electrical quandry for the GURU

“quandry” instersting word, isnt it.

Here is what I have:

New construction. Square D 400 amp rate Box. The plans call for the main disconnect to be 22,000 Vac. Max (whatever that is).The listed breaker for that is a QO-OH breaker. There area two distribution panels, and one service pane. There is one 125 amp QB125 breaker and one 150 amp QB150 breaker.

On the plans it states that the interrupting short circuit amps at the meter bank = 17812 AMPS. Use 22,00 ALL Minumuim

According to the electrical inspector, these breakers will not trip if there is a power surge to the box but they will burn, leaving the house unprotected.

Any thoughts on this.

Interupting capacity has to do with the ability of the breaker to “clear” a short circuit and is rated in Amps.
The plan is calling for breakers rated to intterupt a 22,000A short circuit.

This refers to the maximum current the breaker can “break” without burning up or arcing over.
This is the same thing I was talking to Marc about a couple days ago. Service conductors are not really protected against overcurrent so the utility will provide the installer information about the maximum available fault current if you have a dead short. This is based on the service conductor size, length and transformer serving the building.
It is not uncommon to get a blanket answer of “greater than 10,000a, less than 22,000” if you are not willing to buy “engineering” and that precludes the regular 10ka breaker.

So then they are using the wrong breakers at the service panel??

So far, you’re giving us about half of what we’d need to say for sure, and some of what you’ve provided is bad information. You talk about QB breakers. A type QB breaker is a Zinsco breaker. Perhaps you mean QOB? There is a 22K AIC rated version of a QOB. What is the catalog number of the panel that concerns you?


I do not have the catalogue number of the panel. The breaker i listed was copied from the panel cover and it was a Square D. If it helps, this is a 400 amp rated panel that was installed within the last year and a half. I wish I had pictures, but I left my camera home today. The concern is if these breakers are the right size for the feeds if there is a short. As you can tell, this is not my area of expertise. Let me know what information you need and I will try and get it.

Looks like an ENG has already done a one line diagram for the installation and has listed the SCC as 17812 for that figure. If this is the case the 22KVA AIC is fine for the application on THAT bank…now it is important to have a full and detailed one line diagram drawing as I am guessing this is an industrial application or high end commerical layout.

What you need to do is contact the manufacturer or supply house and they can give you the rating of the breakers in question. But depending on their location the ratings can change…the 17812 is at the first main I am guessing…once it gets down stream the SCC changes from point to point and it will need to be calculated out…

Square D renamed it’s Q2 frame brakers
QB is 10k interupt
QD is 25 k interupt
a QBL200 is a 200 amp 10 k breaker.

I have been on the phone with Square D for two days. The information first listed above is correct. The service breakers are not rated for the system or the plans. Looks like the electrician tried to take the cheap way out, as the installed breakers are much less expensive than what should be in this box

Interesting point here also to be considered…since many commerical and industrial buildings are changing their transformers and so on to more low eff. rated units…we are seeing more transformers going from 5% imp. to low 2% and 3% imp…and the result of less impedance is higher ampacity and thus higher AIC flash issues…so important to remember this…

If you know they change the transformers or make additions to the layout the entire safety plan needs to be updated, single line drawings need to be updated and so on…it becomes a chain of events.

While we are making transformers BETTER…we have to remember if someone changes the unit…it effects everything else down stream even the AIC issues.

To add to Paul’s post, the transformers are also being installed closer to buildings (including the larger homes) and this lowers the impedance as well.
I think that not enough effort by some in our industry is being applied just to this sort of scenario.

I do not think that the electrical contractor was ignoring the ratings specified, maybe he just is not used to the differences, especially if this is a large home. Is it a large home or commercial?
I also see he is going to have an expensive change coming his way. :shock:

You know Pierre…I agree so much with that statement my friend. The sad thing in our OWN industry is the lack of education on this growing concern with NFPA 70E and Electrical Safety…the facts remain it is still the most neglected area looked at in safety concern today in many industrial and commerical settings and it is even growing more of a concern with like you said CLOSE installations to dwellings…

I noticed a HUGE jump in my NFPA 70E class requests as the years went on…people are slowly starting to understand the concerns…