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

[Strathead]

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.

I am not so sure what you are looking at is an answer to your first question. Beware.

 

[electrofelon]

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.

I find that hard to believe, since a panelboard does not have a AIC rating. I suppose they could have labeled it with the lowest AIC of the breakers that were factory installed, but I have never seen that.

 

[odyssey1]

Sorry, I was referencing the SCCR.

 

[Phil Corso]

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

Since you're "Smart", figure it out!:happyyes:

Phil

 

[Jraef]

Sorry, I was referencing the SCCR.

That still doesn't help when picking out a breaker to go in it. For example Sq. D I-Line panels all have an SCCR of 100kA stand alone. That doesn't mean any breaker you plug into it will be good for 100kA. You still have to pick the breakers (or series combo) based on the available fault current.

 

[jim dungar]

Sorry, I was referencing the SCCR.

You are correct, in a manner of speaking.

When Square D ships a 'factory assembled' panelboard or switchboard, it will include a label with an SCCR based on the project specifications and it will include a list of the breakers that can be used to maintain that SCCR.

 

[electrofelon]

You are correct, in a manner of speaking.

When Square D ships a 'factory assembled' panelboard or switchboard, it will include a label with an SCCR based on the project specifications and it will include a list of the breakers that can be used to maintain that SCCR.

Will the SCCR ever be less than the AIC of an a available breaker that fits?

 

[jim dungar]

Will the SCCR ever be less than the AIC of an a available breaker that fits?

Yes.

For example, an I-Line panel can hold breakers with AIC ratings from 14,000 all the way up to 100,000. Many engineer's seem to default specify 65kAIC for their breakers, so the factory would put a65kA SCCR label on the panel.

But when a contractor goes down to the local supply house and simply orders a breaker off the shelf, it will likely only be rated 18kAIC, so installing it will lower the panel's SCCR below that of the other installed breakers.

This is a real concern, so all manufacturers use wording like "panel SCCR is dependent upon the lowest AIC rating of any installed breaker".

 

[electrofelon]

Yes.

For example, an I-Line panel can hold breakers with AIC ratings from 14,000 all the way up to 100,000. Many engineer's seem to default specify 65kAIC for their breakers, so the factory would put a65kA SCCR label on the panel.

But when a contractor goes down to the local supply house and simply orders a breaker off the shelf, it will likely only be rated 18kAIC, so installing it will lower the panel's SCCR below that of the other installed breakers.

This is a real concern, so all manufacturers use wording like "panel SCCR is dependent upon the lowest AIC rating of any installed breaker".

Very interesting - but confusing. I dont see why the choice of breaker would change the SCCR. It seems like that is what it is no matter what. How about this. Say I have an I-line with all 18kAIC breakers and there is a SCCR label that says 18K. I replace all the breakers with 65kAIC. Can I not use this new assembly with a fault current over 18k? IF not, is there anything actually different about the panelboard over a 65K or is it just a technicality?

 

[jim dungar]

Very interesting - but confusing. I dont see why the choice of breaker would change the SCCR. It seems like that is what it is no matter what. How about this. Say I have an I-line with all 18kAIC breakers and there is a SCCR label that says 18K. I replace all the breakers with 65kAIC. Can I not use this new assembly with a fault current over 18k? IF not, is there anything actually different about the panelboard over a 65K or is it just a technicality?

In the case of the typical I-Line panel the SCCR is always equal to the lowest AIC of the installed breakers. So in your example your panel now has a higher SCCR.

However, sometimes there are possible constructions that may limit the SCCR, like bus bar bracing in MCCs and switchboards.

 

[GoldDigger]

In the case of the typical I-Line panel the SCCR is always equal to the lowest AIC of the installed breakers. So in your example your panel now has a higher SCCR.

However, sometimes there are possible constructions that may limit the SCCR, like bus bar bracing in MCCs and switchboards.

If the panel has a main breaker that can limit the fault current that hits the bus, then you might be able to run it with a source that is capable of a high fault current if you have a main breaker with an appropriate SCCR and AIC yetnot be able to run the same source into the same panelboard as an MLO.

Rephrased: If the individual breakers are only OK with a low AIC when series rated with an upstream breaker, you had better have an upstream breaker.
Said that way it seems trivial.

 

[electrofelon]

In the case of the typical I-Line panel the SCCR is always equal to the lowest AIC of the installed breakers. So in your example your panel now has a higher SCCR.

If the individual breakers are only OK with a low AIC when series rated with an upstream breaker, you had better have an upstream breaker.

So to be complete and thorough, can we state:

the SCCR is always equal to the lowest AIC of the installed breakers, or the lowest series rated combination

?

 

[jim dungar]

So to be complete and thorough, can we state:

the SCCR is always equal to the lowest AIC of the installed breakers, or the lowest series rated combination

:thumbsup:
Yes you can.

 

[brantmacga]

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 changing amperage, send the part number of the existing to your square d distributor and they can help you out. They might ask for the label on the panelboard and main breaker also.


Sent from my iPhone using Tapatalk

 

[rhamblin]

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

Sorry for the long reply everyone. For some reason I wasn't getting email notifications. I've reread all of this and this seems to be a pretty good explanation of what needs to be done.

Here is our situation. We are a medium sized printing plant. We have something like 13,500 volts coming in from the utilities feeding 3 onsite transformers, all are of which create 480/277 Volt secondary. Each one then feeds through a main switchgear switch, through some bus bar and then to a breaker that feeds a Bus Duct. This bus duct feeds many machines throughout the plant.

Of course we feed off the bus duct with a bus switch, which has fuses. Didn't I read somewhere that fuses have a really high AIC rating 100-200,000? This fused switch feeds a 400 Amp I-line panel. Which feeds the machine as well as a 480 V 200 amp subpanel and a small 480>120/240 transformer (I don't know the transformer size, but we feed it with a 30 amp 480 breaker).

Getting back to the question of the SCA of the xfmr, would I be taking the measurements from the 13,500 volt xfmr? Seems like that would be a large number, as I believe the secondary currents are 1200 amps, 2000 amps and 3000 amps. Although I haven't checked the impedance values of these xfmrs yet.

Secondly, does having the fuses in line between the xfmr and the I-line panel, change the AIC or AIR SCA to make it a smaller number that what is calculated from the xfmr nameplate?

 

[kwired]

Sorry for the long reply everyone. For some reason I wasn't getting email notifications. I've reread all of this and this seems to be a pretty good explanation of what needs to be done.

Here is our situation. We are a medium sized printing plant. We have something like 13,500 volts coming in from the utilities feeding 3 onsite transformers, all are of which create 480/277 Volt secondary. Each one then feeds through a main switchgear switch, through some bus bar and then to a breaker that feeds a Bus Duct. This bus duct feeds many machines throughout the plant.

Of course we feed off the bus duct with a bus switch, which has fuses. Didn't I read somewhere that fuses have a really high AIC rating 100-200,000? This fused switch feeds a 400 Amp I-line panel. Which feeds the machine as well as a 480 V 200 amp subpanel and a small 480>120/240 transformer (I don't know the transformer size, but we feed it with a 30 amp 480 breaker).

Getting back to the question of the SCA of the xfmr, would I be taking the measurements from the 13,500 volt xfmr? Seems like that would be a large number, as I believe the secondary currents are 1200 amps, 2000 amps and 3000 amps. Although I haven't checked the impedance values of these xfmrs yet.

Secondly, does having the fuses in line between the xfmr and the I-line panel, change the AIC or AIR SCA to make it a smaller number that what is calculated from the xfmr nameplate?

If you assume a transformer has unlimited power available from the primary circuit - the only thing limiting available fault current at the secondary terminals is the impedance of the transformer. Add some conductor between transformer and whatever gear you have in question, you also have resistance in those conductors that put limitations on how much current will flow, short runs of large conductors don't have nearly the limiting effect as long runs or simply having small conductors does have a limiting effect also, but with really high available fault currents you still may need 20-50 feet of conductor to get below some of the lower AIC breaker ratings.

Many do assume unlimited primary ability from the utility just to come up with a worst case scenario, plus they don't really know what is available on the primary side, but reality is there will likley be some limitation to it as well.

Some fuses have pretty high AIC ratings, some don't. Your typical RK-5 fuses will usually be 200kA. A class H fuse I believe is only 10kA. Many breakers are series rated for having RK-5 fuses installed ahead of them.

 

[Jraef]

If you assume a transformer has unlimited power available from the primary circuit - the only thing limiting available fault current at the secondary terminals is the impedance of the transformer. Add some conductor between transformer and whatever gear you have in question, you also have resistance in those conductors that put limitations on how much current will flow, short runs of large conductors don't have nearly the limiting effect as long runs or simply having small conductors does have a limiting effect also, but with really high available fault currents you still may need 20-50 feet of conductor to get below some of the lower AIC breaker ratings.

Many do assume unlimited primary ability from the utility just to come up with a worst case scenario, plus they don't really know what is available on the primary side, but reality is there will likley be some limitation to it as well.

Some fuses have pretty high AIC ratings, some don't. Your typical RK-5 fuses will usually be 200kA. A class H fuse I believe is only 10kA. Many breakers are series rated for having RK-5 fuses installed ahead of them.

And to finish that with relation to the last part of your recent question:

Secondly, does having the fuses in line between the xfmr and the I-line panel, change the AIC or AIR SCA to make it a smaller number that what is calculated from the xfmr nameplate?

Technically, the fuses don't CHANGE the maximum Available Fault Current (AFC) of the transformer secondary, but as kwired said, if using specific fuse types that the Sq. D panel has been SERIES LISTED with, you may end up with what's called a "Series Combination" listing of the panels and fuses that is higher than the panels alone. This is information that you cannot do on your own, the specific components must have been determined and tested by Sq. D, and they can tell you what you you can attain and exactly what fuses you need to use.

That is, IF it is even necessary. FIRST you must start off with determining the AFC on the secondary of those transformers. All decisions must spring from that data.

 

[packersparky]

And to finish that with relation to the last part of your recent question:

Technically, the fuses don't CHANGE the maximum Available Fault Current (AFC) of the transformer secondary, but as kwired said, if using specific fuse types that the Sq. D panel has been SERIES LISTED with, you may end up with what's called a "Series Combination" listing of the panels and fuses that is higher than the panels alone. This is information that you cannot do on your own, the specific components must have been determined and tested by Sq. D, and they can tell you what you you can attain and exactly what fuses you need to use.

That is, IF it is even necessary. FIRST you must start off with determining the AFC on the secondary of those transformers. All decisions must spring from that data.

:thumbsup::thumbsup::thumbsup::thumbsup:

I really don't understand how somebody can bid and order equipment for a job when they don't know the available fault current because the utility doesn't even know that they might have a new service to connect to. As Jraef said ALL calculations come from the utility fault current number.

 

[Jraef]

:thumbsup::thumbsup::thumbsup::thumbsup:

I really don't understand how somebody can bid and order equipment for a job when they don't know the available fault current because the utility doesn't even know that they might have a new service to connect to. As Jraef said ALL calculations come from the utility fault current number.

And to THAT point...

https://www.mikeholt.com/freestuff-shortcircuitcalculations.php

Because lately, I've seen a trend where the Utilities no longer give you the actual number, they just give you the transformer size and impedance. The rest they expect you to do on your own, because from their standpoint, you are a their cheapest labor rate...

 

[electrofelon]

Because lately, I've seen a trend where the Utilities no longer give you the actual number, they just give you the transformer size and impedance. The rest they expect you to do on your own, because from their standpoint, you are a their cheapest labor rate...

Which IMO is good because then you can calculate the ACTUAL number. In another recent thread I complained about routinely getting unrealistic AFC values from the utility, often three times higher than the actual value (computed assuming infinite buss).