How to Calculate KAIC Rating Downstream

[Electricalhelp]

The MDP is 208V/3P/2000A with a AIC of 96k.

I have an electrical Panel feeding from the MDP about 200 Feet away with 208V/3P/400A.

The wire size is 2 Sets (4# 350 KCMIL +1 # 2/0 G in 4"C)

How do I calculate the KAIC rating for the Panel Downstream.

 

[david luchini]

There are various on-line calculators to help with that.

Or you could search for "MVA Method" which is a simple way to do the calculation yourself.

 

[texie]

I like the Bussman app. Android or Apple. Painless to use.

 

[jim dungar]

Terminology is important. You calculate the amount of available short circuit amps (SCA) then you select an AIC rating that is larger than your result.

Almost every breaker or fuse manufacturer has an on-line calculator you can use. Most of them take into consideration the conductor insulation as well as the material of the raceway.

 

[main197]

The MDP is 208V/3P/2000A with a AIC of 96k.

How did you get this number? 96,000 A of available fault current seems high to me. What is the SCCR of the MDP? or are you saying the panel is rated 96kA?

 

[Electricalhelp]

How did you get this number? 96,000 A of available fault current seems high to me. What is the SCCR of the MDP? or are you saying the panel is rated 96kA?

This label was located on the Panel

 

[Electricalhelp]

I like the Bussman app. Android or Apple. Painless to use.

How do I use the Bussman app if I already know the KAIC rating of the MDP Panel?

Since on the app, it ask for Primary Fault Current?

 

[jim dungar]

This label was located on the Panel

That says you need to supply minimum 96kAIC rated devices at that location. It does not tell you why.
It does not provide you with enough information to determine an appropriate AIC rating at some other location.

You need to know the available fault current.

 

[retirede]

How did you get this number? 96,000 A of available fault current seems high to me. What is the SCCR of the MDP? or are you saying the panel is rated 96kA?

AIC is not available fault current. Like Jim said, terminology is important!

 

[Electromatic]

Use the 96kA as your primary, then "add conductors to the system" which would be your feeder length, conductor size, conduit type and the app should spit out a number for your downstream panel.

As others have intimated, the app is pretty plug and chug. If the 96kAIC rating of the MDP is exaggerated (higher than it needs to be), then you'll end up with a number for your other panel that is higher than it needs to be.

 

[main197]

AIC is not available fault current. Like Jim said, terminology is important!

Yes, I know that. Just wanted to make sure what they meant.

 

[main197]

That says you need to supply minimum 96kAIC rated devices at that location. It does not tell you why.
It does not provide you with enough information to determine an appropriate AIC rating at some other location.

You need to know the available fault current.

I second this. You need to know the available fault current. If that's not available, you can use the infinite bus method if you know the specs of the transformer.

 

[electrofelon]

This label was located on the Panel

I would be cautious about using that value. That label looks field applied, and I'm not even sure what they are trying to say honestly. I suspect they meant to say available fault current not AIC. I advise determining the available fault current. The AIC rating of that equipment doesn't really matter for what you're doing, unless you want to confirm that it's not under dutied.

The AIC rating of the downstream breakers need to be fully rated for the available fault current at that point, or series rated with the main breaker in the mdp

 

[Electricalhelp]

I second this. You need to know the available fault current. If that's not available, you can use the infinite bus method if you know the specs of the transformer.

I do not know the specs of the transformer. But I am trying to collect that information from the utility company.
I am also trying to get available fault from the utility company as well but still have no luck yet.

 

[Electricalhelp]

Please let me know if I am using the Bussman app correctly now.
I was told the Transformer was 500KVA witha Z% = 4.7%

The distance to the Transformer to the MDP is 35 feet. The distance from the MDP to the Panel is about 350 feet.

I added the Transformer as a infinite bus, but do I do a bus run or a conductor run next?

For the panel, I did a conductor run first then added a bus run, with a assumption of 5ft.

Not sure how they got 96k still

 

[jim dungar]

Please let me know if I am using the Bussman app correctly now.
I was told the Transformer was 500KVA witha Z% = 4.7%

The distance to the Transformer to the MDP is 35 feet. The distance from the MDP to the Panel is about 350 feet.

I added the Transformer as a infinite bus, but do I do a bus run or a conductor run next?

For the panel, I did a conductor run first then added a bus run, with a assumption of 5ft.

Not sure how they got 96k still

Use cables.
Do not worry about the length of bus inside the panel.

 

[Jraef]

This label was located on the Panel

Hand made label done by someone who was unfamiliar with the proper terminology. You don’t “calculate” the IC rating of devices, they are determined through testing done by the nanufacturer, and “panels” can’t have an “AIC” rating. Panels have an SCCR (Short Circuit Current Rating), formerly known as a “withstand rating”. As others have said, you can calculate the Available Fault (Short Circuit) Current /Amps.

I would also be suspicious of their ability to properly calculate the AFC if they came up with 96kA. Although not impossible, it’s improbable. They likely made a mistake or made some assumptions that resulted in an unnecessarily high value.

 

[Cartoon1]

The electrical utility should be able to give you the maximum short at the service transformer on the secondary side. That number can be used to do the calculation.

 

[WA_Sparky]

Everytime i go through this process i reach out to the electrical utility. They have a chart for the infinite buss max available fault for transformers of each voltage type. If its existing building they have the transformer info already, if not you'll need to tell them the anticipated load and service size.

Ask them what the max Available Fault Current (AFC) is at the secondary side of the transformer.
Locate distance from transformer to building disconnect or panel.
Enter in utility provided info, feeder sizes, and number of conductor info into calculator. For distances i would use point to point (hypotenuse) not likely route for installed conductors cause shorter distance is worst case scenario (I do typically add 5' on either end for rise and drop of conductors into equipment).
If your AFC at MDP comes in at 66,340A... 65,000 AIC breaker wont work so someone might over shoot and put 96,000 AIC rated breakers. Personally I would do your own verification on the numbers. Every once in a while I'll be doing a design on an existing building, and see all breakers are called out for 65kAIC to be conservative. Then find out the utility max fault is only 19kA? Technically yes 65KAIC works but pretty overkill. Its like wearing -40° winter gear in the fall cause it might get cold.

One other interesting thing people dont always grasp, the rating of the panel is equal to the rating of the lowest AIC rated breaker in the panel unless series rated combinations are used. Meaning if they have a main breaker thats rated for 96KAIC, but the remaining breakers in that panel are only 42KAIC, 65KAIC, and 50KAIC, that panel is technically only protected to 42KAIC unless the manufacturer tested and approve the branch and main breakers in combination and shows their combined higher rating.

Rarely have I ever seen a project needing higher than 65KAIC but it has happened, once. There were 3 parallel 14kV transformers inside a building with exposed bus. If the utility comes back with a max of 32kA fault, your available fault will only go down from there.

 

[PE (always learning)]

Attached is a quick calculation based off the variables you gave using SKM power tools.

Assumptions noted below:

1) I assumed all conductors are aluminum and below grade using PVC conduit.

2) I did not provide motor load which could be a big factor in calculating the available fault current depending on how much you have. I don't have any information on this so it is excluded.

3) I used a high available fault current on the primary side of the utility transformer which should reflect a more conservative value on the secondary.

4) I used 200 feet for the distance from the main distribution panel to the downstream panel which I called "P1". You provided several distances so I used the shortest one provided.

5) I assumed 6 sets of 750 AL cable for the conductor sizes from the utility transformer secondary to the main distribution panel. You mentioned this panel was 2000 amps so I matched the feeder size to the panel size.


I am showing about 12,103 amps worth of available fault current at the downstream panel. Again this could be subject to change based on the motor loads.