short circuit MVA rating

[matt111111]

I am trying to determine what the 3 phase and 1 phase MVA short circuit rating is for my shop. Do I need to request this information from my utiliy provider or is there another way of determining this?

Also if the utility enters a main building and that main building supplies multiple buildings under it will the short circuit rating be the same for all sub buildings or will they be different ?

 

[davidbeach]

You'll have to get it from the utility. All of the sub buildings will have a lower fault current from the utility than the main building, but the utility will only give you the fault current at the main building. Unless you have lots of very large motors, you will do fine assuming the same fault current for all buildings.

 

[matt111111]

The sub building I'm looking at has a main breaker with a rating of 400A at 600v. Would I be safe to say the short circuit current is the breaker maximum of 400A? and if so then I could just assume the short circuit MVA rating for the building would be = (sqrt 3 * 600v * 400A)/6

 

[rbulsara]

As stated earlier, the available scc at the point of service will need to be obtained from the utiltiy co or at least know the size of the utility transformer feeding the building, to make an "conservative" estimate.

For a more accurate estimate, a calculation need be performed and necessary source and circuit impedance and other related information need to be known.

The cables and transformers between the main building and the sub-building will reduce the available SC current at the sub building.




Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

I would expect that the maximum fault current would be in excess of 20kA. Maximum fault current is way above maximum load current.

 

[racobb]

The 400 amps is the continuous rating of the breaker and has nothing to do with its short circuit rating.

Sqrt 3 x 600 x 400 = the VA rating of the circuit without the division by 6.

Also am curious why you decided to divide by 6?

As was stated before, you should either get the info from the utility or get some help to come up with a conservative estimate of the available fault duty.

Alan

 

[matt111111]

I got the info from the utility which was stated as

5.425 MVA sc L-L-L
3.744 MVA sc L-G

Xl/Rl 3.74
X0/R0 3.63


These are the ratings at the main. From the main it then feeds into a distribution panel and on the panel is one 400A breaker that feeds the building in question. If I did know the cable size, length, and type feeding the building from the 400A breaker how would I get a closer aproximation of this buildings fault rating?

 

[matt111111]

sorry

5.425 MVA sc
4.697 MVA sc

 

[racobb]

Those numbers seem extremely low!

Alan

 

[rbulsara]

It depends on the size of the transformer, which we do not know. 5.425MVA sounds about right for a 150kVA service transformer at 600V.

Matt:
If you asked your question correctly to the utility co, go with their answer. If in doubt, consult some professional engineer with all the info on hand.

Rafiq Bulsara

http://www.srengineersct.com

 

[matt111111]

Double checked with the utility they have stated that the
fault current is:
5224A three phase and 4524A line to ground fault.

I have used these parameters in the modeling software I am using and they have returned the results of

5.425 MVA sc and 4.697 MVA sc.

 

[rbulsara]

If at 600V.

Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

OK, so what's the question? It seems that you have what you are looking for.

 

[dpc]

You need to make sure you know what voltage the utility data is based on. If your shop takes delivery at primary voltage, they will probably give you the data at primary point of connection. If you take service at low voltage, the fault current they provide is generally referenced to the low voltage side and includes the transformer impedance.

But they have to tell you.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[matt111111]

I have determined that the data sent to me was based on the 12.5 kv side of the transformer. Is there any way to determine what the fault current rating is on the 600v side if the transformer impedance is unknown?

 

[rbulsara]

NO. You at least need to know the xfmr size to get in the ball park.



Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

matt:

With all due respect, you need to get the understanding of the basic of short circuit calculations. I suggest you get a good book such as IEEE Red Book and familiarize your self with the basics or pay someone to help you out.

Even some google search on the topic will get you information that is useful, if you know enough to separate good info from the bad one.



Rafiq Bulsara

http://www.srengineersct.com

 

[racobb]

5224 amps seems much more believeable at 12.5 kV. About 113 MVA SC.

As Rafiq said, you are going to need more help than what you will be able to get here.

Alan

 

[dpc]

If you can determine the transformer kVA size, you can get in the ballpark for short circuit current on the low side.

It would help if we knew what you plan to do with the short circuit data when you get it.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)