Inspector and AIC ratings

[titan1021]

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

 

[iwire]

Unless you can get an engineer to sign off that the available fault current at your subs is under 10K you are pretty much stuck.

110.9 Interrupting Rating. Equipment intended to interrupt
current at fault levels shall have an interrupting rating
not less than the nominal circuit voltage and the current that
is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault
levels shall have an interrupting rating at nominal circuit voltage
not less than the current that must be interrupted.

 

[mike7330]

What is the rating of the 1200 amp main breaker?

 

[Jraef]

Unless you can get an engineer to sign off that the available fault current at your subs is under 10K you are pretty much stuck.

And to further disappoint you, just changing the main alone will not solve the problem, unless your specific panels can be shown by the manufacturer to have a Series Rating of 42kAIC+ with the 42kAIC main and 10kAIC branch breakers. If not, you will have to change out the entire panel to one that has that Series Rating, or use 42kAIC rated branch breakers too.

 

[petersonra]

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

 

[augie47]

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

Wow! I envy you if the "average electrician" in your area can make fault-current calculations. The "better than average" one in my area barely understands the concept.

110.22 seems to define your choices which, in case case, looks like will require an engineering sign-off.

 

[jim dungar]

You definitely need to talk to your supplier.
Most manufacturers of multi-metering equipment have published series-combination ratings. It is routine that multi-metering even has 3 levels of series ratings; Main-meter breaker-tenant panel.


Articles 110.9 and 110.10 have been in the NEC since at least 1976 (when I bought my first copy), yet I am surprised how many electricians do not follow them. Aren't electricians taught that they do not apply 'just at the service entrance'?

When we perform power system studies, like arc flash, this is probably our most commonly found problem. My team alone, recommends $200K-$300K of equipment replacements due to inadequate fault handling ability, with a substantial amount traceable back to additions by electricians.

 

[petersonra]

Wow! I envy you if the "average electrician" in your area can make fault-current calculations. The "better than average" one in my area barely understands the concept.

110.22 seems to define your choices which, in case case, looks like will require an engineering sign-off.

I was referring to taking advantage of the resistance in the wiring between the breakers located in separate panels, and not coming up with a series rating for a panel board as 110.22(b) seems to allow.

A fault current calculation of this type is not much different than calculating voltage drop.

 

[Dennis Alwon]

Mike Holt has a fault calculator that I have used in a pinch.

Jim I do mostly resi work and nobody taught me about AIC ratings. I learned about it years ago on the forum. I would bet most electricians are not aware of how the system works. I have heard about series rating but I don't know much about it.

 

[petersonra]

Mike Holt has a fault calculator that I have used in a pinch.

Jim I do mostly resi work and nobody taught me about AIC ratings. I learned about it years ago on the forum. I would bet most electricians are not aware of how the system works. I have heard about series rating but I don't know much about it.

series ratings allow you to have a breaker that is rated lower than the available SCC in a circuit because the combination of it and some upstream device has been tested and found satisfactory for more SCC.

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.

ETA: Clarification - we can only do this with devices that have SCCR. Since breakers don't have a SCCR, we can't do it with them.

 

[cowboyjwc]

Two things, first I would ask the utility to run a real calc and not what it is at the transformer, it will drop off fairly quick depending on the run. Second, that still may not help you as it may only drop it down to, say, 22K at the main and you will still need the calcs done by an engineer to see if you can get it below 10k at the subs.

The panel is probably rated for more than 10k, but finding breakers that are, can be tricky unless you have a really good wholesale house.

Sorry, there was a third thing. That's something that should have been looked at at plan check and not after the system is installed.

 

[jim dungar]

Jim I do mostly resi work and nobody taught me about AIC ratings. I learned about it years ago on the forum. I would bet most electricians are not aware of how the system works. I have heard about series rating but I don't know much about it.

Major loadcenter manufactures started offering series rated "22/10" equipment so long ago (early 80's) that many people may no longer pay attention to the label.

Very simply put: Series ratings are a tested combination of devices that allows equipment to be applied on systems that appear to exceed their AIC or SCCR ratings.
There is a little bit of 'voodoo' associated with series-ratings, that is why the testing is pretty much required. It is extremely hard to meet the requirements for the engineering methodologies.
It is the physical interconnection and interaction of equipment during faults that creates what is effectively a current limitation. There is no requirement, nor assurance, that either series rated device clears the fault in any particular order.

One word of warning series ratings cannot be used when the is an appreciable amount of motor load after the upstream device. This may be why series ratings are more popular for residential and lighting/receptacle panels. The actual limitation is 1% of the downstream device's normal AIC, which means 100A of motor load for standard 10kAIC breakers.

 

[charlie b]

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.

 

[Fulthrotl]

And to further disappoint you, just changing the main alone will not solve the problem, unless your specific panels can be shown by the manufacturer to have a Series Rating of 42kAIC+ with the 42kAIC main and 10kAIC branch breakers. If not, you will have to change out the entire panel to one that has that Series Rating, or use 42kAIC rated branch breakers too.

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?

 

[jcbabb]

Mike Holt has a fault calculator that I have used in a pinch.

Jim I do mostly resi work and nobody taught me about AIC ratings. I learned about it years ago on the forum. I would bet most electricians are not aware of how the system works. I have heard about series rating but I don't know much about it.

I have found this calculator in particular to be nearly identical to the calculations I have done by hand.

 

[RICK NAPIER]

This issue comes up often and should be addressed at the plan review stage as repairs after the fact can be costly.

 

[Dennis Alwon]

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,

Isn't that because all fuses have an AIC rating of 200K???

We had an inspector turn down a temp pole here because the trany AIC was 45,000 and they install a 200 amp fuse. He said the 200K rating was to high.... I said I didn't think higher was a problem.

 

[Dennis Alwon]

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?

 

[augie47]

The series rating has to be with equipment the manufacturer list as being series rated. I have never seen a situation where it mattered if some of the equipment is older as long as it is on the "list".
I did inspect an old school complex where the original gear was only 5k rated. The only solution the engineers were able to come up with on that job was adding reactors in the service entrance.

 

[petersonra]

Isn't that because all fuses have an AIC rating of 200K???

We had an inspector turn down a temp pole here because the trany AIC was 45,000 and they install a 200 amp fuse. He said the 200K rating was to high.... I said I didn't think higher was a problem.

Not all fuses have a 200kAIC rating.

I maybe should have been clearer. I don't get why it is Ok to put the Cl device in a feeder circuit and than take advantage of the CL providied, but I can't put the same CL device in a branch circuit and take advantage of the CL.

An example as i understand the UL508a rules.

I can put a CL CB in a feeder and take advantage of that in a downstream branch circuit. Say my CL CB limits the feeder current to 10 K. I can use that to protect devices with a SCCR of 10k in a downstream BC.

I cannot use the same CL CB to protect the same device if both are in a BC, unless it has been tested.