Fault Current on Electrical Permit

[jetlag]

On an electrical permit in my county there is a line that just says fault current . Are they wanting to know the fault current of POCO transformer ? How do they expect some one to know that until the POCO comes out to connect the drop to the house , so you can ask ? Will the POCO make a trip out just to look at the transformer and tell you the available fault current ? I just always put 100 K amps for the fault current and no one ever questioned it .

 

[ptonsparky]

For a house?
Call the POCO and ask them what it is a their transformer. You can use that or do a calc. I suspect it is a lot less than 100k. Are you installing 100k equipment?

 

[ptonsparky]

Look at the transformer. Ours will have 25 painted on them for 25kva. Do the math. Mike has a spreadsheet available in his 'free stuff' area.

 

[tom baker]

There is a requirement in the NEC to post fault current and date of calculation.

 

[Hv&Lv]

 

[kwired]

There is a requirement in the NEC to post fault current and date of calculation.

Exception for single family and maybe even two family dwellings though.

You still are supposed to have equipment that can withstand what can be delivered though.

 

[ptonsparky]

In essence when OP declared 100K, the materials installed should have changed.
Obviously, no one is looking or really cares.

 

[ggunn]

To do the available fault current (AFC) calculation you will need the kVA of the transformer as well as its %Z (impedance). The POCO can give you that or the AFC itself.

 

[jetlag]

For a house?
Call the POCO and ask them what it is a their transformer. You can use that or do a calc. I suspect it is a lot less than 100k. Are you installing 100k equipment?

Thanks ptonsparky . Yes it is a house .and it will surely be less than 100k . The transformer is a 25 KVA

 

[ptonsparky]

That Homeline panel should be good then.

 

[jetlag]

View attachment 2563377

Thanks or the chart H

View attachment 2563377

Thanks for the chart Hv8 . It is a house with a regular 200 a GE meter base/Disconnect combo like they sale at Home Depot .. The transformer is single phase 25 KVA 120/240 volt . Would I be safe just to turn in the 6944 amps showing in the chart ?

 

[ptonsparky]

Thanks or the chart H

Thanks for the chart Hv8 . It is a house with a regular 200 a GE meter base/Disconnect combo like they sale at Home Depot .. The transformer is single phase 25 KVA 120/240 volt . Would I be safe just to turn in the 6944 amps showing in the chart ?

Yes.

 

[jetlag]

Exception for single family and maybe even two family dwellings though.

You still are supposed to have equipment that can withsta nd what can be delivered though.s Kwired

Thanks Kwired, It it is single family dwelling . I was not using the formula correctly . I think the fault current is around 6.9 k

 

[kwired]

Thanks or the chart H

Thanks for the chart Hv8 . It is a house with a regular 200 a GE meter base/Disconnect combo like they sale at Home Depot .. The transformer is single phase 25 KVA 120/240 volt . Would I be safe just to turn in the 6944 amps showing in the chart ?

I used the fault current calculator that is available in Mike Holt's "free stuff" section.

Entered a transformer of 25kVA and a "worst case" impedance of 1.5% and 6944 amps was exactly what the available current at transformer terminals came out to be.

I then entered 100 feet of 4/0 aluminum for conductor and the resulting fault current at the load end of those conductors came out to be 4575.

 

[augie47]

In essence when OP declared 100K, the materials installed should have changed.
Obviously, no one is looking or really cares.

I agree. IF you have been listing 100k and no one has had any questions, that info is being ignored by whomever reviews it,.

 

[kwired]

I agree. IF you have been listing 100k and no one has had any questions, that info is being ignored by whomever reviews it,.

Standard application form and whoever reviews it only pays attention when they possibly have good reason to be concerned?

 

[jetlag]

I used the fault current calculator that is available in Mike Holt's "free stuff" section.

Entered a transformer of 25kVA and a "worst case" impedance of 1.5% and 6944 amps was exactly what the available current at transformer terminals came out to be.

I then entered 100 feet of 4/0 aluminum for conductor and the resulting fault current at the load end of those conductors came out to be 4575.

Thanks Kwired , That is a big help . I will turn in the 4575 . Do you count the POCO to the point of attachment and and then on through the outside disconnect and on into the panel box . I doubt the POCO drop is much larger than 1/0 but the service entrance and seu feeder will be 4/0 .. there will probably be 100 ft of the drop and then 35 ft of the 4/0 aluminum but the 4575 will be close enough for all the 200 amp dwellings I do .

 

[augie47]

Since your100k number didn't cause any concern they don't seem to take notice anyway. Any number less than 10k will likely satisfy any review since that;s the threshold number for a lot of equipment.

 

[kwired]

Thanks Kwired , That is a big help . I will turn in the 4575 . Do you count the POCO to the point of attachment and and then on through the outside disconnect and on into the panel box . I doubt the POCO drop is much larger than 1/0 but the service entrance and seu feeder will be 4/0 .. there will probably be 100 ft of the drop and then 35 ft of the 4/0 aluminum but the 4575 will be close enough for all the 200 amp dwellings I do .

Most likely the intent of including this entry on the application is to put whatever the available fault current at the service disconnect is in that entry.

 

[AAR]

I always just use the Eaton Bussman FC2 Available Fault current calculator phone app after I reach out to the utility for the specs on their transformer. If they only supply you with the secondary available fault current you might have to do a little math to pinpoint the impedance. But normally they should give you any spec you request. After your done it emails you the calculation breakdown report and a label to put on the gear. Pretty slick and free last time I checked.