# AIC. what to do?



## A Good Electrician (May 29, 2011)

So, I'm here in LA and all i need to do is add a new panel off of a distribution from an existing spare 200amp breaker. I haven't seen the job yet, this is all on paper. But, my question is, shouldn't the AIC rating of the new panel and breakers be engineered? I've looked up some ways to determine the AIC but they are all VERY in depth and complicated. 
Outside of engineering it, is there a rule of thumb or a simple way of making sure that it's safe? For example, if the AIC at a distribution is 42Kv, can i just match that or make it greater?


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## Dennis Alwon (May 9, 2009)

If you match the AIC of the distribution panel then you should be fine however that is very costly.  Normally an engineer does the calculation but there are some online calculators that may be helpful.


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## Big John (May 23, 2010)

A Good Electrician said:


> ...Outside of engineering it, is there a rule of thumb or a simple way of making sure that it's safe...?


 You can ball-park it. 

Divide 100 by the transformer impedance, and multiply the result by the transformer single-phase full load current.

E.g.:
480V 75kVA 2% impedance
156A full load current
100 / 2% * 156A = 7,812A short-circuit current

This method makes a lot of assumptions: 

That the utility supply on the transformer high side is an infinite bus and that there is zero impedance in the cables between the transformer and your service. Changing either of those will cause your SCC to go _down._

Also that you have nominal line voltage or less, and assuming your loads are near unity power factor (there is no huge imbalance of motors/transformers/capacitive loads, which is almost always true for residential and light commercial). Changing either of those will cause your SCC to go _up. 
_


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## micromind (Aug 11, 2007)

We need a bit more info; is it single or 3Ø? What voltage?

What is feeding the 200 amp breaker? 

If possible, get the KVA rating of the POCO transformer. If it's pole-mount, it's usually printed on the front, if it's pad-mount, sometimes it's printed. 

If nothing else, if the 200 is in a switchgear, the amp rating of the gear would help. 

The AIC rating of the 200 very likely doesn't mean much.


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## A Good Electrician (May 29, 2011)

Thank you Big John. That helps. What is SCC?


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## A Good Electrician (May 29, 2011)

Micromind, Its a 3 phase. Its being feed from a 120/208 3 phase 4 wire 800amp Distribution Board. There are (5) 100amp panels already being feed from this Board as well as (1) 225amp Panel. This will be a new 200amp panel.


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## A Good Electrician (May 29, 2011)

Also, what is POCO?


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## dthurmond (Feb 7, 2011)

A Good Electrician said:


> Also, what is POCO?


Power Company


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## Big John (May 23, 2010)

A Good Electrician said:


> Thank you Big John. That helps. What is SCC?


 Short Circuit Current. 

That said, for some reason I thought you were putting in a new service. If you're not, just match the AIC for whatever is upstream of your distribution board.


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## micromind (Aug 11, 2007)

Assuming the POCO fed it with a 225KVA transformer, with an impedance of 5%, there would be 12,500 amps available at the secondary bushings at the transformer. 

By the time it gets to the breaker, it's be more like 10,000 amps. 

If the impedance is 3%, there's be 20,833 amps at the bushings, maybe 18,000 at the breaker. 

If the transformer is 150KVA (around here, it most likely would be), then at 3% impedance there'd be 13,866 amps at the bushings, maybe 11,000 at the breaker. 

3% is fairly low, but certainly not unheard of. Most POCO transformers of that size around here are 4 - 6%. 

I'd be real surprised if the fault current at the new breaker exceeded 25,000 amps. 

Around here, you can ask the POCO what the available fault current is, and they'll give the figure at the switchboard terminals. It's usually lower than you'd think.


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## Cl906um (Jul 21, 2012)

Copper vs aluminum and distance changes a lot too


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## RIVETER (Sep 26, 2009)

A Good Electrician said:


> So, I'm here in LA and all i need to do is add a new panel off of a distribution from an existing spare 200amp breaker. I haven't seen the job yet, this is all on paper. But, my question is, shouldn't the AIC rating of the new panel and breakers be engineered? I've looked up some ways to determine the AIC but they are all VERY in depth and complicated.
> Outside of engineering it, is there a rule of thumb or a simple way of making sure that it's safe? For example, if the AIC at a distribution is 42Kv, can i just match that or make it greater?


I am not an engineer but if you are coming off of an already engineered panel on the 200 amp breaker just match the 200 amp breaker rating. What you are wanting to do anyway is to use breakers that can withstand the maximum amount of current that the source transformer can produce in a fault condition without exploding or causing a fire.


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## OaklandElec (Jan 4, 2011)

A Good Electrician said:


> So, I'm here in LA and all i need to do is add a new panel off of a distribution from an existing spare 200amp breaker. I haven't seen the job yet, this is all on paper. But, my question is, shouldn't the AIC rating of the new panel and breakers be engineered? I've looked up some ways to determine the AIC but they are all VERY in depth and complicated.
> Outside of engineering it, is there a rule of thumb or a simple way of making sure that it's safe? For example, if the AIC at a distribution is 42Kv, can i just match that or make it greater?


PG&E provides you with a letter telling you the AIC for your service. Contact them, ask for an AIC letter.


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## Meadow (Jan 14, 2011)

This might help:

http://www.alabamapower.com/business/services/architects-engineers/pdf/A%20E%20Fault%20Currents%20Tables%20FINAL%208%202003.pdf


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## A Good Electrician (May 29, 2011)

Thank you Micromind. this is very helpful.


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## A Good Electrician (May 29, 2011)

Thank you Riveter. I'm going to now look into the formula I need to work out the problem. This is very helpful.


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## Bad Electrician (May 20, 2014)

You are a Good Electrician and I am a Bad Electrician we should team up, we'd make one average electrician.:thumbsup:


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## A Good Electrician (May 29, 2011)

So, what I've gathered from all this guys is Phasing is important to know, impedence, type of wire, amps at my main feed BUT I have another question then. In my situation where I'm adding a 200amp from the distribution board (I just want to be clear here) I need to get all of that information from the utility connection point ALL the way to my New Panel?
Or am I getting that information just for anything from my main breaker feeding my panel and All in between?


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## A Good Electrician (May 29, 2011)

I'm in.


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## don_resqcapt19 (Jul 18, 2010)

Big John said:


> Short Circuit Current.
> 
> That said, for some reason I thought you were putting in a new service. If you're not, just match the AIC for whatever is upstream of your distribution board.


That assumes that the original equipment was correctly selected and/or that the utility has not made changes that have increased the available fault current since the original installation.


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## Southeast Power (Jan 18, 2009)

A Good Electrician said:


> So, I'm here in LA and all i need to do is add a new panel off of a distribution from an existing spare 200amp breaker. I haven't seen the job yet, this is all on paper. But, my question is, shouldn't the AIC rating of the new panel and breakers be engineered? I've looked up some ways to determine the AIC but they are all VERY in depth and complicated.
> Outside of engineering it, is there a rule of thumb or a simple way of making sure that it's safe? For example, if the AIC at a distribution is 42Kv, can i just match that or make it greater?


If you have fuses coming in, don't worry about it.
If you have 42k upstream, don't worry about it.
Make it smaller than what you have between you and the POCO.


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## Big John (May 23, 2010)

don_resqcapt19 said:


> That assumes that the original equipment was correctly selected and/or that the utility has not made changes that have increased the available fault current since the original installation.


 Yep. Personally, that's a risk I'm comfortable taking: I don't go back to the utility for a AFC calculation every time I add a subpanel.


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## OaklandElec (Jan 4, 2011)

If you are installing a new main, you have to use the AIC letter rating from PG&E. The inspector will want to see the letter. If the equipment is 25'+ from the main, you can go 10k.


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## tates1882 (Sep 3, 2010)

I have a pretty slick excel worksheet with all the cells formatted with the correct formulas. I f someone can tell me how, I'll share it with everyone.


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## don_resqcapt19 (Jul 18, 2010)

Big John said:


> Yep. Personally, that's a risk I'm comfortable taking: I don't go back to the utility for a AFC calculation every time I add a subpanel.


It would depend on how old the original is for me. Around here they only started looking at this issue about 10 years ago.


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## 123electric (Jun 3, 2012)

Big John said:


> You can ball-park it.
> 
> Divide 100 by the transformer impedance, and multiply the result by the transformer single-phase full load current.
> 
> ...


Big John,
Correct me if I'm wrong but isn't the short circuit rating of a 75kva 480volt transformer @ 2% impedance 4,500 Amps?


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## Meadow (Jan 14, 2011)

tates1882 said:


> I have a pretty slick excel worksheet with all the cells formatted with the correct formulas. I f someone can tell me how, I'll share it with everyone.




I think this page allows for xcels of a certain size to be linked like pics.


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## chicken steve (Mar 22, 2011)

A Good Electrician said:


> So, what I've gathered from all this guys is Phasing is important to know, impedence, type of wire, amps at my main feed BUT I have another question then. In my situation where I'm adding a 200amp from the distribution board (I just want to be clear here) I need to get all of that information from the utility connection point ALL the way to my New Panel?
> Or am I getting that information just for anything from my main breaker feeding my panel and All in between?


Most calcs are line/load >xformer to breaker/fuse _(fuses being more aic for the $$, as Jrannis alludes) _ , but if all serves a honker of a 3ph motor downstream the calc could _(theoretically)_ work the other way load / line....

~CS~


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## micromind (Aug 11, 2007)

123electric said:


> Big John,
> Correct me if I'm wrong but isn't the short circuit rating of a 75kva 480volt transformer @ 2% impedance 4,500 Amps?


It is if it's 3Ø, but John stated that it's single phase.


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## micromind (Aug 11, 2007)

chicken steve said:


> Most calcs are line/load >xformer to breaker/fuse _(fuses being more aic for the $$, as Jrannis alludes) _ , but if all serves a honker of a 3ph motor downstream the calc could _(theoretically)_ work the other way load / line....
> 
> ~CS~


This is true, a motor running across the lines will become a generator very quickly during a fault, and it certainly will contribute to the fault current.


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## 123electric (Jun 3, 2012)

micromind said:


> It is if it's 3Ø, but John stated that it's single phase.


Read his calculation not his first his written paragraph.

Tell me what he wrote for the voltage?


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## Big John (May 23, 2010)

123electric said:


> Correct me if I'm wrong but isn't the short circuit rating of a 75kva 480volt transformer @ 2% impedance 4,500 Amps?


 The reason it's not is because if you calculate based on L-G voltage then you end up underestimating how much current the transformer is capable of pushing during a L-L fault. The system needs to be designed based on the worst-case-scenario current flow, even though a line-to-ground fault is the most likely mode of failure.

Confusing wording in my original example, I meant 480V "single phase" as in "any two phase conductors."


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## 123electric (Jun 3, 2012)

Big John said:


> The reason it's not is because if you calculate based on L-G voltage then you end up underestimating how much current the transformer is capable of pushing during a L-L fault. The system needs to be designed based on the worst-case-scenario current flow, even though a line-to-ground fault is the most likely mode of failure.
> 
> Confusing wording in my original example, I meant 480V "single phase" as in "any two phase conductors."


Big John,
Thank you for clearing the confusion on this. 
Enjoy the rest of your Sunday!


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## chicken steve (Mar 22, 2011)

It's all about that R factor if one runs the calcs. I often dumb it down to looking out the window to see how far the Xformer is. A while ago I wired a phase to phase SC , which lit up the whole room when activated.  It made quite the impression on my apprentice....

~CS~


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