# Available Fault Current 110.24



## Zog (Apr 15, 2009)

NJELEC said:


> What are you guys doing to comply with 110.24 (marking equipment with available fault current). Have you guys have seen a program that does the calculations and prints the labels. If the AHJ's enforce this, engineering cost could add up quickly.


We use SKM power tools. 

But typically this just means you have to get the fault current info from the utility.


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## RePhase277 (Feb 5, 2008)

Zog said:


> We use SKM power tools.
> 
> But typically this just means you have to get the fault current info from the utility.


Yep. Run out and pick up a $20,000 copy of SKM Power Tools.

Or.... Get the fault calculating spreadsheet at Mike Holt's. Then learn how to make your own labels.


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## micromind (Aug 11, 2007)

95% of the time, if you look at the POCOs transformer and calculate fault current from the impedance listed, you're under the maximum allowed for the equipment in question. 

Often, quite a bit under.


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## Zog (Apr 15, 2009)

micromind said:


> 95% of the time, if you look at the POCOs transformer and calculate fault current from the impedance listed, you're under the maximum allowed for the equipment in question.
> 
> Often, quite a bit under.


Maybe, that does not account for motor contribution which can be a huge factor.


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## Bbsound (Dec 16, 2011)

micromind said:


> 95% of the time, if you look at the POCOs transformer and calculate fault current from the impedance listed, you're under the maximum allowed for the equipment in question.
> 
> Often, quite a bit under.


That only gives you some info, and only for the first piece of equipment. Many factors come into play when calculating fault currents including conductor lengths and properties.


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## micromind (Aug 11, 2007)

Zog said:


> Maybe, that does not account for motor contribution which can be a huge factor.


True.

A running motor becomes a generator very quickly during a fault.


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## micromind (Aug 11, 2007)

Bbsound said:


> That only gives you some info, and only for the first piece of equipment. Many factors come into play when calculating fault currents including conductor lengths and properties.


Different jurisdictions have different requirements; around here all they seem to care about is the main and the feeder breakers in the main switchgear. 

I've never had an inspector ask about anything downstream from the main gear.


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## don_resqcapt19 (Jul 18, 2010)

Bbsound said:


> That only gives you some info, and only for the first piece of equipment. ...


The code requirement asked about in this thread only applies to the service equipment.


> 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.


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## don_resqcapt19 (Jul 18, 2010)

Zog said:


> Maybe, that does not account for motor contribution which can be a huge factor.


I would expect that in a non-engineered job, that the motor contributions would not be a big factor. I don't think too many non-engineered jobs have a lot of motors. If the job has been engineered, then the engineer should have done the calculations as part of the design.


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## hardworkingstiff (Jan 22, 2007)

How much H.P. of motor load would it take to make a significant contribution to the fault current?


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## Zog (Apr 15, 2009)

hardworkingstiff said:


> How much H.P. of motor load would it take to make a significant contribution to the fault current?


Define signifigant


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## hardworkingstiff (Jan 22, 2007)

Zog said:


> Define signifigant


I would guess a 15% increase in available fault current might be significant. I guess also the amount of fault current available w/out considering the motors would come into play.

I'm just trying to get a "feel" for this stuff. I don't usually work in areas where it's more than 22K and it's usually less.

So, I guess I'm trying to get a feel for how much impact the motors have. Do you need more than 50 HP of motors to make an impact, or more than 10 HP has an impact? Just trying to get a better feel.

Thanks


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## don_resqcapt19 (Jul 18, 2010)

hardworkingstiff said:


> I would guess a 15% increase in available fault current might be significant. I guess also the amount of fault current available w/out considering the motors would come into play.
> 
> I'm just trying to get a "feel" for this stuff. I don't usually work in areas where it's more than 22K and it's usually less.
> 
> ...





> When there are motors in the system, motor short circuit contribution is also a very important factor that must be included in any short-circuit current analysis. When a short circuit occurs, motor contribution adds to the magnitude of the short-circuit current; running motors contribute 4 to 6 times their normal full load current. In addition, series rated combinations can not be used in specific situations due to motor short circuit contributions (see the section on Series Ratings in this book).


The above is from the Bussmann Short Circuit Calculation publication


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## micromind (Aug 11, 2007)

hardworkingstiff said:


> How much H.P. of motor load would it take to make a significant contribution to the fault current?


A rough estimate would be to add 20 amps per HP to a 208 or 240 3ø system, and 10 amps per HP to a 480.

This is a very rough estimate; the actual values will usually come in at about 2/3 or 3/4 of the figures above. 

10 HP doesn't make much difference, but 100 HP could easily cause the fault current to be higher than the breaker can stand.


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## don_resqcapt19 (Jul 18, 2010)

micromind said:


> A rough estimate would be to add 20 amps per HP to a 208 or 240 3ø system, and 10 amps per HP to a 480.
> 
> This is a very rough estimate; the actual values will usually come in at about 2/3 or 3/4 of the figures above.
> 
> 10 HP doesn't make much difference, but 100 HP could easily cause the fault current to be higher than the breaker can stand.


That is almost twice the motor contribution current that Bussmann uses in the fault current calculation publication, and given Bussmann's bias towards fuses, I would expect that their number would be the higher one.


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## hardworkingstiff (Jan 22, 2007)

Zog said:


> Maybe, that does not account for motor contribution which can be a huge factor.


Would you define huge?


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## GA3sparks (Jul 2, 2013)

I am trying to determine fault current for a small office bldg. 1200 sq.ft 150 BR main exterior disconnect main breaker rated at 25k aic.
POCO's said we have a 50kva pad mounted trans. 1.6 impedance, 21460 fault current on 120v, and 13020 on 240v. Inspector says I can do a calculation or just put a sticker on the panel any help would be appreciated.


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