AIC panel ratings

[wiremen]

I have a 120 208 3ph switch gear rated 65,000AIC.i need to figure out what my AIC rating needs to be for 2-200 amp 3ph 120 208 main lug panels..is THERE a calculation to use.i went to building department.they did not help at all.

 

[Dennis Alwon]

You can get an ee to do the calc or you can try the free calculator that mike holt has on his website. Download it here https://www.mikeholt.com/technical-calculations-formulas.php

Choose the calculator fault current

BTW, welcome to the forum

 

[kingpb]

I have a 120 208 3ph switch gear rated 65,000AIC.i need to figure out what my AIC rating needs to be for 2-200 amp 3ph 120 208 main lug panels..is THERE a calculation to use.i went to building department.they did not help at all.

Building department isn't going to tell you that; its the utility that would have the service information at point of connection.

You can base it on the rating of the gear as worst case; assuming the gear was originally sized properly, but you will still need to know the size of conductors and the length for each main lug panel. This approach may likely cause you to buy gear that has a higher SC rating than is actually needed.

 

[drktmplr12]

Building department isn't going to tell you that; its the utility that would have the service information at point of connection.

You can base it on the rating of the gear as worst case; assuming the gear was originally sized properly, but you will still need to know the size of conductors and the length for each main lug panel. This approach may likely cause you to buy gear that has a higher SC rating than is actually needed.

many building departments will require you demonstrate that the AIC rating of new equipment exceeds the available fault current seen at the line side of the new equipment.

cannot assume the original gear was specified correctly without taking on liability.

 

[wbdvt]

When utilities are asked for short circuit, they will provide you with the infinite bus short circuit on the transformer secondary terminals based on transformer size and impedance. This will provide for the worst case conditions to size the main electrical gear with. Since your gear is rated at 65kA, the utility probably provided a value greater than 42kA but less than 65kA. The issue now becomes determining the short circuit values downstream from the main gear. Options are:

1. Use the rating of the main gear to size the downstream equipment. Easiest but most costly.
2. Use the rating of the main gear (65kA) as the starting fault current and model the conductor impedance to the equipment to determine fault current. Will most likely be high.
3. Obtain the infinite bus fault current from the utility that was provided to size the main gear as it most likely will be lower such as 50kA. Use that to model as per number 2 above.
4. Obtain the available fault current at the primary side of the transformer, transformer kVA, impedance, secondary txf conductors to main gear and use that to model the fault current.

While it is more work to model everything, it could lead to smaller rated equipment at a cost savings. The other benefit is if you have modeled this far, it is not that much more to determine the incident energy levels at the equipment.

 

[kingpb]

many building departments will require you demonstrate that the AIC rating of new equipment exceeds the available fault current seen at the line side of the new equipment.

The OP did not mention he needs to demonstrate to the building department the AIC rating selected/used; I prefer not to speculate, he wanted to select downstream equipment.

cannot assume the original gear was specified correctly without taking on liability.

Hence why I said, ASSUMING it was originally sized properly

Just for fun, we can make a whole bunch of assumptions:
Utility transformer installed stepping distribution voltage down to 208Y/120V service.
Service side sees system as an infinite bus.
Utility transformer as low as 1.7% impedance.

65KA @ 1.7%Z; guessing 1200A main breaker; MVAsc from system is 23.42MVAsc
200A feeder breakers sized on current; i.e. no up-size in cable due to VD. #3/0AWG 75degC Col.
Cable run in EMT, Table 9: Xl = 0.052 ohms/1000ft
Ckt length 50ft (conservative) = MVAsc of cable is = 16.64MVAsc

So, MVAsc at new MLO panel = 9.73MVAsc, or 27kA

 

[Jraef]

I have a 120 208 3ph switch gear rated 65,000AIC.i need to figure out what my AIC rating needs to be for 2-200 amp 3ph 120 208 main lug panels..is THERE a calculation to use.i went to building department.they did not help at all.

Technically, you cannot have an "AIC" rating for a Main Lug Only panel at all... the "IC" in AIC mean "Interrupting Capacity" and lugs do not interrupt.

What you CAN have is a "Short Circuit Current rating" which is what we used to call a "withstand rating" for the bus bars in the MLO panel, then each individual breaker in the panel must have an IC rating equal to or greater than the Available Short Circuit Current of the system at that point. This is what the calculators can tell you.

The IC rating of the branch breakers in these MLO panels MIGHT be able to be "series listed" with the feeders in your up-stream 65kA panel, but that usually will ONLY apply to products of the same manufacturer, because they must be tested and listed in series with each other. If not, then another option may actually be to use a panel with it's OWN main CB, because it is often easier to get a Series Listing on the entire panel that way since the main is definitely going to be the same. So for example lets say you actually have 65kA at the main switchgear, then after calculating, you still have 40kA at the down stream panels. You would have to either use branch breakers that have at least 40kAIC, or if the panel mfr is the same as the switchgear, AND that mfr has a series listing with the feeder in the switchgear and the branch breakers in the MLO panels, those branch breakers might be able to be 22kAIC rated. But if they are not the same mfr or there is no series listing, you might be able to get a MCB panel for the branches that has a 42kAIC main and 22kAIC branches, that has a series listing at 42kAIC.

Bottom line there are many ways to skin this cat.