calculate aic

[don marshall]

what will be acceptable to the ahj to conform to the 2011 nec to calculate aic per new 110.24

 

[charlie b]

I don't have a copy of the new code yet, so I don't know anything about the requirement you mention. But I will say that in order for a system to be properly designed, the amount of current a device can handle (i.e., without being destroyed) has to be greater than the amount of current that the power system could ever inflict upon that device. The term "amps interrupting capacity," or AIC, has to do with the first part - the rating of the device itself. You don't calculate AIC; rather, the manufacturer tells you the AIC.

If you are asking about calculating the amount of current that a system can inflict upon any device installed in the facility, that is a significant math problem that generally requires the use of an analysis software package. If you are asking about what any given AHJ will accept, you will have to ask that particular person.

 

[jim dungar]

I just finished a meeting with one of the those that attended an NEC meeting on this section.

He says, the panel's comments clearly indicate their intention was for this label to contain the fault current value provided by the utility.

 

[bob]

He says, the panel's comments clearly indicate their intention was for this label to contain the fault current value provided by the utility.

At what point along the circuit was the fault value to be taken for the label?
Is the utility going to calculate this value?

 

[bluesmoke...oops]

He says, the panel's comments clearly indicate their intention was for this label to contain the fault current value provided by the utility.

So you are saying the fault current value provided by the utility (which will be from the transformer secondary in my happy little design world) needs to be labeled on the panel?

I read it to mean you just have to put the number your calc shows at each panel in your system.

 

[zog]

Can someone post what the new article says?

 

[bluesmoke...oops]

Ask and you shall receive

110.24 Available Fault Current.
(A) Field Marking. Service equipment in other than
dwelling units shall be legibly marked in the field with the
maximum available fault current. The field marking(s) shall
include the date the fault current calculation was performed
and be of sufficient durability to withstand the environment
involved.

(B) Modifications. When modifications to the electrical
installation occur that affect the maximum available fault
current at the service, the maximum available fault current
shall be verified or recalculated as necessary to ensure the
service equipment ratings are sufficient for the maximum
available fault current at the line terminals of the equipment.
The required field marking(s) in 110.24(A) shall be
adjusted to reflect the new level of maximum available
fault current.

Exception: The field marking requirements in 110.24(A)
and 110.24(B) shall not be required in industrial installations
where conditions of maintenance and supervision ensure
that only qualified persons service the equipment.

 

[Jraef]

Wow, field labeling now... another PITA.

Looks like an opportunity to get into the "custom field nameplates of sufficient durability to withstand the environment involved" business. P-Touch labels will probably not be deemed acceptable...

I'm sure Brady is already on it (if not behind the lobbying effort for the new requirement).

 

[charlie b]

. . . the 2011 nec (requirement) to calculate aic per new 110.24

Now that bluesmoke has been kind enough to post the actual text, it is clear that the requirement is not related to the AIC. It has to do with the available fault current, not the capacity of the equipment to withstand a fault.

He says, the panel's comments clearly indicate their intention was for this label to contain the fault current value provided by the utility.

If the utility provides the service transformer, they should be able to determine the fault current available at its secondary terminals. But if the utility service point is on the primary side of the transformer, then the owner will have to arrange for a calculation to be performed.

I read it to mean you just have to put the number your calc shows at each panel in your system.

Not at each panel, but only at the service panel.

Wow, field labeling now... another PITA.

There is already a requirement for field labeling (i.e., arc flash information), and this one applies to all panels, not just the service panel.

 

[zog]

Ask and you shall receive

Thanks, I find the exception interesting. One thing I have learned on this forum is that different people interpret "Industrial" differently.

I would have liked to see something in there about the methods used. If the utility provided fault current is used at the panels it could snowball into some bad assumptions down the road. Overestimating your fault current is good for AIC ratings but can have dangerous concenquences if applied to an arc flash study.

 

[jim dungar]

But if the utility service point is on the primary side of the transformer, then the owner will have to arrange for a calculation to be performed.

Not at each panel, but only at the service panel.

If the customer owns the transformer, the service entrance is on the primary side, so the secondary panels would not require a label.

The problem is still in getting a value from the utility. Most of the ones I have talked to will be providing their "maximum design level". This is the point they guarantee their system will never exceed and usually bears little to no relationship to what is currently available.

 

[rbalex]

Yes, the Exception appears to be an "industrial exemption" to me. It's only institutional, commercial and "Mom and Pop" operations that would seem to need the label. Except for large campus institutional operations, the utility AIC would probably be enough. Also note application is only for ?services;? other types of distribution equipment seem to be exempt.

 

[rbalex]

I also find the "Modifications" section interesting. How would anyone know there has been a (utility) modification that would affect the available fault(s).

 

[zog]

The problem is still in getting a value from the utility. Most of the ones I have talked to will be providing their "maximum design level". This is the point they guarantee their system will never exceed and usually bears little to no relationship to what is currently available.

Exactly, and if used for an arc flash study could lead to false faster clearing times and lower Ei's than actual. Eventually, someone wearing the PPE listed on a label from these assumptions is going to get burned.

 

[zog]

I also find the "Modifications" section interesting. How would anyone know there has been a (utility) modification that would affect the available fault(s).

That could be an in house modification, like adding motors, but I have a hard time believing if the utility makes a modification they will inform everyone that is affected.

 

[jim dungar]

Exactly, and if used for an arc flash study could lead to false faster clearing times and lower Ei's than actual. Eventually, someone wearing the PPE listed on a label from these assumptions is going to get burned.

This is a training issue.

Right now we deal with enough poorly trained people that try to perform AF calculations using the AIC rating of the equipment.
This label has one purpose in life. To make it easier for inspectors, and others, to verify compliance with 110.9 and 110.10. Any thing else is a misuse.

 

[jim dungar]

That could be an in house modification, like adding motors, but I have a hard time believing if the utility makes a modification they will inform everyone that is affected.

No.

The label is intended to reflect only amps from the utility. There is not even a requirement for X/R ratio.
Several utilities already provide this information in their service manuals.

From Xcel Energies (my emphaisis):

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.

 

[jghrist]

The qualifications on the industrial exemption are interesting.

Exception: The field marking requirements in 110.24(A) and 110.24(B) shall not be required in industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.

How many companies that allow unqualified persons to service the equipment will admit it and tell the installation contractor that labels are needed?

 

[bluesmoke...oops]

Not at each panel, but only at the service panel.

Yeah my eyes skipped right over service.

 

[skeshesh]

This is a training issue.

Right now we deal with enough poorly trained people that try to perform AF calculations using the AIC rating of the equipment.
This label has one purpose in life. To make it easier for inspectors, and others, to verify compliance with 110.9 and 110.10. Any thing else is a misuse.

I don't think any experienced and qualified engineer performing AF would trust just the label. First of all they would certainly request a recent fault level from the utility. Also they would likely do a short circuit study on the entire distribution system past that point anyway. The only time I have seen this not done was when a very recent short circuit study was done by another consulting firm - of course the engineering doing the AF did go through the study and documents to do a sanity check before trusting the result. In addition the system models used in the previous study (in SKM if i remember correctly but it could've been ETAP, etc.) were provided so it was really double checked. Bottom line I would not trust someoen using from a number off of a label in their AF calcs.