kAIC Ratings for Service Disconnect & Main Distribution Panel

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Hello all,

I am a licensed electrician / electrical contractor in Maui, Hawaii, and am currently reviewing an engineered drawing to prepare a bid on a residential project. The property will consist of a 7,000 square foot main dwelling, 1,000 square foot additional dwelling (cottage), a detached garage, swimming pool, etc.. The engineered drawing that I am looking at specifies that the service will be a 600A service. The service conductors will energize a 600A Fusible Disconnect rated for 63kAIC. The feeders on the load side of the service disconnect will energize a 600A main distribution panel. The MDP does not show having a 600A main breaker, but the number of disconnecting means within the enclosure is less than 6, so no problems there. The 600A MDP is also specified at 63kAIC. All Sub-panels after the MDP - house panels, pool panels, detached garage panel, etc. - are specified at 38kAIC.

In my opinion, the kAIC ratings of 63kAIC & 38kAIC are overkill for a residential application. However, this is a multi-acre, multi-million dollar, custom, high-end, luxury home here on Maui, so maybe we're just building the best of the best.

I am currently building a 7,500 square foot structure with a 300A electrical service. It's high-end and nice, yada yada, but not as nice as the above mentioned. My main question is this:

Are the kAIC ratings mentioned above indeed overkill & unnecessary, or are they legit? Does anyone have experience building residential dwellings with 600A services? I believe the engineer was hired to just get through the permitting phase with the county, and the service request phase with the utility company. I don't think he's pouring his heart into these drawings, and I have already found other inconsistencies that he has admitted to be 'Boiler-Print' type specifications. Thanks for reading this. Hope to hear from you all soon. Have a great weekend. Aloha.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
...In my opinion, the kAIC ratings of 63kAIC & 38kAIC are overkill for a residential application....
Being residential is irrelevant. The ONLY thing that matters is that the rating is equal to or greater than the available short circuit current at the line terminals. If someone performed a study on what is going to be available from the utility and it requires a high AIC rating, then it doesn’t matter if it’s residential, commercial, industrial, recreational or a glorified outhouse.

That said, 63kA seems a bit unusual for a 600A service, but it could be that the utility only stocks larger transformers so they end up this way. Could be other reasons as well.
 
Thanks for the response. I am pulling my hair out trying to make a bid for this drawing. The service disconnect is located on a pedestal at the property entrance. The drawing shows the MDP located inside a pool equipment vault - at a distance of 268 feet from the service disconnect. I would then have to run another set of parallel feeders from the MDP another 250 - 300 feet to feed the main dwelling (and all additional buildings & structures) with another 400 amp distribution panel. In total, the feeders need to run approximately 550 - 600 feet before actually energizing the branch circuits, except the pool equipment. It seems like voltage drop is going to require all my feeders to be exceptionally large, and I think I am going to call the general contractor tomorrow and ask him to reconsider this design.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Thanks for the response. I am pulling my hair out trying to make a bid for this drawing. The service disconnect is located on a pedestal at the property entrance. The drawing shows the MDP located inside a pool equipment vault - at a distance of 268 feet from the service disconnect. I would then have to run another set of parallel feeders from the MDP another 250 - 300 feet to feed the main dwelling (and all additional buildings & structures) with another 400 amp distribution panel. In total, the feeders need to run approximately 550 - 600 feet before actually energizing the branch circuits, except the pool equipment. It seems like voltage drop is going to require all my feeders to be exceptionally large, and I think I am going to call the general contractor tomorrow and ask him to reconsider this design.

Well, the good news is you won't have much fault current left at the end of a run that long!

Kidding aside, I would design differently for something like this.
 

Dzboyce

Senior Member
Location
Royal City, WA
Occupation
Washington 03 Electrician & plumber
My local utility has issued a sheet giving the KAIC of various transformers. Is is then an easy task to use Mike Holt’s spreadsheet or the Eaton’s FC2 app to make my KAIC calculations.


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tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
..The engineered drawing that I am looking at specifies that the service will be a 600A service. The service conductors will energize a 600A Fusible Disconnect rated for 63kAIC....
Was this done by an practicing Electrical Engineer?
Post the drawing if you can.
My guess is any fused 600A disconnect will be rated for 200kAIC and so will the fuses after that your calcs will be down to 22kAIC or less.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Read through all of the notes, the information on the panel schedule, and the specs. Look for the words "fully rated" or "series rated". If there is a note that specifically states, series rate your panels you are golden, if a note specifically states, fully rate, then you just have to bite the bullet and rate per the engineer. You can always offer a VE to series rate. If you don't find notes either way, then series rate and note that in your proposal.
 

kwired

Electron manager
Location
NE Nebraska
Was this done by an practicing Electrical Engineer?
Post the drawing if you can.
My guess is any fused 600A disconnect will be rated for 200kAIC and so will the fuses after that your calcs will be down to 22kAIC or less.
My thoughts also, 200kA is common for fused disconnects, with proper fuse rated 200kA. The mentioned 63kA is maybe the designer's calculated available fault current? If so 200kA is greater and no problem. Seems likely the breakers in the MDP will likely be series rated with the fuses, but double check just to be sure.

As you get further from the source the available fault current will drop. If 63kA is available at the service disconnect and there is 268 feet to the MDP, my guess without calculating is that there is easily less than 10kA available at the MDP regardless of what size conductors you have installed.
 

Carultch

Senior Member
Location
Massachusetts
It is then an easy task to use Mike Holt’s spreadsheet or the Eaton’s FC2 app to make my KAIC calculations.

I use that spreadsheet all the time, but my concern is that transformer impedance is a bit of a wildcard. You can specify it, but can't know its actual value until after the transformer is built. For existing units, most of the time, the detailed nameplate is covered by a dead front (meaning the typical site visit will not get this information).

Is there a list of standard minimum impedance based on transformer type and KVA, for a worst case scenario calculation?
 

Dzboyce

Senior Member
Location
Royal City, WA
Occupation
Washington 03 Electrician & plumber
Is there a list of standard minimum impedance based on transformer type and KVA, for a worst case scenario calculation?

that is what I posted. That is what my local utility supply’s us with. We don’t have to call them for every installation and ask them about a specific set of transformers.
 

pv_n00b

Senior Member
Location
CA, USA
If you want the worst case I can't say I've seen a distribution transformer with an impedance of less than 4%, so maybe use 3% or 3.5%.
 
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If you want the worst case I can't say I've seen a distribution transformer with an impedance of less than 4%, so maybe use 3% or 3.5%.

Maybe for a padmount, but pole units are very often high 1's or low 2's.

These AIC figures utilities give are notoriously high. I dont always get to see the transformer data plate, but when I do there is usually 2-3 times less fault current than the chart shows.
 
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