How do i estimate or figure out the AIR rating for a panel?

[rhamblin]

I need to replace a breaker in an I-Line panel. I'm looking at the Square D catalog and the first thing it says is "Select AIR rating". I don't know what that is. I'm thinking this can be calculated by an engineer exactly, but is there a way to estimate on the high side? Otherwise how is this normally done?

 

[Smart $]

I need to replace a breaker in an I-Line panel. I'm looking at the Square D catalog and the first thing it says is "Select AIR rating". I don't know what that is. I'm thinking this can be calculated by an engineer exactly, but is there a way to estimate on the high side? Otherwise how is this normally done?

Perhaps a typographical error. Should have been AICR (Amperes Interrupting Current Rating).

AICR must equal or exceed the MAFC (Maximum Available Fault Current) at the supply terminals.

 

[jim dungar]

AIR is actually the more correct term for AIC, but for all purposes they are used interchangeably.

The Amps Interrupting Rating takes into account some 'quirks' in the AIC testing process. Bussmann has 'talked' about AIR rating for years.

 

[jim dungar]

Perhaps a typographical error. Should have been AICR (Amperes Interrupting Current Rating).

I have never seen that term used ever in my years in the industry.

 

[tkb]

I need to replace a breaker in an I-Line panel. I'm looking at the Square D catalog and the first thing it says is "Select AIR rating". I don't know what that is. I'm thinking this can be calculated by an engineer exactly, but is there a way to estimate on the high side? Otherwise how is this normally done?

If you are replacing the breaker, get the info off of the existing breaker.

 

[Smart $]

I have never seen that term used ever in my years in the industry.

My bad. Should have been Amperes Interrupting Capacity Rating... usually shortened to Amps Interrupting Capacity or AIC for short.

 

[Jraef]

About a decade ago there was a movement to change AIC (Amps Interrupting Capacity) to AIR (Amps Interrupting Rating), but the effort fell a little flat. The issue was, as I recall, that something cannot have a capacity to interrupt unless it is going to actually interrupt, so only a fuse or breaker could have an AIC rating, but not the panelboard or switch holding them. Those parts of the system had what was called a "Withstand Rating". The change to "AI Rating" was then meant to combine Interrupt Capacity and Withstand Rating to apply to everything. I still see AIC on breakers and fuses though, the only place I see AIR is panelboards.

 

[ActionDave]

This is a fascinating exploration of electrical terms. Does anybody besides Tim have anything to add that addresses the OPs question?

 

[Sahib]

is there a way to estimate on the high side?

To estimate AIR : Connect a suitable load across panel disconnector terminals, say 1kW heater. Suppose voltage change is 0.2V. Then AIR=1/0.2=5kA.

 

[GoldDigger]

To estimate AIR : Connect a suitable load across panel disconnector terminals, say 1kW heater. Suppose voltage change is 0.2V. Then AIR=1/0.2=5kA.

That might tell you something about the internal resistance of the disconnect, but that, AFAIK, has little or nothing to do with the interrupting capacity or withstand rating of the device. It also depends on the voltage applied across the 1kW load and therefore the current produced.
And the resistance of the closed contacts tells you nothing about how the contacts open or what they do to extinguish the resulting arc.
Seems like snake oil to me.

 

[electrofelon]

If you are replacing the breaker, get the info off of the existing breaker.

I guess it depends on how thorough one wants to be. Going with the same value as what is/was there of course would not take into account things like the calculation being done correctly in the first place, changes in equipment, elimination or addition of a series rating, etc.....


The calculation itself is very easy, its getting the transformer information that can be problematic. To get the Available fault current at the transformer secondary, its transformer full load current divided by the impedance. Then google "mike holt free stuff" and get the fault current calculator (it has the transformer part in there too) and you enter your conductor size and length which will drop it some more.

 

[GoldDigger]

A quick clarification may be in order:
I thought the OP was trying to get a rough idea of the listed AIC of the old breaker.
This would not necessarily be a good indication of the required AIC, because the existing breaker might be overspecced or even underspecified.
The other possibility is that the OP was asking about a way to estimate the required AIC, which is the same as the available fault current.
Knowing the measured existing fault current is not necessarily sufficient to meet NEC requirements since POCO may change their equipment at any time.

mobile

 

[wbdvt]

I would say one way is to get the information off of the existing breaker and that way you are replacing in like and kind or get the rating of the panel or main breaker and use that. But that does not mean the breaker or the whole panel for that matter is undersized for the available fault current. It is also possible that this is a breaker that was added later after the initial install and was bought based on price alone. For example, panel and all other breakers are rated at 18kA and this breaker is a 14kA but when panel installed fault current was 15kA.

Of course, the above assumes that the equipment was sized correctly to begin with and the available fault current has not changed.

In order to know the available fault current at the panel, the system would have to be analyzed. Others things could be helpful in answering the OPs question such as is this a single or 3 phase panel, is it the first panel where the utility connects or is it in an industrial facility where it is supplied by a transformer that steps the plant distribution voltage down to 480Y/277V.

 

[Strathead]

I need to replace a breaker in an I-Line panel. I'm looking at the Square D catalog and the first thing it says is "Select AIR rating". I don't know what that is. I'm thinking this can be calculated by an engineer exactly, but is there a way to estimate on the high side? Otherwise how is this normally done?

There is valuable information above. I will try to summarize and expand. The AIC or AIR rating is as defined above. It is calculated base on the fault current available upstream. This starts with the Utility transformer. The resistance of the wire is then taken in to account which is calculated using the NEC and length. Once the AIC rating of the main circuit breaker or fuses is properly determined, subsequent breakers can be selected that are fully rated, ( the above transformer and any wire resistance to that breaker is used to determine) or when allowed by the designer, a series rating can also be used (breakers or fuses that are tested and listed as acceptable when used together usually all from the same manufacturer because manufacturers have no vested interest in testing their breakers with other manufacturers equipment).

That is the background as to how AIC is actually determined. One thing I didn't see mentioned above. A panel as an assembly will carry a rating based on the lowest AIC rated breaker in the panel. Another thing I don't think I saw above... The AIC rating is on the label of the breaker. If it is not on the labeled, then the breaker must be considered to be 5KAIC.

After everything above is said. I and I assume many others would check all the breakers in a panel. If they are all close to each other in their AIC rating, I would buy a new breaker that matches that. But as others mention unless the AIC rating required is on a label on the panel, the only right way to do it is to calculate the AIC yourself.

Hope this helps.

 

[kwired]

Series ratings complicates this some. There may be lower ratings on the branch breakers, higher rating on the main or feeder, but is acceptable if those combinations are series rated to be used together.

This is commonly seen on 120/240 "miniature" breakers like we see for general lighting and appliances. Main breaker in those panels is often 22 -25 kA rated, but the "standard" branch breaker used the majority of the time is only rated 10 kA. They are series rated and you can supply the main with a circuit capable of delivering 22 or 25 kA.

 

[jim dungar]

To estimate AIR : Connect a suitable load across panel disconnector terminals, say 1kW heater. Suppose voltage change is 0.2V. Then AIR=1/0.2=5kA.

This is absolutely false. The AIR can only come from the manufacturer.

The required AIR or AIC level is based on available SCA, fault current, which is determined by field estimates, actual measurements, or most typically from the POCO.
We deal with other situations, of similar terms, like this when we deal with the ampacity of a conductor and the amps drawn by the load. The ampacity must exceed the amps.

 

[Jraef]

I need to replace a breaker in an I-Line panel. I'm looking at the Square D catalog and the first thing it says is "Select AIR rating". I don't know what that is. I'm thinking this can be calculated by an engineer exactly, but is there a way to estimate on the high side? Otherwise how is this normally done?

Ok, back to this.

What Sq. D. is saying indirectly is that the breakers that you can plug into an I-Line panel can have anything from 10kAIC to 100kAIC, so it's up to you to make sure to pick the right one. Yes, you could just look at what is already in there and match it, but as others pointed out that runs a risk tht the last person to do this was wrong and you just repeated his mistake, but being the last one to touch it, the mistake becomes yours.

So as you asked, yes there IS a simple way of over estimating this, and that is to calculate the Available Fault Current (or SCA, Short Circuit Amps) at the transformer feeding this panel and use that value. In reality the SCA will always be LESS than that at the terminals of the panel because of wire resistance, but you asked or a quick and dirty way to OVER estimate, so that's it.
http://www.schneider-electric.us/en/faqs/FA105006/

Now, if that value is higher than what the other breakers have in that panel, it warrants a more full investigation because as was also said, things may have changed from when it was originally installed. For example the utility may have installed a bigger transformer somewhere up the line. In that case, it may fall upon your responsibility to at least inform your higher ups that a full coordination study should be done to properly determine if what is in there now is still suitable. It might very well turn out that because you over estimated it, there is no problem, or there is some sort of series rating going on that keeps everything else valid. That's not what you asked for, that's the reality and complexity of it, so take that into consideration. Remember, the last guy to touch it owns the responsibility for it

 

[Phil Corso]

Gentlepeople,

Just a reminder... "Peak", "Average", and "RMS" are three distinct values!

Regards, Phil Corso

 

[Smart $]

Gentlepeople,

Just a reminder... "Peak", "Average", and "RMS" are three distinct values!

Regards, Phil Corso

Hmmm. The relevance to this thread is...?

 

[odyssey1]

Square D Panel

Square D Panel

I have found AIC ratings for Square D panels on the back side of front panel. you have to physically take it off to get to the rating information.