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Modeling a Transformer as Utility
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Author:  Lucy_2701 [ Tue Jul 30, 2024 9:34 am ]
Post subject:  Modeling a Transformer as Utility
If I'm taping from the secondary of a 480V transformer (750KVA, 12.47kv-480V) Z=5.25%) to power my low voltage MCC, do I still need the information from the utility which is powering that transformer? Or should I just model my transformer as the "utility" ?
Author:  bbaumer [ Tue Jul 30, 2024 12:27 pm ]
Post subject:  Re: Modeling a Transformer as Utility
Depends on what you're trying to determine. If you're just looking to calculate maximum bolted fault current then assume an infinite bus primary and have all your motor loads accounted for.

If you're doing an arc flash study s well, I'd recommend obtaining the utility company data and modeling it as accurately as possible.
Author:  Lucy_2701 [ Tue Jul 30, 2024 1:36 pm ]
Post subject:  Re: Modeling a Transformer as Utility
Thank you bbaumer.
Now when I asked for the system equivalent at the point of interconnection, or the short circuit, someone asked: "well the point of interconnection for us is the disconnect switch at the pole from the transformer. Why don't you use the ratings for that disconnect?"
And I don't know the answer to that. Could you, or someone, please explain?
Author:  bbaumer [ Tue Jul 30, 2024 2:22 pm ]
Post subject:  Re: Modeling a Transformer as Utility
I don't understand whoever said that and I don't think they do either.

Are you / they talking about the primary side cutouts or is there an exterior, 480V service disconnect?

Different utilities have different rules or requirements for their customers as far as where their stuff ends and the customers begins.

Your maximum available at the transformer secondary is going to be something like 750000/480/1.732/0.0525 = 17, 184 amps plus motor contribution. I would probably spec a 65kA MCC if it were me but you could go lower.

Again, if you're doing an arc flash study, it's best to obtain the actual available fault current data from the utility at whatever point on the system they determine and go from there.
Author:  Lucy_2701 [ Wed Jul 31, 2024 8:25 am ]
Post subject:  Re: Modeling a Transformer as Utility
bbrent thak you for your reply.

I'm attaching a drawing to better explain myself:

1)Do I ONLY need the short circuit from the 480VAC EXISTING POWER DISTRIBUTION PANEL? or do I need the info from where that POWER DISTRIBUTION panel is connected to?

2) The question asked was: since we are connecting the panel to a delta delta transformer and that to a 100A fused disconnect at that to another fused disconnect, he is asking if our point of connection is the first fused disconnect instead of the 480V EXISTING POWER DISTRIBUTION PANEL and thus we only need the info of that first fused disconnect instead of the 480V EXISTING POWER DISTRIBUTION PANEL

Attachments:
One Line question.pdf [27.93 KiB]
Downloaded 821 times
Author:  bbaumer [ Wed Jul 31, 2024 1:03 pm ]
Post subject:  Re: Modeling a Transformer as Utility
1. If you can get the available fault current at that distribution panel that will be fine an you can model that as the utility, you still need the feeder size and length from that distribution panel to the first disconnect though.

2. Not sure I understand question 2 but if so, you need the fault current at the upstream distribution panel and feeder data as described in 1. above OR if they can tell you the available fault current at the first fused disconnect switch instead that would work too as your "UTILITY".

Attachment:
AVAILABLE FAULT CURRENT.jpg
AVAILABLE FAULT CURRENT.jpg [ 59.18 KiB | Viewed 9648 times ]
Author:  Lucy_2701 [ Wed Jul 31, 2024 1:08 pm ]
Post subject:  Re: Modeling a Transformer as Utility
bbaumer Everything has been clearly explained and hence clearly comprehended.

THANK YOU!
Author:  bbaumer [ Wed Jul 31, 2024 1:14 pm ]
Post subject:  Re: Modeling a Transformer as Utility
Glad I could help.
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