Inspector and AIC ratings

[jim dungar]

I have never quite understood why, but UL508a allows us to put a current limiting device such as a fuse or current limiting circuit breaker in a feeder and take advantage of that for the downstream branch circuits, but it is generally not allowed, or at least generally considered not allowed in NEC installations. One would think the electrons would not know or care that they were in a UL508a panel or not.

UL508A does have restrictions on what can be protected in this manner, for the most part protective devices are excluded.
From Bussmanns SPD discussion of UL508A"...other than branch circuit overcurrent protection devices such as fuses, circuit breakers, instantaneous trip circuit breakers or motor circuit protectors - MCPs - and self-protected combination starters..."

 

[Sierrasparky]

let me embarrass myself by asking a stupid question:

my assumption is that putting a lower AIC breaker
than required lowers the entire panel AIC rating?
including a branch circuit breaker used after a main
of the correct AIC?

unless the panelboard is engineered sufficiently
to allow the main AIC rating to apply to all downstream
breakers?

my other assumption is that i can only use breakers
listed for THAT panelboard, or i cancel the AIC rating
of the entire thing?

Yes that would be my understanding.

It is awful that such a safety issue is disregarded so often. I was working for a company on a High rise Condo. I was off doing work on another project and moved to this condo job because the need to keep on sched and wrap up the house panels and equipment to get the place online and off temp. All the House sub panels were already installed as well as breakers installed and landed. The screwed up part was that the install had the lowest AIC rated equipment in the first position and the 65k stuff at the end. I knew that the first panel needed 65k and the rest were sieries rated 22k and then 10k from the subs of the subs. Well it was all backwards. The inspector was a all in one and did not know the problem or how to deal with it. The main office of the company had on staff engineers were told my me to them as well as the VP of the company also a EE and EC. They never fixed the problem it is still that way today. They had a company wide solution however. The only purchased equipment with main breakers and sieries rated to 10k. for future jobs. I guess they could not get the point out to the installers so they threw money at equipment instead of training.


Go figure

 

[cowboyjwc]

So if you had an old school building that you were adding panels to would you not use series ratings? Does that series rating have to be testing with the existing equipment?

You could as long as you used the same manufacture, other wise you can do a calc and see if you can get down to where you need to.

The reason it was suggested a couple of times to talk to the utility again is that it drops off pretty quick, depending on the length of the run and if it's passing through transformers.

 

[titan1021]

Before you walk too far down the wrong path, which may well be the case here, are you certain that you asked the right question of the utility, and that they gave you the information that you actually needed (and not just thought you needed)? AIC stands for "amps interrupting capability." It is a rating of the overcurrent protection equipment, a measure of how much current a breaker can interrupt without the breaker being destroyed in the process. Unless the utility knows something about your main breaker, they won't be able to answer the question of "what is the AIC rating of the service?"

The information needed from the utility is the available short circuit current ("SCCA") at the location of the point of separation between their equipment and the owner's equipment. That will generally be just upstream of the service transformer. If they are supplying the building at 120/208 (i.e., if the utility owns the transformer and the point of service is at the service switchboard), then they should be able to give you the SCCA at the service switchboard. Is that, in fact, the 42,000 amp value that you were given?

Let?s assume the utility meant to say that the available fault current at the service switchboard is 42K. Then the main breaker, the bus bars, and all other breakers in the service panel need to be rated to handle 42K.

Ohm?s law is not going to give you all you need to know, in order to calculate the required rating of the downstream panels and their breakers. If for example there are any large motors, such as elevator motors or large HVAC equipment, then the calculation will need to take into account the amount of current those motors would contribute to a fault. Now I won?t say that you need to be an engineer in order to be able to perform such a calculation. But you would need to be an engineer in order to be able to take legal responsibility for the results of the calculation.

That is a good point! When I met with the rep from the utility company at the sight, the first thing he did was locate their in ground transformer located approx. 100yds away. I first contacted the utility provider to have them assess the existing service to determine if it could handle the jump to 200amps from the existing 100amps. They gave me the all clear, never mentioning anything about the AIC ratings. When I pulled the permit from the city, again nothing mentioned about the AIC ratings. I installed the new equipment based on the AIC rating of the 23 other existing services. It doesn't makes sense to change this one new panel to 42k, when everything else is 10k. This is a good size electrical system, and I am sure it was all engineered and the 10k rating was approved or determined at that time. Very frustrating ! I will take your advice and ask for clarification on where this rating is coming from.

Thanks

 

[cowboyjwc]

That is a good point! When I met with the rep from the utility company at the sight, the first thing he did was locate their in ground transformer located approx. 100yds away. I first contacted the utility provider to have them assess the existing service to determine if it could handle the jump to 200amps from the existing 100amps. They gave me the all clear, never mentioning anything about the AIC ratings. When I pulled the permit from the city, again nothing mentioned about the AIC ratings. I installed the new equipment based on the AIC rating of the 23 other existing services. It doesn't makes sense to change this one new panel to 42k, when everything else is 10k. This is a good size electrical system, and I am sure it was all engineered and the 10k rating was approved or determined at that time. Very frustrating ! I will take your advice and ask for clarification on where this rating is coming from.

Thanks

Along the lines of one thing you just said, you can do everything just right and then some where along the line the utility comes along and changes the transformer and it can cause the AIC to go up.

 

[Dennis Alwon]

Along the lines of one thing you just said, you can do everything just right and then some where along the line the utility comes along and changes the transformer and it can cause the AIC to go up.

This is a potential big problem when malls or small shopping areas expand. What do you do make every shop owner and mall owner change their panels and breakers????

 

[Strathead]

This is a potential big problem when malls or small shopping areas expand. What do you do make every shop owner and mall owner change their panels and breakers????

My two cents worth. This issue really irritates me. If it really is that important, then it should be truly dealt with. Once the AIC is determined, kits should be designed and installed that prevent breakers that are smaller from being installed. Perhaps not irreversible, but certainly something that requires knowledge and conscious effort. More often than not, it seems that laziness is used to determine what must be installed, kind of like the load calculations often used by Engineers that don't get questioned. I don't worry, because they certainly have a large enough service, but I wonder how often the customer is paying WAY too much so less technical figuring has to be done.I do understand though, that some utilities are limiting transformer changes without prior investigation.

 

[kwired]

Is the 10K gear in close proximity to the 42K gear?

Remember the longer the run and smaller the conductors are the faster the SCCR will drop when going to an additional device.

 

[RB1]

Did I understand you to say that the transformer is located 100 yards away?

 

[jim dungar]

This is a potential big problem when malls or small shopping areas expand. What do you do make every shop owner and mall owner change their panels and breakers????

All the major POCOs in this area, require you to install equipment with AIC ratings based on a 'design' level SCA. They then purchase and install transformers so that this deign level SCA is never achieved.

This is from Xcel Energy (arc flash is handled differently):
"Tables IA through V in this Section show the available RMS symmetrical fault currents that may be expected at the secondary terminals of distribution transformers. Each fault current value listed in the tables is based on the lowest percent impedance transformer that might be set initially or as a replacement. No primary source or secondary line impedance has been included since it is generally relatively small, may change, and cannot be accurately forecasted.

Note: Because an overloaded transformer is typically replaced by the next larger size transformer, and an under-loaded transformer may be replaced by the next smaller size transformer, the customer is encouraged to use this range of transformers to perform their studies and select equipment such as current limiting fuses, breakers and switchgear bus bar bracing."

 

[cowboyjwc]

This is a potential big problem when malls or small shopping areas expand. What do you do make every shop owner and mall owner change their panels and breakers????

My two cents worth. This issue really irritates me. If it really is that important, then it should be truly dealt with. Once the AIC is determined, kits should be designed and installed that prevent breakers that are smaller from being installed. Perhaps not irreversible, but certainly something that requires knowledge and conscious effort. More often than not, it seems that laziness is used to determine what must be installed, kind of like the load calculations often used by Engineers that don't get questioned. I don't worry, because they certainly have a large enough service, but I wonder how often the customer is paying WAY too much so less technical figuring has to be done.I do understand though, that some utilities are limiting transformer changes without prior investigation.

I wish I had the answer and I agree with Strathead there should be a way that it couldn't be changed.

One question I have, is that about the highest AIC we'll see around here is 42K, so fairly easy to design around, but I've heard that there are places where the AIC is as high as 100K. Those would have to be tough to get down. Anyone have that issue?

 

[kwired]

I recently upgraded a commercial service from 100amp 120/208Y to 200amp 120/208Y, and also replace the 100amp sub-panel the service feeds with a new 200amp sub-panel. The building has a 1200amp main service coming in that serves (17) metered apartment units and (5) separately metered commercial spaces.
All (22) of the existing services have main breakers rated at 10k, as is the upgraded service and sub-panel I installed.
The inspector comes out for the final inspection and says that he wants me to get a letter from the utility provider stating the AIC rating of the service. The utility provider comes back with a rating of approximately 42k, now according to the inspector I will have to change both of the main breakers I installed to 42k breakers. I believe that I should only have to change the main breaker at the service to 42k, but not the sub-panel's main. I couldn't find a particular code section that stated that specifically, just wondering if I am on the right track.

Thanks

Based on the service of 1200 amps the transformer is not likely to be any larger than 500kVA, and likely is smaller than that, unless this service is loaded to the max. A quick look at charts in my American Electricians Handbook says a 500kVA with 4.5% impedance only has 33273 amps available at the transformer output, and that is assuming an infinite ability on the primary side of the transformer. 100 yards of conductor are going to make a significant drop at the service equipment, but just exactly how much will depend on size, type and number of parallel sets of conductors. The chart shows fault current to be about 22.6kA with four 750kcmil 200 feet long. Three 350kcmils shows about 18kA for 200 feet. 300 feet (100 yds) was not a length in the table but the 500 foot lengths were getting results under 10kA until you start reaching the column for four - 750kcmil conductors. Those were for copper conductors, if you have aluminum the values would be less.

 

[Ragin Cajun]

Having done numerous apartment complexes, fault rating on the service equipment, meter centers, etc. must be addressed up front.

I clearly list the requirements on the drawings and verify same with the shop drawings. Most of the suppliers in my area know I am a bear about shop drawings and I rarely have an issue.

My point is that it's the engineer's responsibility to be sure the equipment is specified properly from the get go. And for him to be firm on shop drawings. Unfortunately, the engineer isn't always in the shop drawing loop.

Problems can occur, of course, when additions are cobbled up without proper drawings. Then, whoever installes the mess is libel. But isn't that what typically happens? I see this in older industrial buildings where the incoming service is upgraded and fault current is out the roof, but the original equipment remains. It's a wonder that more incidents don't occur. Length of cable runs is our friend!

RC

 

[electricguy61]

I would just like to say that this kind of discussion is why I read the forum.

I learned more about AIC in a few minutes then I have in years.

Thanks folks!:thumbsup:

 

[tortuga]

Interesting discussion
To answer

where does it say an engineer has to sign off on it? ohms law can be figured out by the average electrician.

240.86 States
[QUOTE="NEC 2011 (C) NFPA](A) Selected Under Engineering Supervision in Existing
Installations. The series rated combination devices shall be
selected by a licensed professional engineer engaged pri-
marily in the design or maintenance of electrical installa-
tions. The selection shall be documented and stamped by the
professional engineer. This documentation shall be available
[/QUOTE]

However whats interesting is states I am familiar with Oregon and California exempt Licensed electricians from having to use an engineer to design there own projects I wonder which takes precedence in this case?

References to California and Oregon engineer license exemptions:
http://www.bpelsg.ca.gov/laws/pe_act.pdf see -> 6737.3. Exemption of contractors
ORS 672.060 (12) http://www.leg.state.or.us/ors/672.html
ORS 479.860 (2)

 

[jumper]

I would just like to say that this kind of discussion is why I read the forum.

I learned more about AIC in a few minutes then I have in years.

Thanks folks!:thumbsup:

Ditto for me on all three statements.

 

[augie47]

I wish I had the answer and I agree with Strathead there should be a way that it couldn't be changed.

One question I have, is that about the highest AIC we'll see around here is 42K, so fairly easy to design around, but I've heard that there are places where the AIC is as high as 100K. Those would have to be tough to get down. Anyone have that issue?

The larger cities in this areas supply the downtown area off an underground "grid" where the utility transformers for many square blocks are paralleled. SCCA for some of the buildings supplied by this grid is often in the 100k range and higher.

 

[petersonra]

Interesting discussion
To answer
240.86 States


However whats interesting is states I am familiar with Oregon and California exempt Licensed electricians from having to use an engineer to design there own projects I wonder which takes precedence in this case?

References to California and Oregon engineer license exemptions:
http://www.bpelsg.ca.gov/laws/pe_act.pdf see -> 6737.3. Exemption of contractors
ORS 672.060 (12) http://www.leg.state.or.us/ors/672.html
ORS 479.860 (2)

are you claiming that 240.86 prohibits an electrician from selecting a UL listed series rated combination from a manufacturer?

I agree that if you are making up your own series rated combination breaker system that it is well outside of the reach of anyone who does not have the expertise to make that work. But a UL listed system?

And I do not see anything in that provision that prohibits an electrician from using Ohm's law to determine what the SCC will be at the point where the CB is actually located.

 

[Dennis Alwon]

Have any commercial guys had problems with their temp poles that are set next to a trany. In Durham they turned down a job because the contractor installed a standard temp pole with 10kva breakers, The trany was 50,000 or so. Cost a bundle to correct

 

[texie]

Have any commercial guys had problems with their temp poles that are set next to a trany. In Durham they turned down a job because the contractor installed a standard temp pole with 10kva breakers, The trany was 50,000 or so. Cost a bundle to correct

I would say the inspector made the correct call. This issue of AIC ratings, as others have said, is a real can of worms. All to often it is overlooked by EC's and AHJ's.