Question on fault current

[BJAP]

We have an inspector that has asked us to find the fault current for him. The service size is 600 amps, the under ground is 100 foot. We are using 250mcm with a 3/0 neutral parallel. 2.5 PVC pipe. He wants the fault current from the transformer to the service, then from the service to the outlets. Any help appreciated! thanks in advance.

 

[augie47]

The first thing you must do is determine the available fault current at your source (transformer). If it is a utility transformer, the utiliy should be able to give you a value.
Once you have this, you can do the calculations. I suggest you go to Mike Holt's home page and look at some of the fault current formulas in his technical section.

 

[wirenut1980]

He wants the fault current at the outlets? That seems a little out of place.

 

[davidr43229]

After you have gathered all the necessary information, you can go here and figure it out.
http://cooperbussmann.com/software/index.aspx
Just my $.02

 

[wbalsam1]

We have an inspector that has asked us to find the fault current for him. The service size is 600 amps, the under ground is 100 foot. We are using 250mcm with a 3/0 neutral parallel. 2.5 PVC pipe. He wants the fault current from the transformer to the service, then from the service to the outlets. Any help appreciated! thanks in advance.

Find the FLA
FLA = kVA x 1,000 / V

Find IC (interrupting current)
IC = FLA / impedance :smile:

 

[Pierre C Belarge]

What kind of occupancy is this building?
I can understand the request for the service, but I am curious as to why he is asking for the the fault current at receptacles???

 

[kingpb]

*Cheap and dirty method*

Step 1:
Assume infinite bus on the HV side of the service transformer.

Step 2:
Divide rated current of the transformer (unlikely 600A, will probably be less) by the transformer impedance, example with rated current = 600A ,and impedance of 5.75%, therefore 600A/0.0575 = 10,435A

Step 3:
Determine what the rating of the equipment is, if it's higher then this, tell the inspector it's 10,435A, if not go to step 4. (Usually not required for small systems)

Step 4:
Determine impedance of the cable 100' of parallel 250KCMIL (using effect Z from Table 9) = (0.033 ohms/1000ft) x 100 ft = 0.0033 ohms and reduce the amount of fault current by combining in accordance with engineering principles. (You can do what is called the MVA method)
The outcome of which will give yo about 8.3KAIC at the panelboard


Of course this was all based on a lot of assumptions regarding your actual system.

 

[BLUEDEVIL]

if it is coming from utility transformer you can get the incoming fault from the utility, if not use "kingpb" calculation just check your transformer "%Z"

 

[BJAP]

The value at the transformer is 171,800. This is a residence, and yes, he said he wants the fault current from 2 locations. From the xfmr to service, then from the service to outlets. thanks

 

[coulter]

Tell us about the MVA method.

carl

 

[don_resqcapt19]

BJAP,

The value at the transformer is 171,800.

That number can't be right for a residential service.
Don

 

[coulter]

Good thinking Don.

Let's see:
Residential
600A service
Parallel 250kcmil service conductors (Al or CU?)
SCC at xfm = 171KA (?)

Single phase, 240V (? - you didn't say)

If this is all true, makes perfect sense to me why the inspector is interested in fault current calcs. There isn't any equipment on the market that I know of that will handle it.

Edited to add "Parallel" to service conductors - had to reread OP
carl

 

[BJAP]

The utility company told us that it was 171,800 amps semetrical 120/240. Does that make sense?

 

[coulter]

.... Does that make sense?

Not a bit. 240V, 600A = 144KVA. So if the utility installed a 150KVA transformer, and it were a 2%Z (unlikely that low), that's 31kA SSC - nowhere near 171KA.

The worst I have ever seen (and I haven't seen everything:roll: ) was a 500kva, 208, 2%Z. SSC was 69KA

Now if the customer had supplied the transformer and the utility said the primary was 171MVA, that would make sense.

carl

 

[spsnyder]

This is from our local POCO, National Grid....

http://www.nationalgridus.com/niagaramohawk/non_html/constr_esb750.pdf

Check out page 63 at the bottom. Specifically Table 10 under 9.3.2. It gives available SSC for single phase 240V. Not in the same ballpark as given to you.

Regards

 

[kingpb]

Tell us about the MVA method.

carl

The MVA method developed by Moon Yuen at Bechtel in the 60?s is a modification of the Ohmic method where the impedance of a circuit equals the sum of the impedances of components constituting the circuit. The method uses the admittance of each component expressed as the Short Circuit MVA (SC MVA)that could flow through that component from an infinite bus to a short circuit. It was published through IEEE but never gained much recognition. It is very useful for small systems, but as the number of busses and motors significantly increases, it will not be as accurate as a computer program.

I was fortunate enough to get hold of a copy of the paper, otherwise you an get it from IEEE if your a member. Don't know if a copy is floating around the internet.

 

[coulter]

...you an get it from IEEE if your a member. ...

I am that - got a number?

carl

 

[kingpb]

Conference Catalog No. 73 CHO 769-01A
Paper No. PCI-73-7

A search on Google will also direct you to IEEE.

 

[BJAP]

New info gathered. We met the power consultant from the utility company. He says the Ultimate Fault Current is 171,800 amps 3 Phase. We are running single phase, 600 amps, 250 mcm, 3/0 neutral. 100 foot under ground. So we are now looking for the fault current from the transformer to the service, then the service to the outlets. I know it has come up that the 171,800 number is crazy, but it looks like this is it. Can someone give me a figure on these fault current numbers. thanks in advance

 

[coulter]

Conference Catalog No. 73 CHO 769-01A
Paper No. PCI-73-7

A search on Google will also direct you to IEEE.

Thanks king. I'll go look for it

carl