Location of Available Short Circuit Current

Status
Not open for further replies.
MyCleveland said:
Doesn't every transformer get shipped with an X/R on its build tag.
My point was they give you all this information they had to calculate, and leave out something they could get easily.
Click to expand...
yes, but how easy it is for you to view that tag or get someone from POCO to get info from it for you will vary from pretty easily to nearly impossible.
 
PE (done with training) said:
Yes, this information is very helpful even without the transformer X/R.

The transformer X/R is typically not available from the utility and I am lucky to get the information that I do.

I can use resources to make an educated assumption on the transformer X/R if need be.
Click to expand...
I would say this is exceptional information to receive. I’ve got some quick replies with good info but nothing this hood.
Curious what the benefit is to request the sca at the primary?. I always thought It’s a little more straight forward to prove the proposed equipment is adequate. Even if you were doing a more thorough study would getting utility info at the primary be better ?
I guess whether the utility xfmr is existing or not may come into play.
I thought it was interesting for Arizona utilizes where they provide tables, they give you the value to use at your service equipment. I guess it’s a safe approach to make sure things are good.
 
MyCleveland said:
I am recalling sales reps telling me they do...recall is getting shadier by the day.

What is the % split and magnitude they assign for X & R ?
Click to expand...
I sold transformers starting back in the 70s, except for special order units, with custom testing, specific X/R values were not routine.

You can get X/R, from %Z, by using symmetrical components.
 
Last edited:
I asked a similar question regarding maximum short circuit labeling requirements in my last post.

Can anyone confirm if the calculation for this is

transformer KVA rating divided by secondary voltage for single phase or secondary voltage x 1.732 for 3 phase?

Then result divided by transformer impedance?

I still don’t know if secondary conductor impedance has to be figured in

Also how is equipment selected according to this figure or what is the significance to this labeling rule? Thanks
 
Grouch1980 said:
When the utility provides you with their available short circuit current, where exactly in the incoming service to the building is this amount located? Is it right at the service point, where your building connects / splices to the utility, right before the service equipment?
Click to expand...

Maybe this depends on where the short is located, whether phase to phase secondary prior to first point of disconnect or after first point of disconnect to feeders. Either way I think the main panel is where the label is to be located? If this is what you meant?


It would make sense if fault was in primary service, then cutouts in utility lines would be location of short and therefore primary would open at utility cutouts (and or first point of disconnect) and fault thereafter would not be seen at main panel once cleared by these cutouts.

If short is in secondary then the first point of disconnect Breaker would open and this would be short location

There is also possibility of both primary and secondary opening
 
MyCleveland said:
Doesn't every transformer get shipped with an X/R on its build tag.
My point was they give you all this information they had to calculate, and leave out something they could get easily.
Click to expand...
It's usually covered by the doors of the transformer, so you have to open it and expose live parts to see this information. The transformer also could be locked, so only the utility can open its doors, once a service lateral is complete.
 
Carultch said:
It's usually covered by the doors of the transformer, so you have to open it and expose live parts to see this information. The transformer also could be locked, so only the utility can open its doors, once a service lateral is complete.
Click to expand...

You mean like this picture? We have access to our delta delta transformer at our job site and the specs are 300,000 VA TRANSFORMER with 4800 volts primary and 480 volts secondary

but maximum available fault current is not provided on nameplate label and has to be calculated
 

Attachments

  • 9F7B9908-1B3F-4701-91D6-D0F5506E2EBB.jpeg
    9F7B9908-1B3F-4701-91D6-D0F5506E2EBB.jpeg
    32.9 KB · Views: 8
Jpflex said:
You mean like this picture? We have access to our delta delta transformer at our job site and the specs are 300,000 VA TRANSFORMER with 4800 volts primary and 480 volts secondary

but maximum available fault current is not provided on nameplate label and has to be calculated
Click to expand...
And that is because the available current on the supply side is not constant from install to another.
Assuming infinite bus ability is somewhat common thing to do but is a worst case value and not the actual available fault current.
 
kwired said:
And that is because the available current on the supply side is not constant from install to another.
Assuming infinite bus ability is somewhat common thing to do but is a worst case value and not the actual available fault current.
Click to expand...

What is meant by infinite bus? If this is not actual fault current then how do you detrmine it?
 
Jpflex said:
What is meant by infinite bus? If this is not actual fault current then how do you detrmine it?
Click to expand...
petersonra said:
Infinite bus just means that you start with the assumption that the primary side of the utility transformer has no impedance so there is an infinite amount of short circuit current available on the primary side.
Click to expand...
Yes. Primary conductors back to the substation do have impedance and will limit what the primary can deliver.

Infinite bus is just presuming there is no impedance in the supply circuit which gives you worst possible scenario for available fault current on the secondary.
 
Status
Not open for further replies.
Top