# AIC Series Rating



## Black Dog (Oct 16, 2011)

Achilles said:


> ***Backstory***
> 480V Service / with Fuses for OCP. Fuses are series rated accordingly with SqD as such all branch circuits are 10KAIC. We are providing a 500KVA transformer for all building 120/208v Power.
> 
> ***Question***
> Does series ratings carry through a transformer? Is 10KAIC acceptable for the transformers secondary OCP?


I say it is..:thumbsup:


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## don_resqcapt19 (Jul 18, 2010)

I think you need to do a calculation based on the available fault current at the primary of the transformer and the impedance of the transformer.


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## micromind (Aug 11, 2007)

Since the secondary of a transformer is a separately derived system, you'll need to calculate its available fault current.

Assuming an infinite primary, in order for a 500 KVA 120/208 to have less than 10,000 amps of fault current it'd have to have an impedance of 14% or higher. 

Not many transformers are that high.........

Of course, a true infinite primary doesn't exist, but if it's connected to a 480 system that has a ton of available fault current, it'll be close to infinite. 

The next standard 120/240 volt breaker AIC is 22,000 amps. In this case (again, assuming an infinite primary), you'd need an impedance of 6 1/2% or higher.


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## eddy current (Feb 28, 2009)

I'd bet the impedance of the tranny is not more than 5%

Are u asking about the AIC of the fuses for the tranny, or the AIC of the breakers for the branch circuits on the secondary?


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## hardworkingstiff (Jan 22, 2007)

Achilles said:


> ***Backstory***
> 480V Service / with Fuses for OCP. Fuses are series rated accordingly with SqD as such all branch circuits are 10KAIC. We are providing a 500KVA transformer for all building 120/208v Power.
> 
> ***Question***
> Does series ratings carry through a transformer? Is 10KAIC acceptable for the transformers secondary OCP?


I'm interpreting this to mean you are picking up a 480V circuit from a breaker than has a 10kaic rating and feeding a transformer. The 10kaic rating is of the breaker is allowed because of series rated fusing ahead of it.

Is this accurate?


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## Big John (May 23, 2010)

micromind said:


> Since the secondary of a transformer is a separately derived system, you'll need to calculate be available fault current...


 Agreed.

I will preface this by saying that I don't recall ever seeing a 480:208 transformer that required a higher interrupting rating on secondary side.

That said you AIC depends entirely on the amount of current the system can supply, and we all know when you step down voltage, current goes up. For you trasformer you would theoretically have more than double the primary fault current. But in reality it is very likely that the circuit impedances and upstream OCP will still keep you below 10kAIC.


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## don_resqcapt19 (Jul 18, 2010)

Big John said:


> Agreed.
> 
> I will preface this by saying that I don't recall ever seeing a 480:208 transformer that required a higher interrupting rating on secondary side.
> 
> That said you AIC depends entirely on the amount of current the system can supply, and we all know when you step down voltage, current goes up. For you trasformer you would theoretically have more than double the primary fault current. But in reality it is very likely that the circuit impedances and upstream OCP will still keep you below 10kAIC.


But a series rated system on the primary side of the transformer does not mean that the available fault current on the supply side is less than the 10k rated breaker. It just means that the series combination of breakers is suitable for the actual available fault current.

We really need to know the available current at the primary and the impedance of the transformer.


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## Big John (May 23, 2010)

don_resqcapt19 said:


> But a series rated system on the primary side of the transformer does not mean that the available fault current on the supply side is less than the 10k rated breaker....


 How would they be allowed to use 10kAIC equipment if more than that were being permitted past the main during a fault?


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## don_resqcapt19 (Jul 18, 2010)

Big John said:


> How would they be allowed to use 10kAIC equipment if more than that were being permitted past the main during a fault?


You are probably correct, but I haven't seen anything that tells me a series rated system is a current limiting system.


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## hardworkingstiff (Jan 22, 2007)

don_resqcapt19 said:


> You are probably correct, but I haven't seen anything that tells me a series rated system is a current limiting system.


I thought the whole theory behind a series rated system is that the 1st OCP will limit the current to the 2nd OCP to its lower AIC rating. 

Am I mistaken?


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## macmikeman (Jan 23, 2007)

hardworkingstiff said:


> I thought the whole theory behind a series rated system is that the 1st OCP will limit the current to the 2nd OCP to its lower AIC rating.
> 
> Am I mistaken?


Yes . The whole theory behind the cascade is to interrupt the circuit at the lowest point so as to limit upstream downtime, but still maintaining protection upstream that can handle interrupting the high fault levels available as a backup plan.


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## hardworkingstiff (Jan 22, 2007)

macmikeman said:


> Yes . The whole theory behind the cascade is to interrupt the circuit at the lowest point so as to limit upstream downtime, but still maintaining protection upstream that can handle interrupting the high fault levels available as a backup plan.


If I'm understanding you properly, a series rated system would never be allowed.


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## chicken steve (Mar 22, 2011)

> micromind said:
> 
> 
> > Since the secondary of a transformer is a separately derived system, you'll need to calculate its available fault current.
> ...


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## macmikeman (Jan 23, 2007)

hardworkingstiff said:


> If I'm understanding you properly, a series rated system would never be allowed.


Then you are not understanding properly. Nothing new there.


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## don_resqcapt19 (Jul 18, 2010)

hardworkingstiff said:


> I thought the whole theory behind a series rated system is that the 1st OCP will limit the current to the 2nd OCP to its lower AIC rating.
> 
> Am I mistaken?


I don't really know. My understanding is that the combination of the two breakers will permit the series circuit to interrupt a fault current that is higher than the interrupting rating of the lower rated device. If it does that by limiting the fault current, fine, but I am not sure it really does.


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## Achilles (Sep 11, 2014)

My question is directed at series rating the secondary side of a transformer. Fused OCP allows me to series rate down to 10KAIC. I'm uncertain if I can carry the series rating through the transformer at 10KAIC, or if I need to calculate the fault current at the transformer and restart my AIC series for my secondary Mains.

Thanks for everyone's input.


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## IEC (Sep 20, 2015)

Achilles said:


> My question is directed at series rating the secondary side of a transformer. Fused OCP allows me to series rate down to 10KAIC. I'm uncertain if I can carry the series rating through the transformer at 10KAIC, or if I need to calculate the fault current at the transformer and restart my AIC series for my secondary Mains.
> 
> Thanks for everyone's input.


The primary and secondary are always calculated separately on an SDS. Your AIC rating of the primary has no bearing on downstreaming the fault protection on the secondary. 

Calculate your secondary potential and that's your first AIC OCPD rating. Then series rate from there downstream. FWIW, I can't remember putting a 10k AIC on a secondary side. The available fault current is almost always more than that. 



hardworkingstiff said:


> I thought the whole theory behind a series rated system is that the 1st OCP will limit the current to the 2nd OCP to its lower AIC rating.
> 
> Am I mistaken?


Yes. 

The reason we series rate is to provide fault protection higher "upstream" (closer to the origination of the current) than "downstream" (closest to the load). 

The idea is thus:

If you have three motors coming off three fused disconnects and one leg of one motor goes into OL, then only that OCPD on that one leg will open. The result is that the rest of the motors continue to run as they are supposed to. If the AIC exceeds the one leg or the one motor, then your primary OCPD will open, protecting the feeders and the other motor loads. 

We use this method so that, for instance, the production line of a plant doesn't completely shut down for an isolated problem. Thinking about this in its utmost simplicity, consider three production lines churning out a product off of one transformer. One production line goes into OC. If all the protective devices in the circuit had the same AIC rating, then the primary OCPD would trip and shut down all three lines. If, on the other hand, the primary is 22 rated and the fuses in the proverbial disconnect are 10 rated, then only the one line shuts down and power remains to the other two lines.

Make sense?


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## Achilles (Sep 11, 2014)

240.86 Series Rating: Does not restrict series rating a transformer.

240.86 (B) Tested Combinations: If the manufacture has not tested series rating through a transformer maybe this is where the code restricts the application.

Either way, a 500KVA transformer even assuming infinite current (which is not considering it's not connected to utility on the primary side) can only provide 46,000A @ 3%

________________________________________________________________

Question #2

How do I calculate the fault current of a 500KVA 120/208V transformer? I know the available fault current on the 480V side of the transformer is 15,941A


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## AK_sparky (Aug 13, 2013)

It sounds like there is a confusion here of Series Rating and Selective Coordination. I may be wrong, but my understanding is that they are two different things.


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## hardworkingstiff (Jan 22, 2007)

IEC said:


> Make sense?


Not really. My understanding is that the AIC concern is for the available current during a short circuit. If you have (like in your example) a fault in the circuit to one motor (and the available AIC current was let's say 14,000A) the main could have a 22k rated breaker and the branch breaker could have a 10k rated breaker if they are series rated. The theory (as I understood it) was that the main would open the circuit during a fault before the available (14k) current would damage the 10k rated breaker. If the 10k breaker was used w/out the 22k series rated breaker ahead of it then a fault with 14k available could damage the 10k rated breaker and fuse it closed and damage to the breaker and equipment and possible personnel could occur. 

It is my understanding that the main purpose of the series rated breakers was to save money.

Where am I wrong?


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## hardworkingstiff (Jan 22, 2007)

IEC said:


> Your AIC rating of the primary has no bearing on downstreaming the fault protection on the secondary.


How can that be? The current available on the primary is not infinite, so the secondary current is limited by the available current on the primary, right?

I know people calculate the maximum available fault current on the secondary of a transformer, but that's usually just so they know if they need to deal with it or not. When they calculate the max available fault current of a transformer like that they use an infinite source current which in not real world.


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## don_resqcapt19 (Jul 18, 2010)

Achilles said:


> ....
> Question #2
> 
> How do I calculate the fault current of a 500KVA 120/208V transformer? I know the available fault current on the 480V side of the transformer is 15,941A


Download the Bussmann app.
Using a 3% impedance the app says you would have 20,492 amps of available fault current at the secondary of the transformer.


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## don_resqcapt19 (Jul 18, 2010)

IEC said:


> ...
> ... If all the protective devices in the circuit had the same AIC rating, then the primary OCPD would trip and shut down all three lines. If, on the other hand, the primary is 22 rated and the fuses in the proverbial disconnect are 10 rated, then only the one line shuts down and power remains to the other two lines.
> 
> Make sense?


Not even close to making sense. The AIC rating has nothing to do with the trip curves.


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## IEC (Sep 20, 2015)

OK, it looks like I either have a poor understanding of something I thought I knew well, used a poor explanation to describe it or am confusing two separate things. 

Obviously an XFR does not have an infinite primary current, and I have never once calc'd one as though it does. The formula is fourth-grade math, so even I get that one, lol. 

I always understood that, basically speaking--fault calcs have to be done, of course, but as a general rule--the closer you are to the service (SDS in this case) the higher the available fault current potential will be. As such, assuming you don't exceed said calcs, it is fairly common to have, again as an example only, a 22k rating on the first OCPD while others downstream may only have 10k. 

In that theory, the time curve has little to do with anything. 

Where am I going wrong? I am certainly not above reproach, but if I am incorrect, meaning the one who taught me was incorrect, then I'd rather educate myself than just be told that I'm wrong without any substantiation to the contrary. 

Thanks in advance.


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## AK_sparky (Aug 13, 2013)

When there is a fault (for example a line shorts to a ground) there exists the possibility for 1000's of amps of current to flow. This current is the available fault current. It is generally the same regardless of fuse curve. What the fuse curve changes is the amount of time that fault current exists.

AIC ratings indicate how much a fuse or circuit breaker can SAFELY interrupt. If your supply has 22kA available fault current, but your fuse or CB is only rated for 10kA, it may blow up or light on fire or whatever when it tries to clear a fault with such high current.

Adding a higher AIC rated fuse upstream generally doesn't change the available fault current.

The time curves are used for selective coordination. Selective coordination ensures that a fault only opens the closest fuse. You want this so that your whole plant electricity doesn't go down when you short out one lightbulb socket. At any given current, the downstream fuse should open before the melting point of the upstream fuses. Basically you don't want their curves to overlap. Selective coordination is generally not related to AIC and available fault current.


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## IEC (Sep 20, 2015)

AK_sparky said:


> When there is a fault (for example a line shorts to a ground) there exists the possibility for 1000's of amps of current to flow. This current is the available fault current. It is generally the same regardless of fuse curve. What the fuse curve changes is the amount of time that fault current exists.
> 
> AIC ratings indicate how much a fuse or circuit breaker can SAFELY interrupt. If your supply has 22kA available fault current, but your fuse or CB is only rated for 10kA, it may blow up or light on fire or whatever when it tries to clear a fault with such high current.
> 
> ...


That's exactly what I was trying to say. But I was confusing selective coordination with AIC. Time to do some reading on it.

Thanks for not hammering me over sounding like a blithering idiot.


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## micromind (Aug 11, 2007)

As I understand it, a series-rated system is nothing more than more than one (usually 2) breakers that open at roughly the same time during a fault. The reason they can clear a fault with higher available current than either one is rated for is because there is more than one set of contacts opening at the same time. 

A 2 pole breaker with internal common trip is an example of a series-rated system. If a single pole breaker can interrupt 10KA at 120 volts, and 10KA at 240 volts is applied, it'll blow up. This is because the heat generated across the contacts is too great for it to handle. 

But if the same heat is across 2 sets of contacts, then each set effectively sees 1/2 of the energy. It would be like a single set of contacts opening twice as far at twice the speed. 

This is why a series-rated system needs to be listed as such. Both breakers need to trip at roughly the same time and both sets of contacts need to open at roughly the same speed. 

I've never seen a 'series-rated' system that involved fuses. Typically, a fuse has a much higher interrupting rating than a breaker and it can stand alone to clear the fault. The breaker in this case is nothing more than a switch that most likely doesn't open during a fault. Or if it does, the fuses have already cleared the fault and the breaker sees very little current. 

I have seen several systems that had fuses downstream of a breaker though, and the available fault current exceeded the breakers rating but not that of the fuses. 

Note; this is not Gospel truth, it is only the result of knowledge I have amassed over the years.


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## hardworkingstiff (Jan 22, 2007)

micromind said:


> As I understand it, a series-rated system is nothing more than more than one (usually 2) breakers that open at roughly the same time during a fault. The reason they can clear a fault with higher available current than either one is rated for is because there is more than one set of contacts opening at the same time.


I have a different understanding. A series rated system is typically a panel with a main that has an interrupting rating higher than the highest possible fault current, and the branch breakers have a lower interrupting rating. The higher rated OCP device (main) will open under a fault condition before any damage can be done to the lower rated OCP device (branch).

It's not that the two breakers combined open a higher rated fault, it's the higher rated one protects the lower rated one from damage. Since the trip curves need to be similar, the breakers need to be listed as a series-rated system.


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