Article 110.24 of 2011 NEC

[Ray Holder]

As per article 110.24 of the 2011 NEC, non-dwelling service equipment is required to be field marked with the amount of available fault current when installed or modified.

Most electricians do not know how to do this calculation. Please give us an example of this calculation so we can get a better understanding on how to apply Article 110.24.

 

[raider1]

Welcome to the forum.

Most of the time for non-dwelling units an electrical engineer is involved and they will preform the necessary calculations.

If you want to do the calculations yourself then you will use the infinite bus method and the utility transformer to come up with the available fault current at the load end of the transformer. Then you would use the point to point method to come up with the available fault current at the service equipment.

Mike has a free available fault current calculator available HERE. Scroll down the page to where you see the fault current calculator. This is a free Excel spread sheet.

Chris

 

[augie47]

Calculating the actual AIC in many cases will require engineering studies which will take into account motor contribution, etc.
In most cases, the utility can provide the AIC at the transformer or some point such as the end of the service drop or lateral.
That number (the utility) is often accepted by the AHJ.
Service cable size and length will effect the utility figure and guides to those calculations can be found in Mike's free stuff:
http://www.mikeholt.com/freestuff.php?id=freegeneral

(sorry Chris,, stepped on you..I was looking for references)

 

[jim dungar]

Maybe +99% of the time this value will need to be provided by the utility.
It is very rare that you will have access to all of the information needed to make the calculation yourself.

Free software is available from several locations.

There are several methods for estimating fault currents (i.e. infinite bus) but what confidence level does the result have when you estimate more then 50% of the information.

Info includes
Utility available fault current at the primary of their transformer.
Transformer size
Transformer impedance
Secondary conductor size and length.

 

[jim dungar]

Calculating the actual AIC in many cases will require engineering studies which will take into account motor contribution, etc.

Sorry, just my time to be insufferably picky again.:roll:

AIC is provided by the manufacturer of the protective device, it cannot not field calculated.
Available short circuit amps, often called SCA, is what is required for the NEC label.

 

[augie47]

Sorry, just my time to be insufferably picky again.:roll:

AIC is provided by the manufacturer of the protective device, it cannot not field calculated.
Available short circuit amps, often called SCA, is what is required for the NEC label.

If I had checked to see if you were on line I would not have taken the chance and answered In the back of my mind, I said "I will AGAIN say something Jim will catch"..... I am trying to get my terminology in shape. I'm just not sure you ans I will live that long..
Thanks one more time

 

[don_resqcapt19]

As long as the available fault current given to me by the utility is less than the rating of my service equipment, I see no real reason to do any calculations. I will just post the utility infinite buss fault current on my service equipment.

 

[jim dungar]

As long as the available fault current given to me by the utility is less than the rating of my service equipment, I see no real reason to do any calculations. I will just post the utility infinite buss fault current on my service equipment.

I believe that meets the actual intent of the NEC.

 

[Smart $]

Sorry, just my time to be insufferably picky again.:roll:

AIC is provided by the manufacturer of the protective device, it cannot not field calculated.
Available short circuit amps, often called SCA, is what is required for the NEC label.

A little behind in timing, but I was thinking the same as I read Gus' post.

Now as a matter of being picky, SCA may be a manufacturers term, but the NEC uses the term short-circuit current, and short-circuit current rating (SCCR), with the latter actually defined in Article 100.

 

[jim dungar]

Now as a matter of being picky, SCA may be a manufacturers term, but the NEC uses the term short-circuit current, and short-circuit current rating (SCCR), with the latter actually defined in Article 100.

Yes, it is too bad that there is no single universally accepted acronym for available Short Circuit Current. SCA has been used, in my part of the country, for many decades.

SCCR, short circuit current rating, is something that is applied to equipment, it is not a measurement of "how much is possible", but rather of "how much can I tolerate".

 

[Smart $]

Yes, it is too bad that there is no single universally accepted acronym for available Short Circuit Current. SCA has been used, in my part of the country, for many decades.

...

The most common usage of short-circuit current in the NEC is short-circuit current available. Seems SCCA would distinguish itself nicely from SCCR. SCA (short-circuit amperes) is a bit ambiguous in that it could be taken as either SCC(A) or SCCR in an NEC discussion, or implementation of the labeling requirement.

 

[jim dungar]

The most common usage of short-circuit current in the NEC is short-circuit current available. Seems SCCA would distinguish itself nicely from SCCR. SCA (short-circuit amperes) is a bit ambiguous in that it could be taken as either SCC(A) or SCCR in an NEC discussion, or implementation of the labeling requirement.

Feel free to try to persuade others of your point of view, although SCA is more common than you may think (i.e. both Bussmann and Mike Holt use SCA in their fault current formulas). The deadline for make suggestions to the 2014NEC is approaching.

 

[ron]

As long as the available fault current given to me by the utility is less than the rating of my service equipment, I see no real reason to do any calculations. I will just post the utility infinite buss fault current on my service equipment.

Unfortunately, the utility value may be conservatively high (infinite primary), but thought needs to be given to the amount of motor and stand-by generator (if any) contribution, and determine if the posted amount needs to be higher than that given by the utility..

 

[Smart $]

Feel free to try to persuade others of your point of view, although SCA is more common than you may think (i.e. both Bussmann and Mike Holt use SCA in their fault current formulas). The deadline for make suggestions to the 2014NEC is approaching.

Noted.

However, I'm of the impression the NEC text tends to avoid acronyms (excluding the 2011 adoption of SSBJ). We practitioners using NEC terminology on a frequent basis are the ones who "coin" the acronyms. Typically we use the terminology of the NEC in doing so. After reconsideration, with the NEC using the title and terminology Available Fault Current, I believe AFC would be more appropriate. From here it is simply a matter of using this acronym at each opportunity and others will also.

 

[jim dungar]

Unfortunately, the utility value may be conservatively high (infinite primary), but thought needs to be given to the amount of motor and stand-by generator (if any) contribution, and determine if the posted amount needs to be higher than that given by the utility..

The NEC label requirement is not concerned with contribution from any 'internal' sources.

 

[jim dungar]

After reconsideration, with the NEC using the title and terminology Available Fault Current, I believe AFC would be more appropriate. From here it is simply a matter of using this acronym at each opportunity and others will also.

Be aware that this one will cause confusion with Arc Fault current, as in Arcing Fault Circuit Interrupter (AFCI).

 

[don_resqcapt19]

Unfortunately, the utility value may be conservatively high (infinite primary), but thought needs to be given to the amount of motor and stand-by generator (if any) contribution, and determine if the posted amount needs to be higher than that given by the utility..

Yes, it may be high, but as long as that value is less than the AIC rating of my equipment there is no problem.

You are correct that you would have to add for motor loads, but in other than industrial facilites which are exepmt from this requirement, the motor load is unlikely to make a substantial difference.

As far as a stand-by generator, it would be unlikely that it could supply more fault current than the utility and a stand-by generator does not run in parallel with the utility so it does not add to the available fault current.

 

[Smart $]

Be aware that this one will cause confusion with Arc Fault current, as in Arcing Fault Circuit Interrupter (AFCI).

In that case, a proposal to change the 110.24 title to Available Short Circuit Current (ASCC), Available Fault Amperes (AFA), or Short Circuit Amperes (SCA), or something similar is arguably warranted.

FWIW, it will not be me making the proposal.