When calculated short circuit current exceeds AIC rating

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SaraL

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Washington, DC
I'm working on a small new construction project where panelboards were purchased and installed before the short circuit analysis was completed for the system (buildng construction is still in progress, though). The analysis has found that one panel, which has an AIC rating of 14k, has an available short circuit current of 14,556AIC. While my specs do require the electrician to confirm available short circuit current and ensure that the ratings of the equipment they install meet or exceed that, I'm wondering if there are any options to remedy this situation short of replacing the installed panelboard with a new panel that has a higher AIC rating. The panelboard with the issue is the main panel for the building and is 400A, 277/480V 3ph 4W. The AIC ratings for the rest of the downstream panels look fine.

The only two options I'm aware would be looking into the use of line reactors or increasing the length of the feeder. I'm going to check on how much longer the feeder length would need to be to reduce the available current to within 14k. Since the analysis is showing that the current is only slightly over that 14k rating, I'm thinking that the additional length needed to bring the rating down to 14k might not be too bad. Any thoughts? (as an aside, I read this forum all the time...thanks to everyone who contributes!)
 

jim dungar

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Wisconsin
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PE (Retired) - Power Systems
Line reactors are, more often than not, considered as a last option, rather than a first choice.

The bracing of your panel 'interior' is rarely a concern, most of them are rated for 'the highest installed breaker'. So it is ofeten easiest to replace the branch breakers themselves (almost all 14kAIC breakers have versions with higher ratings)

Can you change the feeder device to be something that has a series-combination rating with the installed breakers?

How about 'lengthening' the conductor run, by coiling conductors in a large pull box, or taking the long way around a room?
 

SaraL

Member
Location
Washington, DC
Thanks for the fast response. I'm already checking with the person who did the analysis to see how much extra wiring would be needed to bring it down to 14k. Your other ideas for using either a series-rated main breaker (I'm already calling for an MCB) or changing out the branch circuit breakers for ones with a higher rating (provided the interior of the panel has a rating higher than 14k) could also work out well. I'll keep investigating these 3 options to see which would be the most feasible.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
I'm working on a small new construction project where panelboards were purchased and installed before the short circuit analysis was completed for the system (buildng construction is still in progress, though). The analysis has found that one panel, which has an AIC rating of 14k, has an available short circuit current of 14,556AIC. While my specs do require the electrician to confirm available short circuit current and ensure that the ratings of the equipment they install meet or exceed that, I'm wondering if there are any options to remedy this situation short of replacing the installed panelboard with a new panel that has a higher AIC rating. The panelboard with the issue is the main panel for the building and is 400A, 277/480V 3ph 4W. The AIC ratings for the rest of the downstream panels look fine.

The only two options I'm aware would be looking into the use of line reactors or increasing the length of the feeder. I'm going to check on how much longer the feeder length would need to be to reduce the available current to within 14k. Since the analysis is showing that the current is only slightly over that 14k rating, I'm thinking that the additional length needed to bring the rating down to 14k might not be too bad. Any thoughts? (as an aside, I read this forum all the time...thanks to everyone who contributes!)

There is another option. Installing a fused disconnect or a higher AIC main breaker in front of the panel that series rates with it and allows reduction of the available fault current.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
be thankful we have Jim on the Forum. As you can see, he is worth far more than he is paid here :D
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
..While my specs do require the electrician to confirm available short circuit current and ensure that the ratings of the equipment they install meet or exceed that ...
Unless this is a design and build job, I don't see why you would expect the contractor to do engineering work.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Are you sure the panel is only rated for 14K? If I put 14K AIC on the construction drawings, the panelboard shop drawings will come back showing 14K even if the breakers are good for a higher rating.

Also, as others have mentioned, series ratings are a good option. You may already have a breaker upstream of this panel that gives you a series rated of more than 14K.

If neither of those work, I assume the calculation was completed assuming infinite primary on the line side of the main transformer. Since you are so close to the 14K rating, recalculating the currents including a short piece of wire on the transfomers primary would probably bring your calculation to under 14K.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
You might not have to increase the length of the conductors that feed this panel. All you might have to do is to discover what the actual length is, and put that value into the calculation. You might also check the actual impedance of the upstream transformer, and the lengths and sizes of the conductors on its primary side. Just ask the contractor to give you measured values, and not just estimated values. It is entirely possible that this will give the computer software enough of a difference in its input data to give you what you need in the output data.

Welcome to the forum.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Unless this is a design and build job, I don't see why you would expect the contractor to do engineering work.
The way I would ask this is, why is the contractor purchasing the equipment before the engineer has provided the requirements for fault duty?
 

Flapjack

Senior Member
Location
USA
Occupation
EE
You might not have to increase the length of the conductors that feed this panel. All you might have to do is to discover what the actual length is, and put that value into the calculation. You might also check the actual impedance of the upstream transformer, and the lengths and sizes of the conductors on its primary side. Just ask the contractor to give you measured values, and not just estimated values. It is entirely possible that this will give the computer software enough of a difference in its input data to give you what you need in the output data.

As steve said too, if the calc was assuming an infinite source, then you should be fine. I would take the above advice, and you probably won't have to make any changes.
 

SaraL

Member
Location
Washington, DC
As an update, the team doing the short circuit calculation extended the study further upstream beyond the utility xfmr (we were able to get some short circuit info further up from the xmfr) and the calculated short circuit at the main panel is fine now.

And since there has been discussion about why things were happening in this sequence...my specifications usually do include a requirement for the contractor to perform (or contract out for somenoe else to perform) a short circuit analysis and to coordinate the final ratings of the equipment with the results of that study. Because the actual distances of feeders being installed can vary from what was assumed during design and because the final specification/size of the utility transformer is also frequently either an unknown during design or it changes during construction, the short circuit analysis we require is kind of the final check before the AIC ratings of equipment is finalized. I know that there are alot of differences of opinion out there on whether this is appropriate, but it's something I include in my specs.

In the case of this project, the contractor didn't perform this study at all, the client asked our team to do the study instead, and this request came in after everything was installed on-site. Regardless of who performs the study, I think we can all agree that this was not the correct order for things to be done in.
 
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