Calculating the AIC

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Open Neutral

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Given a 1.8%Z transformer with 23,100 Amp fault, a 80 ft secondary run of paralleled 350MCM, how do I calculate the AIC?
(My trust in the utility and EC is less than wholehearted; I want to verify what we're being told....)
 
AIC isn’t something you calculate, it’s the rating of the device.
what you want is Available Short Circuit Current

Isc= XF kVA x 100 / (sq rt 3) x (secondary kV) x (%Z of the XF)

Conductors: %Z wire = ((reactance of wire in table 9) x kVA / voltage squared) x ((length of conductor / number per phase) x 100)
 
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Open Neutral said:
Given a 1.8%Z transformer with 23,100 Amp fault, a 80 ft secondary run of paralleled 350MCM, how do I calculate the AIC?
(My trust in the utility and EC is less than wholehearted; I want to verify what we're being told....)
Click to expand...

A few things are unclear. I assume the 23,100 value is the ASSC at the secondary of the transformer? If so the 1.8%Z is a red herring. Also would need the voltage and system type. If the 23,100 is on the transformer secondary and you want to verify that, we need the size of the transformer and the system type.

This is handy: https://www.mikeholt.com/files/other/Fault_Ver.2016.xlsx
 
Open Neutral said:
The POCO uses the term "23,100A fault current" on their 50KVA 240/120 secondary padmount transformer. But the secondary cabling has some impedance, true?

HB/LV is talking 3 phase I assume, this is single phase.
I'm looking at John S's spreadsheet but am missing some data because there's no result.
Click to expand...

For single phase transformers, the ASSC is typically higher across the 120V winding. It seems a common rule of thumb is assume 50% higher than the L-L SCC. For example, see this document page 27.

https://www.pse.com/-/media/PDFs/Co...hash=261DD91382D90EF7192C85C21413E4DC06D07E63


PSE actually gives the %Z for the entire winding and the 120V winding and you can see the L-N SCC is about 50% higher. This explains why that 23,100 figure seems twice as high as you would think. I get about 17k ASSC after the 80 feet of conductor. Is this an issue? Pretty much anything with a main will have a 22K main that series rates with 10k branches. One exception to look out for is when using a standard 10k back fed as a main like in meter centers. You would likely need the higher AIC (typically "H" version) in those situations.
 
Open Neutral said:
The POCO uses the term "23,100A fault current" on their 50KVA 240/120 secondary padmount transformer. But the secondary cabling has some impedance, true?

HB/LV is talking 3 phase I assume, this is single phase.
I'm looking at John S's spreadsheet but am missing some data because there's no result.
Click to expand...

Can’t you input the voltages on the spreadsheet, or did you get a snapshot only?
I believe put the voltages you should get usable data.
 
electrofelon said:
For single phase transformers, the ASSC is typically higher across the 120V winding. It seems a common rule of thumb is assume 50% higher than the L-L SCC. For example, see this document page 27.

https://www.pse.com/-/media/PDFs/Co...hash=261DD91382D90EF7192C85C21413E4DC06D07E63


PSE actually gives the %Z for the entire winding and the 120V winding and you can see the L-N SCC is about 50% higher. This explains why that 23,100 figure seems twice as high as you would think. I get about 17k ASSC after the 80 feet of conductor. Is this an issue? Pretty much anything with a main will have a 22K main that series rates with 10k branches. One exception to look out for is when using a standard 10k back fed as a main like in meter centers. You would likely need the higher AIC (typically "H" version) in those situations.
Click to expand...

Beat me to it....:p
 
Hv&Lv said:
AIC isn’t something you calculate, it’s the rating of the device.
what you want is Available Short Circuit Current

Isc= XF kVA x 100 / (sq rt 3) x (secondary kV) x (%Z of the XF)

Conductors: %Z wire = ((reactance of wire in table 9) x kVA / voltage squared) x ((length of conductor / number per phase) x 100)
Click to expand...

That is NOT the available short circuit current but rather infinite bus short circuit current. Available short circuit current is the fault current that can be delivered by the utility based on their system. This information has to come from the utility. The infinite bus short circuit current is useful in design to spec the AIC and SCCR equipment ratings. The available short circuit current is needed to comply with NEC 70 labeling for service equipment 1200A or greater, analyzing incident energy values (arc flash) and to determine if installed equipment is not rated properly.
 
wbdvt said:
That is NOT the available short circuit current but rather infinite bus short circuit current. Available short circuit current is the fault current that can be delivered by the utility based on their system. This information has to come from the utility. The infinite bus short circuit current is useful in design to spec the AIC and SCCR equipment ratings. The available short circuit current is needed to comply with NEC 70 labeling for service equipment 1200A or greater, analyzing incident energy values (arc flash) and to determine if installed equipment is not rated properly.
Click to expand...

And that infinite bus value is something we can calculate. Can be difficult to obtain information needed to get true available fault current as we won't necessarily know enough about details on supply side of the transformer, and those details can change with no notification as well. All POCO needs to do is some upgrading to their system and things can easily change but you never know about it in your facility.

POCO can give you information or even a SSC value, but without knowing how it was determined you still don't know how accurate it is or it could even be something like "this is a pretty fair average one may expect, but changes in system loading or other conditions can make it higher or lower.
 
wbdvt said:
That is NOT the available short circuit current but rather infinite bus short circuit current. Available short circuit current is the fault current that can be delivered by the utility based on their system. This information has to come from the utility. The infinite bus short circuit current is useful in design to spec the AIC and SCCR equipment ratings. The available short circuit current is needed to comply with NEC 70 labeling for service equipment 1200A or greater, analyzing incident energy values (arc flash) and to determine if installed equipment is not rated properly.
Click to expand...

not completely...
the conductors Z has to be added also.. I thought I put that in there..

I doubt if I will ever give you our source impedance.
Your going to get infinite bus for the most part unless your WAY down the line...
From there, you need to add your conductor Z.
Like it has been mentioned already, we make changes all the time and aren’t going to tell you about them.
 
Hv&Lv said:
...Your going to get infinite bus for the most part unless your WAY down the line...


Like it has been mentioned already, we make changes all the time and aren’t going to tell you about them.
Click to expand...

Almost all of the utilities I get information from have a similar position for their 'secondary voltage' customers (the utility owns the transformer). However, they will all provide actual, and often design level, fault current for the primary service customers (the customer owns the transformer).

So the only thing the utility changes would be their transformer. Why, adopt the philosophy that your system changes for one type of customer but not the other?
 
jim dungar said:
Almost all of the utilities I get information from have a similar position for their 'secondary voltage' customers (the utility owns the transformer). However, they will all provide actual, and often design level, fault current for the primary service customers (the customer owns the transformer).

So the only thing the utility changes would be their transformer. Why, adopt the philosophy that your system changes for one type of customer but not the other?
Click to expand...

Wire changes, feeds, etc, all have an effect. A normal feed may have 1 mile of conductor, an alternate feed could have three miles.
Transformers aren’t the only change.

why change for one type vs another? No clue why they do that..
 
I would conjecture it is partly due to the size of the customer. I dont see a utility paying their engineers for researching and investigating primary SCC - and informing the customer down the road of any changes - on a secondary metered service where the transformer reduces the SCC to a reasonable value (perhaps not the case for some network services). I would also conjecture that taking into consideration limited primary SCC over infinite has a relatively small effect as compared to that of the transformer.
 
electrofelon said:
I would conjecture it is partly due to the size of the customer. I dont see a utility paying their engineers for researching and investigating primary SCC - and informing the customer down the road of any changes - on a secondary metered service where the transformer reduces the SCC to a reasonable value (perhaps not the case for some network services). I would also conjecture that taking into consideration limited primary SCC over infinite has a relatively small effect as compared to that of the transformer.
Click to expand...

Now modeling does all the work...
click a spot, and the Z is there. That fast.
I know a couple that give exact values based on the modeling.
I don’t like to do that.
Say you base your values on the exact figure your given, there is some type of change by the utility that makes the figure go higher. Say we build a new feed that cuts off a 1/4 mile. Now the values are higher. Someone gets burnt with the wrong PPE, who is to blame??
 
Hv&Lv said:
Now modeling does all the work...
click a spot, and the Z is there. That fast.
I know a couple that give exact values based on the modeling.
I don’t like to do that.
Say you base your values on the exact figure your given, there is some type of change by the utility that makes the figure go higher. Say we build a new feed that cuts off a 1/4 mile. Now the values are higher. Someone gets burnt with the wrong PPE, who is to blame??
Click to expand...

Good point. Here the Poco is doing a wholesale upgrade of a 6 mile section of line, including a brand new route for part of it . I imagine the SCC is going up.
 
I understand that the 'infinite bus' method is generally considered more conservative, because the actual short circuit current on the output of the transformer will be lower than that given by the infinite bus method.

But aren't there some situations where the higher available short circuit current can result in lower incident energy, because protective devices operate faster? Or do the calculations for incident energy take this into account?

Thanks
Jon
 
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