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Series-rating subpanels #40626 07/29/04 01:38 PM
Joined: Jul 2004
Posts: 625
SolarPowered Offline OP
According to, say, the Square D documentation, their QO-series mains and breakers don't series-rate for AIC for more than two levels. That is, a 22,000 AIC main with a 10,000 AIC breaker series rates to 22 kAIC for the combination. But, if you have a 22 kAIC main feeding a 10 kAIC breaker feeding a subpanel which contains 10 kAIC breakers, the combination is only rated for 10 kAIC.

In order to keep the 22 kAIC rating, you need to either use a 22 kAIC breaker feeding the subpanel (QOs series rate with other QOs, local or remote), or use only 22 kAIC breakers in the subpanel.

However, judging from the what's on the shelf at the local retail establishments, I infer that this is rarely done. They don't appear to stock 22 kAIC 70-100 amp breakers with which to feed subpanels. Therefore, it appears to me that there are very few residential subpanel installations around that keep the 22 kAIC rating of the main panel.

Am I missing something here? Like, that all residental services are less than 10 kA SCC? Or that the breakers really do series-rate three levels deep? Or that the wiring to the subpanel is (nearly) always sufficient to reduce the available SCC to < 10 kA?

Or is the SCCR requirement just routinely ignored?

[This message has been edited by SolarPowered (edited 07-29-2004).]

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Re: Series-rating subpanels #40627 07/29/04 02:13 PM
Joined: Jul 2004
Posts: 49
GamecockEE Offline
Solar, I think that for the most part, the SCCR is being ignored, mostly because they don't recognize when it can be an issue. On residential, it would probably show up on a multifamily dwelling where a bigger transformer would be used to serve a number of dwellings. On single family dwellings, at least in my part of thw world, it would never be an issue. It's not uncommon to see a 25KVA xfmr feed 3 houses with 100 feet of service conductor. Where you are at I would expect that the AHJ's to be up on things pretty good.

Re: Series-rating subpanels #40628 07/29/04 05:47 PM
Joined: Jan 2001
Posts: 1,044
Tom Offline
If you do the calculations, it would be a rare dwelling that even approaches 10,000 amps of available fault current. You would need to have a fairly large transformer (100kva or so) or a smaller one with fairly low impedance (2% or less) to even begin to be concerned.

Buss used to have a downloadable short circuit calculator. If you can get it, play with some numbers & you'll see what I mean.


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Re: Series-rating subpanels #40629 07/30/04 12:08 AM
Joined: Jan 2004
Posts: 615
Jps1006 Offline
In order to keep the 22 kAIC rating, you need to either use a 22 kAIC breaker feeding the subpanel (QOs series rate with other QOs, local or remote), or use only 22 kAIC breakers in the subpanel.

or couldn't you use a 10K AIC feeding a 22K AIC main breaker subpanel?

Re: Series-rating subpanels #40630 07/30/04 09:55 AM
Joined: Jul 2001
Posts: 599
JBD Offline
You are misinterpreting the Square D series ratings.

You may series rate only two devices, but they do not have to be electrically consecutive. If the 22K main breaker series rates with a downstream 10K breaker it makes no difference how far downstream the breaker is or if there are additional breakers in between them.

What you can not do is use one valid series rating to "feed" another series rating, such as: Main 400A LA 42K in series with a feeder 200A QD 25K which is then in series with a branch 20A QO 10K.

[This message has been edited by JBD (edited 07-30-2004).]

Re: Series-rating subpanels #40631 07/30/04 10:19 AM
Joined: Mar 2002
Posts: 582
Ron Offline
I agree with your statement except for the part about other OCPD's between the series rated combination.
The series rated combination is tested with those two devices. If other fuses or CB's are placed between the series rated combination devices, then that adds additional dynamic impedance that was not considered during the UL testing.
The main reason to require a tested combination, is dynamic impedance.
See the following article:

Re: Series-rating subpanels #40632 07/31/04 12:56 PM
Joined: Apr 2004
Posts: 201
CharlieE Offline
You might want to rethink the part about the residential services never going over 10 kA. In the front lot services of today and zero lot line construction, the transformers are setting close to the garage (you know, where you are fond of placing the service [Linked Image] ). We are serving a lot of subdivisions that are mostly electric heat with 50 and 75 kVA transformers and either 4/0 or 500 Al. triplex service laterals. The homes that are next to the transformers get 20 to 30 foot service laterals. They have a problem.

Charlie Eldridge, Indianapolis Utility Power Guy
Re: Series-rating subpanels #40633 08/02/04 10:59 AM
Joined: Jul 2001
Posts: 599
JBD Offline
Mea Culpa.

I was too brief in my description.
While, three levels of series ratings are available, they must be listed as such. For example, from Square D Switchboard/Panelboard Short-Circuit Current Ratings : "In order for all OCPDs (A, B, C) to be in a series combination (three-tier), the series combination would have to be marked on the panelboard housing OCPD (C)."

Re: Series-rating subpanels #40634 04/05/06 09:50 AM
Joined: Apr 2004
Posts: 156
rad74ss Offline
I was informed this morning that all of our electrical panels for HVAC equipment are required to have the SCCR rating as of April 25 2006. They are going to have a web seminar on it on May 17. This is required for all UL508A listed panels which accounts for 99% of the panels we build.

We use mostly fuses which jumps up the rating, but keeping track of which device trumps the other and how to make sure everything is kept straight over a thousand components is going to be one heck of a learning curve.

Re: Series-rating subpanels #40635 04/05/06 03:02 PM
Joined: Mar 2002
Posts: 582
Ron Offline
The 2005 version of the NEC requires a SCCR for a control panel. As jurisdictions adopt the 2005 NEC, then the rating will be required for those panels.



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