# 2011 NEC Art. 110.24(A)



## Globe Trotting Spark (May 31, 2015)

I have been working out of the States since 2011 and am curious about how the available fault current or PSC (Prospective Short Circuit) from the British nomenclature is being evaluated for field marking commercial/industrial panelboards back home.

1) Can anyone direct me to an American meter than can evaluate and display the Available Fault Current in kA? The one used in Afghanistan is the Fluke 1653B and it it mistake proof.

2) Has anyones municipality or State require this field marking on services other than residential? 

3) Does one just call the Utility company, give an address and receive the Available Fault Current value for field marking those services associated with the address given?

4) Can anyone post the equation used and an example of how the equation is used for determining the Available Fault Current for a seperately derived system?

I am unable to instantly reply so I thank those for any useful insight before hand?


----------



## Pete m. (Nov 19, 2011)

Typically the available fault current is provided by the serving utility.

If there is no utility the fault current can be calculated based upon the size and impedance of the transformer serving the electrical equipment.

Lastly, yes, it is enforced.

Pete


----------



## RIVETER (Sep 26, 2009)

The available fault current would be supplied by the service transformer. That particular information should be with the transformer and therefore, the utility.


----------



## Meadow (Jan 14, 2011)

If you are looking for perspective fault current you will need to buy a foreign meter, I could be wrong but I have not seen one for the US market. 

http://www.tester.co.uk/megger-mft1720-multifunction-installation-tester


----------



## Meadow (Jan 14, 2011)

And oh, this might help:

http://www.alabamapower.com/business/services/architects-engineers/pdf/A%20E%20Fault%20Currents%20Tables%20FINAL%208%202003.pdf


----------



## don_resqcapt19 (Jul 18, 2010)

meadow said:


> And oh, this might help:
> 
> http://www.alabamapower.com/busines...df/A E Fault Currents Tables FINAL 8 2003.pdf


That works fine for the rule in 110.24 but don't try to use it for arc flash calculations as that table is based on an infinite bus. The infinite bus is the worst case for available fault current and equipment selected on that current will always be suitable. Even when you use the tables that account for the cables on the secondary side, you have not accounted for the actual available fault current on the line side of the transformer.

Since the arc flash incident energy is based on OCPD clearing times, using the infinite bus current for an arc flash calculation may result in a lower incident energy than if you had used the actual available fault current in the calculations.


----------



## don_resqcapt19 (Jul 18, 2010)

Globe Trotting Spark said:


> ...
> 2) Has anyones municipality or State require this field marking on services other than residential?
> ...


The rule in 110.24 does not require the available fault current to be marked on dwelling unit services. It is only required on non-dwelling unit services.


----------



## Meadow (Jan 14, 2011)

don_resqcapt19 said:


> That works fine for the rule in 110.24 but don't try to use it for arc flash calculations as that table is based on an infinite bus. The infinite bus is the worst case for available fault current and equipment selected on that current will always be suitable. Even when you use the tables that account for the cables on the secondary side, you have not accounted for the actual available fault current on the line side of the transformer.
> 
> Since the arc flash incident energy is based on OCPD clearing times, using the infinite bus current for an arc flash calculation may result in a lower incident energy than if you had used the actual available fault current in the calculations.


Is this because lower fault current takes longer for the OCPD to open?


----------



## Globe Trotting Spark (May 31, 2015)

I plan on testing these tables against a few services feed from utility supplied transformers using a Fluke 1653B. I would imagine the values will vary but hopefully not wildly. If the values are close then maybe investing in one of these testers would be wise. Only problem will be having it calibrated due to this meter being from Britain. If one of you had the opportunity to acquire a meter that could calculate the maximum fault-current without having to walk off the service drop for length and finding the value on a chart would you?

Are you guys just using the maximum fault current from utility company charts? If so, how does that jive with 110.24(A)'s statement that the available fault current on services other then residential shall include the date the fault-current calculation was performed? Is pulling a fault-current value from a chart suitable for performing a calculation?

I mean, really, what inspector is going to check if your pulled calculation is correct or not. I suppose that is all they would have to go on given the absence of american meters ability to calculate service fault-currents. Your thought?


----------



## Pete m. (Nov 19, 2011)

Globe Trotting Spark said:


> I mean, really, what inspector is going to check if your pulled calculation is correct or not. I suppose that is all they would have to go on given the absence of american meters ability to calculate service fault-currents. Your thought?


If the fault current values are supplied by the POCO I look no further. It doesn't matter to me if they got the values from a chart or if someone actually performed calculations.

The POCO, in this area, provides the available fault current at the _point of service_ and that is usually on the secondary side of the POCO transformer. The electrician or EE will sometimes use that value to calculate the fault value at the service equipment if it is necessary.

Pete


----------



## don_resqcapt19 (Jul 18, 2010)

meadow said:


> Is this because lower fault current takes longer for the OCPD to open?


Exactly the incident energy is a function of both clearing time and available fault current.

This makes it some what difficult to do the arc flash calculations for the service equipment. The utility system is somewhat dynamic and they do not always have the same available fault current. Many engineers who do these calculations with run them a number of times using a reasonable range of available fault currents, and post the worst case based on that. 

The key is using reasonable available fault currents as using current outside of what is reasonably expected to be available can result with the "dangerous" classification...that is above 40 cal.

That is not to say that some systems are not in the dangerous category even where reasonable currents have been used. We often see this with industrial switch gear that is feed from a large transformer. The rating on the line side of the main breaker is very high.


----------

