AIC rating

hhsting

Senior Member
I asked for calculation of AIC rating per NEC 2014 section 110.9 and 110.10. The engineer coordinated with utility provided pdf from electric utility stating initial fault current are 22kA and these are subject to change anytime.
Engineer based calculation on this and showed AIC rating of equipment.

I sent comment back saying please provide worst case short circuit calculation utility xfmr impedance and size and infinite bus and that initial preliminary calculation is not acceptable and not in compliance 110.9 and 110.10.

I got response back saying they are in compliance.

Would the initial fault currents subject to change at the utility xfmr be acceptable or not? Would not one need wise case short circuit fault current based in poco xfmr impedance, size and infinite bus at primary?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
That has been an issue across the country. Utilities sometime upgrade the tranies to a larger aic rating and it can impact all the equipment in the building. If the 22ka is there at the time of the install then you are compliant but if the engineers think it may be changed they should, imo only, raise the Ka...
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Utilities always say their values are subject to change. After all, if they upgrade their network fault currents may be increased. In some cases an expansion is already being conducted while in others it is only on the 10 or 20 growth plan.

This is why many designers specify AIC ratings that are much higher than the initial available short circuit amps (SCA).
 

kwired

Electron manager
Location
NE Nebraska
Assuming infinite bus primary is somewhat safe worst case - until they increase transformer size sometime down the road, then new calculation would be needed.

If you have 22k gear and the calculation comes out just about right at 22k, probably worth consideration to think about higher rated gear in some cases.

Don't forget that number will drop if there is any conductor between whatever point they calculated and the gear you are concerned about.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
I'm trying to create a chart, but its going over like lead balloon since it involves challenging the code as it stands...
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Everything is subject to change. Code compliance is only guaranteed at the time of the inspection, "what if" is not in the NEC. Ground tests results change by the minute, voltage from a transformer changes constantly, people changing breakers or fuses to larger sizes, etc... I agree with the engineer that he is in compliance.

Roger
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
We should get into the habit of using terms more correctly. AIC is the rating of the device that clears the fault. “I” is for “interrupt”.
The power source has an available fault current, not an AIC.
Bad habit. I have been told about it before but I just don't think about it....
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
....
Would the initial fault currents subject to change at the utility xfmr be acceptable or not? Would not one need wise case short circuit fault current based in poco xfmr impedance, size and infinite bus at primary?
The typical fault current provided by a utility is based on the lowest impedanace transformer of that sise used in their system and an infinite primary. With many utilities, it is difficult to get the real world number that you need for arc flash calculations.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
If it's 22k there a good possibilty your panel is series rated. If so and you follow 110.22 labeling requiremetns the SCA at the time of your install will be evident for future examination if needed.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
The typical fault current provided by a utility is based on the lowest impedanace transformer of that sise used in their system and an infinite bus...
It is not unusual to find the utility is also anticipating the transformer will e entirely be replaced with a larger size unit. Either they undersized the original unit or they are planning for an emergency replacement based on what can fit in the same space and what they can get there hands on.

Yes, this is an inherent problem with conducting arc flash studies.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Ok so what happens if the power company changes a trany that was 22ka to one that is 35ka. Does that mean all the equipment would need to be replaced if the aic is 22ka?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Ok so what happens if the power company changes a trany that was 22ka to one that is 35ka. Does that mean all the equipment would need to be replaced if the aic is 22ka?
Yes, if the utility makes a change that increases the fault current to a value higher than the rating service and/or down stream equipment, the equipment needs to be upgraded to the new fault current. Often, this does not happen because the building owners have no idea that the utility has made a change, even if they get something that shows an increase in the available fault current, the owners often have no idea what that means and take no action.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
and, if I may add to your scenario, Customer A and Customer B are fed from the same transformer.... "A" increases his load causing POCO to install a bigger transformer increasing the SCA for "A" and "B".
"B" has not altered his electrical system but is now non compliant.
If he is required to update .. at who's expense ???? :)
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Yes, if the utility makes a change that increases the fault current to a value higher than the rating service and/or down stream equipment, the equipment needs to be upgraded to the new fault current. Often, this does not happen because the building owners have no idea that the utility has made a change, even if they get something that shows an increase in the available fault current, the owners often have no idea what that means and take no action.

Yeah that is what I had heard so what we have is a utility that needs to figure something out. That could cost hundreds of thousands of dollars. Couldn't they just add another transformer instead of causing this issue
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
and, if I may add to your scenario, Customer A and Customer B are fed from the same transformer.... "A" increases his load causing POCO to install a bigger transformer increasing the SCA for "A" and "B".
"B" has not altered his electrical system but is now non compliant.
If he is required to update .. at who's expense ???? :)
is it common that multiple commercial or industrial customers are served from the same transformer?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
is it common that multiple commercial or industrial customers are served from the same transformer?
This is why some utilities provide what seem to be unreasonably high fault values. They are trying to future proof the service entrance gear.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Ok so what happens if the power company changes a trany that was 22ka to one that is 35ka. Does that mean all the equipment would need to be replaced if the aic is 22ka?

Not necessarily, the impedance of the drop and service conductors knocks down the fault current.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Yeah that is what I had heard so what we have is a utility that needs to figure something out. That could cost hundreds of thousands of dollars. Couldn't they just add another transformer instead of causing this issue
I think all of what the utility does would be covered whatever state agency sets the utility rules.
Sure they could add an addition transformer where they need more capacity, but that may not be cost effective for them and they will not do that unless they are forced to.

Like I said in my previous post, I think that in the majority of these cases, nothing is changed by the customer. I am not sure there is much the AHJ can actually do to force the customer to upgrade his equipment to match the new available fault current. In many cases the AHJ will not even know that the transformer has been changed to one with a higher available fault current.
 
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