AIC ratings and available fault current...

[John L]

Recently had a plan reviewer ask me for "AIC ratings and available fault current" for a small commercial job we are doing. i didnt have a clue, but contacted the power company to find out that the F.C. is 15,660amps for this property. We only have a 600a service so I guess we are OK there.

What is meant by the request for "AIC ratings"? Do they want the manufacturers listed AIC for each proposed new panel and all the breakers contained therein? Or would this be something else?

I am a rookie, Help is appreciated.

 

[LarryFine]

Let's start with what "SS" means.

 

[bob]

Yes. What does SS mean? And while you are checking that out check out this
site http://www.maintenanceresources.com/referencelibrary/ezine/eleccircbreak2.html

Recently had a plan reviewer ask me for "AIC ratings and available fault current" for a small commercial job we are doing. i didn't have a clue, but contacted the power company to find out that the F.C. is 15,660amps for this property. We only have a 600a service so I guess we are OK there.

You can not guess. You must know its ok. The service rating of 600 amps
does not guarantee you are safe.

What is meant by the request for "AIC ratings"? Do they want the manufacturers listed AIC for each proposed new panel and all the breakers contained therein? Or would this be something else?
I am a rookie, Help is appreciated.

AIC Ampere Interrupting Capacity. The MFG does indeed mark their gear with an AIC rating. Its some ones job to make sure its matches the available fault current from the utility transformer.

 

[hmspe]

Recently had a plan reviewer ask me for "AIC ratings and available fault current" for a small commercial job we are doing.

Simplified answer: "Available Fault Current" is the theoretical maximum current the utility can dump into a short circuit at a particular location. The "AIC rating" is the amount of current that can be dumped into a piece of electrical equipment under short circuit conditions without having [SIZE=-1]shrapnel blown out of the enclosure.

The long answer would take a few hours....

The 600A rating of the equipment has nothing to do with the available fault current or the AIC rating of the equipment.

For the most part, equipment must have an AIC rating at least as great as the available fault current at that point.

Martin [/SIZE]

 

[LarryFine]

To add: the available fault current drops as you travel from the transformer secondary terminals, through the drop or lateral lines, the metering, the disconnect, especially the main OCP, the service conductors, and into the distribution equipment panel(s).

In other words, the impedance of each segment of the current pathway reduces the available fault current beyond it. The POCO may have 60kA available, but it may only be 6kA by the time it reaches the breaker panel(s).

 

[sceepe]

I am a rookie, Help is appreciated.

No problem, Welcome to the forum. This is a great place to clarify any questions you may have about the N.E.C.

However, this is not a real good place to learn the basic concepts of electrical design. Available fault current calculations are a little too complicated to be explained properly on an internet forum.

Your best bet may to be to ask the professional engineer who is signing the drawings and who is in direct supervision of the design work (as required by law) to provide you with a fault current analysis of the project. I am sure that since he has been so involved in the project (as required by law) he will have no problem giving you the calculations that the AHJ is looking for.

This is one of those times when you can be glad you have that PE involved with the project and your not just trying to make up the electrical by yourself. Thank goodness you didn't just pay somebody with a liscense to stamp those drawings so you could get your permit.

 

[bphgravity]

See 110.9 and 110.10 of the NEC.

 

[scott thompson]

John L:

Recently had a plan reviewer ask me for "AIC ratings and available fault current" for a small commercial job we are doing. i didnt have a clue, but contacted the power company to find out that the F.C. is 15,660amps for this property. We only have a 600a service so I guess we are OK there.

The Plan Checker asked about the "Available Fault Current", likely because the Single Line Diagram on the Electrical Sheets did not have it listed.

The "Available SCA" (Short Circuit Amperes) is normally quoted by the PoCo (The Utility's Service Design Engineers, to be more correct), but it may also be figured in the Field by You (or any other EC), by using one of the several types of _SCA Calculations_ available. The "Point-To-Point" method is commonly used.
You would need to know the Power Transformer's KVA rating and it's % Impedance value.

What is meant by the request for "AIC ratings"? Do they want the manufacturers listed AIC for each proposed new panel and all the breakers contained therein? Or would this be something else?

The request for Available SCA + the Equipments' rated AIC, is to know if the Equipment + the OCPDs used for the Service Equipment have an AIC rating which is _NOT LESS_ than the Available SCA from the Utility Transformer, when measured at the Service Disconnect.

Example:

1: If the Fault Level at the Service Disconnect is 15,000 Amps L-L-L, and the Service Equipment + OCPD(s) in this Equipment are rated 22,000 AIC, then the System conforms.

2: If the Available SCA at the Service Disconnect is 23,000 Amps L-L-L, and the Equipment is rated as 22,000 AIC, then the System is Non-Conforming, and requires Equipment rated above the 23K Available SCA (in this case, the next step up would be 30KAIC).

I am a rookie, Help is appreciated.

No problem! How else would anyone learn these things, if not to ask questions.

Here is a link to a Thread at ECN (Electrical Contractors' Network), which discusses this stuff briefly:

What Makes A Breaker Trip

Reply if you have more questions.

Scott

 

[LarryFine]

No problem! How else would anyone learn these things, if not to ask questions.

Indeed! Additionally, we are also learning as we research the NEC and other sources to answer these questions. Many of us delve into sections of the code we would otherwise probably never see in the course of our own work, so thank you, too!

 

[wireman3736]

Let me ask this, if the main breaker in a panel is rated for the available fault current, now are the individual breakers in the panel required to be rated the same and every thing downstream or can they be of a lessor aic? the reason for this question is a few years back I was working for a mfg company, They expanded there operation and installed a bunch of large molding machines most using about 300 to 400 amps, it was determined that the disconnects on the machines did not have a rating for the afc, The contractor was allowed to install a junction box in each conduit run coming from the main switchgear and install a fuse block with fuses rated for the afc,

 

[ron]

All equipment, including safety switches, must be rated for the avail. fault current.

 

[hardworkingstiff]

All equipment, including safety switches, must be rated for the avail. fault current.

Doesn't the available fault current get smaller and smaller the farther you get from the transformer? If the fault current at the panel is 20,000 and you run a couple hundred feet to a disconnect with some #2 cu. wire then the fault current available at that point should be much less, right?

 

[wireman3736]

All equipment, including safety switches, must be rated for the avail. fault current.

Does this mean if I install a toggle switch for the boiler in a residential home then it has to meet the afc rating of the circuit? :

 

[hmspe]

Does this mean if I install a toggle switch for the boiler in a residential home then it has to meet the afc rating of the circuit? :

Yes. It only takes anout 15' of #12 NM or 10' of #14 MN to drop 10KAIC at the panel to below 2KAIC.

Martin

 

[LarryFine]

Doesn't the available fault current get smaller and smaller the farther you get from the transformer? If the fault current at the panel is 20,000 and you run a couple hundred feet to a disconnect with some #2 cu. wire then the fault current available at that point should be much less, right?

This is absolutely correct. There are fault-current calculators available that take into account each section of conductor and over-current devices. Current-limiting fuses in particular reduce available fault current.

 

[hmspe]

Current-limiting fuses in particular reduce available fault current.

True in most cases, but not necessarily usable for design. Not too many years ago the "up, over, and down" method for determining "let through" current for fuses was acceptable for just about everything. It's no longer allowed, at least in my area, for any load with "dynamic impedance", which would include fuses supplying down-stream circuit breakers.

Martin

 

[bob]

Let me ask this, if the main breaker in a panel is rated for the available fault current, now are the individual breakers in the panel required to be rated the same and every thing downstream or can they be of a lessor aic?

Yes the breakers in the panel must be rated for the given AIC. Think about it. If there is a fault close to the panel the fault level will be the given available fault current. The main breaker will not change that.

it was determined that the disconnects on the machines did not have a rating for the afc, The contractor was allowed to install a junction box in each conduit run coming from the main switchgear and install a fuse block with fuses rated for the afc,

This is not a proper installation. There is a "series rated" installation with fuses and breakers that would be correct but the this installation but be tested and rated for the available fault current. Just sticking a fuse ahead of the underrated panel will not solve the problem.

 

[scott thompson]

***BUMP-O'-ROONEEE!!!***

Just bumping this thread up again.

Scott

 

[PowerPipe]

Let me ask this, if the main breaker in a panel is rated for the available fault current, now are the individual breakers in the panel required to be rated the same and every thing downstream or can they be of a lessor aic?

There are (2) types of AIC ratings for panelboards. Fully rated and series rated. For a fully rated panel the overall short circuit current rating is equal to the short circuit current rating of the lowest rated device in the panel...If the MCB is rated for 65k @ 480V and there is (1) branch c/b rated for 22k @ 480V, the panelboard SCCR is 22k @ 480V. Series rated panelboards can have the branch c/b's with a SCCR lower than the MCB, but the lower rated branch c/b's have to be UL tested and labeled for series rating with the particular c/b's installed in the panelboard. I attached a link for more explanation on series ratings.

http://ecatalog.squared.com/pubs/Circuit Protection/Molded Case Circuit Breakers/0600DB0108.pdf

 

[bphgravity]

Let's say you have a calculated SCCR of 19,000A at the line side of the service disconnecting means - a main breaker in a 200A panelboard. This precludes that all devices installed in the panelboard including the main is required to be 22,000 AIC rated?

Say you have a SCCR of 9,000A at the line side of the service disconnecting means yet the plans call for a 22,000 AIC rated panel. Would it be inappropriate to install a 22,000 AIC main and 10,000 AIC branch circuit breakers? Keep in mind the SCCR is 9,000A. But by installing the 10,000 AIC devices, does that reduce the rating of the equipment to 10,000 even though the main is 22,000 AIC?