AIC RATING FOR PANELBOARD VS MAIN BREAKER INTERRUPTING RATING

[PE (always learning)]

Hello,

I have an issue where the fault current at my 480V 3 Phase panel is 24,200A and the rating of the panel itself is 22,000 AIC, but the main breaker has an interrupting rating of 35,000 AIC and the branch circuit breakers have interrupting ratings of 25,000 AIC. Does this mean that my panel is still covered for the available fault current?

Best Regards,
Engineer in training

 

[Jraef]

That answer can only come from the panel mfr. Some systems have "Series Ratings", but to attain them, the specific components must be tested together by the mfr of the equipment when it is UL listed.

Just for your edification;

"AIC" means "Amps Interrupting Capacity" and therefore can only apply to devices that actually do the "interrupting" such as circuit breakers or fuses. because they just hold the devices, Panelboards cannot have an "AIC rating". They do have a "withstand rating", meaning what mechanical forces the bus bars are braced to withstand in the event of a high short circuit current event. Combined with the installed breakers, a panelboard, even if only braced for 22kA fault withstand, might possibly have a higher overall "SCCR" (Short Circuit Current Rating) because of the current limiting capabilities of the main breaker. But again, ONLY if the mfr tested and listed it as such.

 

[PE (always learning)]

Thank you for the quick response and clarification on proper nomenclature. As you said, this is an issue of clarifying with the manufacturer on whether or not the main breaker has current limiting capabilities and on whether or not the SCCR of the panel is greater than the available fault current with said main breaker. I will contact the manufacturer and verify this information. Is it plausible to move the panel farther away or provide extra feeder length in order to provide more resistance and choke down the available fault current? Is this a common solution for panels that have already been ordered and are on the cut off for the available fault current?

 

[pcanning87]

Yes you can increase the circuit impedance. You could lengthen the conductor run, reduce the wire gauge (if oversized), change conductor material or potentially change conduit material.

Technically those are all viable, you would just need to weigh the added cost and other effects.


Sent from my iPhone using Tapatalk

 

[charlie b]

To add to what Jraefoffered, if you put too much current through a breaker, the metal contactscould melt (on a microscopic level), essentially welding them together, andpreventing the breaker from ever being able to open. I believe that is the basis for the breaker’sAIC rating.

Now let’s look atthe panel itself. Current flowingthrough the bus bars will create around itself a magnetic field. That is true of all three bus bars, and thethree fields interact with each other. For normal levels of current, the three magnetic fields areinsignificantly small. But during afault condition, the magnetic field resulting from the extremely high currentflowing through each bus bar will tend to push the other two bus bars away fromitself. If the current exceeds the panel’swithstand rating, the bus bars can break away from their mounting. Please note that this has nothing to do withhow well the breakers can handle the current.

 

[charlie b]

Is it plausible to move the panel farther away or provide extra feeder length in order to provide more resistance and choke down the available fault current? Is this a common solution for panels that have already been ordered and are on the cut off for the available fault current?

Before I went down that path, I would have another look at the fault current calculation. You don't have to take it at face value. Look at the input assumptions (fault current available from the utility, cable sizes and lengths, motor contribution, transformer impedances, etc.). You may be able to refine the calculation in order to get a result that is below the 22,000 amp threshold.

 

[adamscb]

The way I understand this is similar to Charlie b, in that when the breakers have a higher rating than the available fault current that means that they're guaranteed to be able to clear the fault. However, when the busbar itself in the panelboard is underrated, to me that means that the busbar could catastrophically fail. It all depends if the breaker can clear the fault fast enough. In my opinion it's best to have breakers rated for the fault current as well as the main bus bars.

 

[electrofelon]

Hello,

I have an issue where the fault current at my 480V 3 Phase panel is 24,200A and the rating of the panel itself is 22,000 AIC, but the main breaker has an interrupting rating of 35,000 AIC and the branch circuit breakers have interrupting ratings of 25,000 AIC. Does this mean that my panel is still covered for the available fault current?

Best Regards,
Engineer in training

I find it kinda hard to believe that at manufacturer would sell you a panel board where the SCCR is lower than the AIC of the provided breaker. I could see maybe if you pieced something together from parts, but even then Ill bet the SCCR of the bussing is all built for the highest typical AIC breaker that can be installed, to minimize their component inventory (as an example, I had some factory ordered 125 and 250 amp panels and there were exactly the same).

 

[topgone]

I find it kinda hard to believe that at manufacturer would sell you a panel board where the SCCR is lower than the AIC of the provided breaker. I could see maybe if you pieced something together from parts, but even then Ill bet the SCCR of the bussing is all built for the highest typical AIC breaker that can be installed, to minimize their component inventory (as an example, I had some factory ordered 125 and 250 amp panels and there were exactly the same).

I agree. Vendor must have done some changes to the listed panel or what?

 

[Ingenieur]

As others have said
verify the equipments actual ratings
confirm the fault calc...infinite bus, pu Z, etc
it looks like an infinite bus calc on a 1 mva 5% pu Z xfmr?
what is the mcb rating?

 

[Jraef]

Hello,

I have an issue where the fault current at my 480V 3 Phase panel is 24,200A and the rating of the panel itself is 22,000 AIC, but the main breaker has an interrupting rating of 35,000 AIC and the branch circuit breakers have interrupting ratings of 25,000 AIC. Does this mean that my panel is still covered for the available fault current?

Best Regards,
Engineer in training

The more I thought about this, the more it bothered me and I took a look. Between Eaton, Square D, GE, and Siemens I could not find a 480V Panelboard rated for anything less than 35kA SCCR using main breakers (GE). Everyone else was at least 50kA. So unless this is some odd off brand or something really old and obsolete, I think there may be a problem with the equipment selection or specification interpretation going on here. 22kA sounds more like a Load Center spec, not a Panelboard and AFAIK there are no 480V rated load centers. Someone may have accidentally chosen a 208/120V 3 phase load center not realizing the need was 480V, or someone is assuming 480V 3 phase when in fact it is 208/120V 3 phase.

 

[PE (always learning)]

The more I thought about this, the more it bothered me and I took a look. Between Eaton, Square D, GE, and Siemens I could not find a 480V Panelboard rated for anything less than 35kA SCCR using main breakers (GE). Everyone else was at least 50kA. So unless this is some odd off brand or something really old and obsolete, I think there may be a problem with the equipment selection or specification interpretation going on here. 22kA sounds more like a Load Center spec, not a Panelboard and AFAIK there are no 480V rated load centers. Someone may have accidentally chosen a 208/120V 3 phase load center not realizing the need was 480V, or someone is assuming 480V 3 phase when in fact it is 208/120V 3 phase.

Hey,

Thanks again everyone for your responses. To put some frame of reference for this project, my company has been hired by the contractor for the project that I'm working on to do the power systems study. We did not do the engineering design for this project and the engineer has already taken notice of the referenced panel's withstand rating and requested on the shop drawings to increase the withstand rating to 65K. The panel was located right next to the main distribution panel, that is why it needed a higher rating and the issue as I have explained has been taken care of.

Also, to answer your question Jraef, the original panel I was referring to was a Siemens panel model number P1E42FX200CTST with a main breaker model number FXD6. The FXD6 breaker has an AIC rating of 35kA at 480V. I have attached pictures of the original shop drawings for the panel to this response if you are curious.


Best Regards,
Engineer In Training

 

[Jraef]

Hey,

Thanks again everyone for your responses. To put some frame of reference for this project, my company has been hired by the contractor for the project that I'm working on to do the power systems study. We did not do the engineering design for this project and the engineer has already taken notice of the referenced panel's withstand rating and requested on the shop drawings to increase the withstand rating to 65K. The panel was located right next to the main distribution panel, that is why it needed a higher rating and the issue as I have explained has been taken care of.

Also, to answer your question Jraef, the original panel I was referring to was a Siemens panel model number P1E42FX200CTST with a main breaker model number FXD6. The FXD6 breaker has an AIC rating of 35kA at 480V. I have attached pictures of the original shop drawings for the panel to this response if you are curious.


Best Regards,
Engineer In Training

OK, so there is something odd with that too. Here is the spec on the P1 panel itself:

Revised Type P1
480Y/277 Vac Maximum
600Y/ 347 Vac Maximum (limited
applications)
400 Ampere Mains
400 Ampere Maximum Branch
UL Short Circuit Rating —
200,000 A. @ 240 Vac / 100,000 A. @
480/277 Vac. IR Maximum

But when you install a Main CB, the panel rating takes on the rating of the Main CB.

Now I did forget that the P1 panels CAN use the cheap-shirt Type BQ breakers, which are basically 240V breakers with some added barriers to make them pass at 480/277V (never straight 4890V though). Those breakers are only rated for 14kAIC, so when you use them in a 480/277V panelboard as a main, you get stuck with a 14kA SCCR (IR). There is nothing different about the PANEL itself though, it is still BUILT for 100kA regardless of what main you put in it. The max IR changes because of the main. So if you just install an HFXD6 as the main, that exact same panel becomes 65kA.

 

[TheTap]

First post ever here on this forum. I use Siemens software to layout Panelboards and it will allow me to have a system interrupt rating lower than the mcb or any branch breakers. My panel can be nameplated at 10k with a 65k main & branches. However, 10k & 22k panel interiors look identical so it probably is braced for a rating higher than 10k.

 

[topgone]

First post ever here on this forum. I use Siemens software to layout Panelboards and it will allow me to have a system interrupt rating lower than the mcb or any branch breakers. My panel can be nameplated at 10k with a 65k main & branches. However, 10k & 22k panel interiors look identical so it probably is braced for a rating higher than 10k.

idk but design-wise, you ought to choose a breaker interrupting rating that is greater than the available fault on the location. Every breaker connected besides another will experience the same amount of fault current. As what others have said, the SCCR rating of the panel will be the rating of the lowest interrupting rating of the components inside the panel.