is it necessary to do a short circuit for this?

Isaiah said:
This is a very large, heavy-industrial facility with big MVA transformers in a client-owned utility SS. They feed several 15kV SWGR which in turn feed numerous 13.8kV - 480V XFMRs
They in turn feed 480V SWGR which has feeders run to local bus bar enclosures that tap down into small dry type XMFR that feed 480-480/277V and 480-208/120V panelboard that feed small loads such as the HVAC unit.
I assume the large XFMRs and SWGR have already been calculated and I should focus on the small XFMRs to Panelboard to HVAC for short circuit cable sizing.
Correct me if I’m wrong here.


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It should be marked somewhere, and should be all the way to the last panel board supplying the branch circuits, and if you can find it you start from that point and then start calculating motor loads additions from there and all other calculation factors. But if this is an old system it may not be marked anywhere, then you would start from scratch.
 
Fred B said:
It should be marked somewhere, and should be all the way to the last panel board supplying the branch circuits, and if you can find it you start from that point and then start calculating motor loads additions from there and all other calculation factors. But if this is an old system it may not be marked anywhere, then you would start from scratch.
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The Panelboard is marked 10kA….there are no motor loads.


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Isaiah said:
I disagree - that would require running ETAP each time you sized a branch circuit - that’s excessive to say the least
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No.
If your system is already modeled in ETAP, it should be easy to determine the available fault current at any of your existing transformer secondaries. These fault currents would already include both utility and internal sources of short circuit current.

Sure you can assume the worst case of an infinite bus for each transformer by why if you already have a model created. This is definitely the fast way to make an estimate, but it often results in saying equipment is 'overdutied' when it really isn't.


Thank you for providing more details about your system.
 
jim dungar said:
That is the default SCCR for 208V panelboards. Your actual fault current could be higher or lower.
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The Upstream Transformer feeding the panel board secondary is 480/277V. The HVAC unit is actually 480 volts three phase. So if I run a hand calc. I come up with 1083A for available fault current at the secondary side of the XFMR source. If I use the BUSSMANN FC2 Ap I get 1082A. Does this look right?


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jim dungar said:
No.
If your system is already modeled in ETAP, it should be easy to determine the available fault current at any of your existing transformer secondaries. These fault currents would already include both utility and internal sources of short circuit current.

Sure you can assume the worst case of an infinite bus for each transformer by why if you already have a model created. This is definitely the fast way to make an estimate, but it often results in saying equipment is 'overdutied' when it really isn't.


Thank you for providing more details about your system.
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Yes but I don’t have access to the ETAP Model. I’m just an inspector


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For that size transformer at 5% impedance it makes sense.

curious...what is the HVAC MCA ??
 
Isaiah said:
45kVA. 480-480/277V


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Then yes your numbers look right for a 5%Z transformer. That seems like a high number for such a small transformer. but, your HVAC would still be okay if the transformer was as low as 1.1%Z.
 
Isaiah said:
Yes but I don’t have access to the ETAP Model. I’m just an inspector


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Then the customer, or contractor, should provide you the proper information for you to do your job.
 
jim dungar said:
Then the customer, or contractor, should provide you the proper information for you to do your job.
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Agreed - just don’t like to ask the engineering firm
Getting info outta them is like pulling 4 embedded wisdom teeth. Kinda painful


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Whole different story but something doesn't add up. A 45kva transformer with a 480 secondary would be 54 amps.
Not sure how its handling that HVAC and other loads.
(Plus the 10ka rating mentioned earlier is more indicative of a 208/120 panel)
But, if it is as you say, you certainly dont have a SCCR problem.
 
Isaiah said:
45kVA. 480-480/277V


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That is a small transformer, I wasn't concerned about having too high of fault current just based on that alone. But I did plug in 45 kVA @5% impedance into the spread sheet that is on Mike Holt's site. The highest fault current available at transformer terminals is the 1083 you mentioned. continue on directly from there with 100 feet of 4 AWG in metal raceway and you are down to 982 amps at the load end. You likely do have some more conductor between transformer and the panelboard which will only make the load end even lower.

I will add this seems like too small of a transformer to feed a HVAC unit with a MCA of 70 amps, let alone anything else that may been on the panelboard already. 45kVA @ 480 volts three phase is only about 54 amps.
 
augie47 said:
Whole different story but something doesn't add up. A 45kva transformer with a 480 secondary would be 54 amps.
Not sure how its handling that HVAC and other loads.
(Plus the 10ka rating mentioned earlier is more indicative of a 208/120 panel)
But, if it is as you say, you certainly dont have a SCCR problem.
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I was writing similar while you posted, happened twice to me today where someone posted same thing I was writing at the time.
 
kwired said:
That is a small transformer, I wasn't concerned about having too high of fault current just based on that alone. But I did plug in 45 kVA @5% impedance into the spread sheet that is on Mike Holt's site. The highest fault current available at transformer terminals is the 1083 you mentioned. continue on directly from there with 100 feet of 4 AWG in metal raceway and you are down to 982 amps at the load end. You likely do have some more conductor between transformer and the panelboard which will only make the load end even lower.

I will add this seems like too small of a transformer to feed a HVAC unit with a MCA of 70 amps, let alone anything else that may been on the panelboard already. 45kVA @ 480 volts three phase is only about 54 amps.
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I’ll have to go out and double check the XFMR nameplate
Thanks for heads-up
This has been a great thread


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I would also confirm the HVAC is being fed by that transformer.
 
I guess if you reverse calculate the MCA that unit could possibly be on a 45 kVA transformer, but there is no room for any other loads.
 
kwired said:
I guess if you reverse calculate the MCA that unit could possibly be on a 45 kVA transformer, but there is no room for any other loads.
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I went out and checked nameplate myself this time - it’s actually 225kVA, not 45kVA. And the impedance is 5.3%
The kid that sent me the info is an apprentice with 6 months experience


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