Thread: Electrical help needed

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 panel. The service panel breakers are a 125 amp QB125 breaker and a 150 amp QB150 breaker.

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

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I am a bit put off by the abbreviations that you used; I believe that you meants to use "AIC," or 'arc interrupting rating.'

AIC is the minimum size -in amps - of current that a switch is rated to interrupt.
Let us, just for a moment, imagine a short circuit. Say, the classic 'kid sticks knife in toaster.' That short will draw lots of current .... much, much more than the 20 amps that the breaker is set for .... until the breaker trips. If the breaker was not able to handle that amount of current, it is possible that the breaker would not be able to shut off the power.

At one time, the industry standard AIC was 10,000 amps. Square D made much of their 'better' breakers having a higher 22,000 AIC rating. Battalions of sales reps went out, with the express purpose of getting architects and engineers to specify the higher AIC rating - thus reducing the competition. Those days are over; most every breaker now has a 22.000 AIC rating, though plenty of older models are out there.

From an inspection point, this is an irrelevant issue. There really is no role for 'designing after the fact.' AIC is pretty much a matter left on the drawing table. All you need to ascertain is whether a panel is UL listed, is being used within it's listing limitations, and is installed correctly.

The inspector is referring to the fault current (short circuit) rating of the breaker. The standard rating for a breaker is 10,000 amps. The design of the electrcial system feeding the house can raise this amperage. Two things that can affect this rating is the distance from the substation and the distance from the house to the transformer. The inspector is correct that the breakers may burn up rather than trip. The higher rated breakers can get very expensive. Fuses are rated at 100,000 amps. You can also install current limiting fuses ahead of the breaker panel. This will allow you to use the standard 10,000 amp breakers.

Current-limiting fuses (the "up-over-down" method) will work, though many jurisdictions do not accept them because that is not a listed method. The rest of the breakers in the service panel should have a 22K rating, or be part of a "series rated" system - not typical for Square D. An engineer should provide a point-to-point calculation for the breakers in the subpanels. Bottom line, inspector was correct not to pass this if they have that high a known short-circuit current available at the meter. JC

All the above posts are basically saying the same thing, only in different ways, some with correct terminology, others with terminology which is close and is meant to mean the same thing.

Also, AIC = amps interrupting current rating.

Except for the end result of a couple of those posts:

"From an inspection point, this is an irrelevant issue. There really is no role for 'designing after the fact.' AIC is pretty much a matter left on the drawing table. All you need to ascertain is whether a panel is UL listed, is being used within it's listing limitations, and is installed correctly."

"Bottom line, inspector was correct not to pass this if they have that high a known short-circuit current available at the meter."

Two opposite conclusions, one is correct:
- From an inspection stand point, the inspector is supposed to be verifying that the installed items meet what the design calls for, and that includes the proper (or higher) AIC rating. If they do no meet the required (specified) AIC rating, the job should not be passed.

Doing this is not "designing after the fact", only "verifying what was specified was installed" - that's the inspectors *only* job (provided it meets code ... minimum).

Turns out the city inspector was right. I contaced Square D with the box in front of me. From my initial conversation with them they thought the breakers were correct. After giving them more information off the panel box, they concluded that the breakers are only rated at 10,000 and not 22000. Looks like the electrician tried to save some money, as the correct breakers are much more expensive.

It's darn near impossible for anyone outside the power company to get the information necessary to perform an arc fault calculation. You need specific transformer information - not something from a handbook. One of the critical figures is the impedance of the transformer ... not to mention the exact length of the service wires.

Heck, design professionals - your EE's and master Electricians - who do these things for a living have a hard time getting a straight answer from the PoCo.

I'd love to see an HI perform such an analysis - one that had any meaning.

"Series rating" is in vogue right now, and I have had no problems with Square D on the issue. But, again, this is something way beyond the competence of any home inspector.

What defines competence? Well, a good place to start is with your AHJ. I am not aware of any AHJ's who will accept an HI's stamp on electrical plans and calculations. You generally need a PE, ME, or EC license for that.

Therefor, evaluating arc fault issues are, by definition, beyond the scope of the HI profession.

John,

You are getting hung up on belittling home inspectors again. you are stuck on the "home inspector" "electrician" "electrical inspector" thing.

Go back and re-read Bill's first and second post with an open mind as to what is written.

The CITY ELECTRICAL INSPECTOR SAID ... go back and re-read it.