

Facts. But would you have a limit? I have seen spec's call for a 65kA when the SCC was low enough to have a 22kA or even a 10kA. There is no benefit going higher, by my understanding.

There is no reason to go higher AIC than needed, except if planning for future increases of the available fault current. I think it is fairly common for specs to say 65k, most likely that is because someone just doesn't want to or doesn't know how to calculate the available fault current, and thinks 65k should cover it (which does in most cases).

 

[don_resqcapt19]

Facts. But would you have a limit? I have seen spec's call for a 65kA when the SCC was low enough to have a 22kA or even a 10kA. There is no benefit going higher, by my understanding.

If it is up to me, that would get 22 AIC rated breaker.

 

[ggunn]

There is no reason to go higher AIC than needed, except if planning for future increases of the available fault current. I think it is fairly common for specs to say 65k, most likely that is because someone just doesn't want to or doesn't know how to calculate the available fault current, and thinks 65k should cover it (which does in most cases).

What I have done in some cases is to set the kAIC to meet or exceed the AFC at the utility transformer secondary terminals because it is sometimes difficult to get all the information needed to do an AFC calc at a point interior to the system. It covers the worst case scenario.

 

[petersonra]

Many of our customers have started specifying 65KA SCCR for control panels because it is a practical limit that can be done cost-effectively. In fact you can pretty much do it for the same price as 5 or 10K in most cases.

 

[Fred B]

Many of our customers have started specifying 65KA SCCR for control panels because it is a practical limit that can be done cost-effectively. In fact you can pretty much do it for the same price as 5 or 10K in most cases.

Supplying a 65 kA rated panel might be cost effective related to a 10kA panel but how price neutral are 65kA breakers compared to the 10kA? I would think substantial difference. A higher level panel than calculated needs might be competitively priced if potential exists for upgrades that would require such.

There is no reason to go higher AIC than needed, except if planning for future increases of the available fault current. I think it is fairly common for specs to say 65k, most likely that is because someone just doesn't want to or doesn't know how to calculate the available fault current, and thinks 65k should cover it (which does in most cases).

Your SCCR is only as high as the lowest rated device installed. Care would be required to ensure everything connected thru the panel can handle the higher faults potential to avoid a rapid unintended disassembly of the equipment of devices when or if your AFC increases.

 

[petersonra]

People who want the lowest cost are mostly not getting any breakers in their control panels. For a control panel rated under about 60 Amps there is usually not a lot of cost difference to go to 65 kA

 

[petersonra]

Your SCCR is only as high as the lowest rated device installed. Care would be required to ensure everything connected thru the panel can handle the higher faults potential to avoid a rapid unintended disassembly of the equipment of devices when or if your AFC increases.

Has this ever actually happened?

The reality is that for the most part the available short circuit current is substantially overstated and most components are unlikely to explode just because they saw slightly more short circuit current than they are actually rated for.

From what I can tell, the most likely problem is a component failure, but some kind of explosion due to excess short circuit current is unlikely, but not impossible.

 

[jim dungar]

Wasn't the OP about a panel board and not a control panel.
Even though both are fruit, apples are applied different than tomatoes.

There is usually a fair price difference when exceeding 10kAIC for fully rated panel boards up to 240V, while some series rated ones can reach 65KAIC without much of an adder. At 480V the pricing for higher full ratings seems to have become not as extreme as it was years ago.

 

[electrofelon]

Your SCCR is only as high as the lowest rated device installed.

Which is silly. That seems just like a dumb meaningless statement anyway, Do manufacturers actually state that? Why would installing a breaker change the sccr of the panelboard? Furthermore why would it even matter because if I put in a breaker with an AIC of x then the fault current is less than that anyway so who cares. Plus how does it work with series ratings? I don't think I've ever seen series ratings documented for the panel board itself which would need to be the case if a breaker is lowering the panel's SCCR

 

[electrofelon]

There is usually a fair price difference when exceeding 10kAIC for fully rated panel boards up to 240V, while some series rated ones can reach 65KAIC without much of an adder. At 480V the pricing for higher full ratings seems to have become not as extreme as it was years ago.

I priced a Siemens 120/208 panel board a few years ago with 10K and 22k branches, and IIRC it was about $6 more per Pole. I also just checked some of the online breaker sites and they are showing a standard b120 at $11, b120h (22k) $20, and a b120hh (65k) about $55. A BQD 120 which is their standard 480 breaker that gets you 65k at 120/208 is the same price as the b120hh. So yeah 65k fully rated is going to be a big jump with 22 k not so bad. Series ratings will of course save a lot of money. 10k branch almost always has a upstream device frame size limitation of 250 amps, but go up to 22k branches and that opens you up to just about any frame size and available fault current for your main and you'll still get a series rating. (For Siemens 250 volt stuff)

 

[jim dungar]

Which is silly. That seems just like a dumb meaningless statement anyway, Do manufacturers actually state that? Why would installing a breaker change the sccr of the panelboard? Furthermore why would it even matter because if I put in a breaker with an AIC of x then the fault current is less than that anyway so who cares. Plus how does it work with series ratings? I don't think I've ever seen series ratings documented for the panel board itself which would need to be the case if a breaker is lowering the panel's SCCR

It is very easy to support/brace the bussing in a panelboard for 200kA fault currents.
But the bussing is only part of a panel's SCCR. As a result the AIC of the breakers becomes the limiting factor in the overall installed SCCR.

Because the panelboard buss rating is usually higher than any possible breaker it is not impacted by any series rating.

 

[electrofelon]

It is very easy to support/brace the bussing in a panelboard for 200kA fault currents.
But the bussing is only part of a panel's SCCR. As a result the AIC of the breakers becomes the limiting factor in the overall installed SCCR.

Ok but again that just seems silly. Why can't the SCCR just stay the same? I see no reason that should change based on what breaker is put in.

 

[jim dungar]

Ok but again that just seems silly. Why can't the SCCR just stay the same? I see no reason that should change based on what breaker is put in.

The breaker is a component of the assembled panelboard, why wouldn't it impact the assembled SCCR?

 

[electrofelon]

The breaker is a component of the assembled panelboard, why wouldn't it impact the assembled SCCR?

. I guess the way I think about it and what seems most logical to me, is that the panel board, which is the bus assembly itself, has a sccr and I don't see why installing one breaker over another should modify that

 

[electrofelon]

Here is the label off a siemens P1 panel. IT is rather hard to follow exactly due to the commas and poor structure. IVe got to run for now, but Ill offer my interpretation of it later.