# Is it normal for AIC/AIR rating to be lower upstream?



## erics37 (May 7, 2009)

Generally speaking, the further away from the service you are, the lower your available fault current will be, and thus a lower AIC device may be adequate; HOWEVER I am not an expert on this subject and could very well be talking out of my ass. A fault current calculation would probably be in order.

There are several members on here who are real experts that could probably answer your question.


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## TOOL_5150 (Aug 27, 2007)

erics37 said:


> Generally speaking, the further away from the service you are, the lower your available fault current will be, and thus a lower AIC device may be adequate; HOWEVER I am not an expert on this subject and could very well be talking out of my ass. A fault current calculation would probably be in order.
> 
> There are several members on here who are real experts that could probably answer your question.


But what he describes, is opposite of what you describe. I see it as:

Service > 10k AIC DP > 22K AIC New Sub panel


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## gv703 (Jan 27, 2013)

TOOL_5150 said:


> But what he describes, is opposite of what you describe. I see it as:
> 
> Service > 10k AIC DP > 22K AIC New Sub panel


You're correct, that's my situation. The Distribution Panel for the floor is about 250' away. The service actually comes into the building to the MDP in another floor much further away. I haven't had a chance to look at it yet.


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## chicken steve (Mar 22, 2011)

haven't had to do the calcs in a long time, but Eric's basic jist is spot on, the Xformer kva & impedance....all matter as you get closer to them .....~CS~


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## erics37 (May 7, 2009)

TOOL_5150 said:


> But what he describes, is opposite of what you describe. I see it as:
> 
> Service > 10k AIC DP > 22K AIC New Sub panel


You're right, I had it backwards.

If they spec 22 kAIC stuff then it's probably not gonna do a lot of good downstream from a 10 kAIC MDP. A fault calculation is in order it seems!


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## 123electric (Jun 3, 2012)

For you to truly determine if the AIC ratting of your MLO panel and breakers are sufficient, you need to determine the impedance ratting of the utility transformer as well as the kva size.
A standard 500 kva 120/208 utility transformer will normally have a short circuit rating of 69,444 amps at 2% impedance. If the transformer were to fault under the right conditions with your panel tapped in on the line side of the meter could send 10k panel into flames.


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## micromind (Aug 11, 2007)

Generally speaking, an 800 amp 120/208 circuit can easily have a fault current in excess of 10KA. Very few of them will have more than 22KA.

I suspect the engineer hasn't looked at the whole system (or maybe he has), and he's trying to cover his behind by specing 22KA equipment. 

Or maybe he's trying to be some sort of a big-shot.......look at me; I'm specing higher rated equipment; all those other idiots don't know as much as I do.


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## chicken steve (Mar 22, 2011)

erics37 said:


> You're right, I had it backwards.
> 
> If they spec 22 kAIC stuff then it's probably not gonna do a lot of good downstream from a 10 kAIC MDP. A fault calculation is in order it seems!


But doesn't it work both ways? For ex, what if huge motors (and huge locked current ratings) exist close to the 10kAIC MDP ? 

~CS~


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## micromind (Aug 11, 2007)

123electric said:


> For you to truly determine if the AIC ratting of your MLO panel and breakers are sufficient, you need to determine the impedance ratting of the utility transformer as well as the kva size.
> A standard 500 kva 120/208 utility transformer will normally have a short circuit rating of 69,444 amps at 2% impedance. If the transformer were to fault under the right conditions with your panel tapped in on the line side of the meter could send 10k panel into flames.


Your utility must use different transformers than the one around here does. Around here, a 500KVA transformer operating at 12.5KV and 120/208 would have an impedance of 6% or so. 

In this case, the fault current would be less than 23,167 amps.


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## micromind (Aug 11, 2007)

chicken steve said:


> But doesn't it work both ways? For ex, what if huge motors (and huge locked current ratings) exist close to the 10kAIC MDP ?
> 
> ~CS~


True. In a fault, a running motor will contribute to the fault current.


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## gv703 (Jan 27, 2013)

micromind said:


> Generally speaking, an 800 amp 120/208 circuit can easily have a fault current in excess of 10KA. Very few of them will have more than 22KA.
> 
> I suspect the engineer hasn't looked at the whole system (or maybe he has), and he's trying to cover his behind by specing 22KA equipment.
> 
> Or maybe he's trying to be some sort of a big-shot.......look at me; I'm specing higher rated equipment; all those other idiots don't know as much as I do.


I'm starting to believe you may be right.... I'm going to have to do some calculations of my own :glare:


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## micromind (Aug 11, 2007)

gv703 said:


> I'm starting to believe you may be right.... I'm going to have to do some calculations of my own :glare:


The first place to start is the transformer that supplies the system. Often it is not capable of more than 10KA, and if so, there's no need to go any further.


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## gv703 (Jan 27, 2013)

micromind said:


> The first place to start is the transformer that supplies the system. Often it is not capable of more than 10KA, and if so, there's no need to go any further.


Ok, I did take some notes on the xfmer and it was a big one, if I'm not mistaken its a 225KVA, 480V 3p delta primary - 208V 3p4w wye secondary. When I get back out there I'll have to verify that.


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## gv703 (Jan 27, 2013)

micromind said:


> The first place to start is the transformer that supplies the system. Often it is not capable of more than 10KA, and if so, there's no need to go any further.


Disregard my last post. I just realized you may be referring to the AIR rating, not the size of the xfmer, sorry . I don't have that in my notes, I'll make note of it on my next visit.


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## micromind (Aug 11, 2007)

To get the fault current, we'll need the KVA rating of the transformer and the impedance. It's expressed as a percent, usually somewhere between 2% and 10%.


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## MDShunk (Jan 7, 2007)

It's entirely possible that the engineer has determined that the equipment that exists is inadequate, and wants the new stuff to have the appropriate AIC rating. I never really saw it as my place, when the plans are otherwise engineered, to fuss with anyone about AIC ratings unless I was fishing for a change order.


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## gv703 (Jan 27, 2013)

micromind said:


> To get the fault current, we'll need the KVA rating of the transformer and the impedance. It's expressed as a percent, usually somewhere between 2% and 10%.


The xfmer is a 225 KVA, 480 delta to 208 wye, with a 5.2% impedance.


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## Meadow (Jan 14, 2011)

The lengeth and guage of the conducters from the utility transformer to the meter also play along with the AIC. The longer the less available fault current.


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## gv703 (Jan 27, 2013)

meadow said:


> The lengeth and guage of the conducters from the utility transformer to the meter also play along with the AIC. The longer the less available fault current.


The xfmer is right next to the panel. I'd say the conductors are 12' long tops, and the gauge (I'd guess) to be parallel 500s, since it's an 800A service. I don't know for sure since I can't open the panel up without shutting it down. Now, if you mean on the primary side, it's feeding off of a Main Service panel, which is about 15' away, off of a 400A breaker I believe.


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## micromind (Aug 11, 2007)

gv703 said:


> The xfmer is a 225 KVA, 480 delta to 208 wye, with a 5.2% impedance.


That transformer can produce 12,019 amps at its terminals provided there's a full 480 volts at the primary at the time of the fault. 

In reality, 10KA breakers will be fine, here's why;

1) If a circuit is indeed a bolted fault (dead short), the voltage at the primary will not be 480. It'll likely be quite a bit less, very likely less enough to get the fault current below 10KA.

2) A considerable amount of fault current will be lost across the 208 main breaker. 

3) Some fault current will be lost in the feeder from the transformer to the main breaker. 

If all 3 of these didn't reduce the fault current to below 10KA, I'd be really surprised.


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## RePhase277 (Feb 5, 2008)

micromind said:


> That transformer can produce 12,019 amps at its terminals provided there's a full 480 volts at the primary at the time of the fault.
> 
> In reality, 10KA breakers will be fine, here's why;
> 
> ...


I'd say it was marginally less. But maybe too close for comfort. 22kAIC equipment may cover his butt, just in case.


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