transformer sizing

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wired480_99

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I have an existing building with 100 A 480V service. Drawing calls for 225A 120/208V panel w/ connected load= 58.3KVA, demand load= 44.69KVA.
There is also a 16.5KW RTU on the existing 480V panel.
1. Do I size the transformer based on demand or actual total load?
2. Assuming it is based on demand, I calculated 124A on secondary44690/208/1.732, 54A on primary 44690/480/1.732
3. Based on connected load, 162A secondary 58300/208/1.732, 70A primary 58300/480/1.732.
4. I then add the RTU load 16500/480/1.732=19.85A
5. If I calculated correctly the existing 100A 480V service is adaquate.
6. My next question is what size transformer do I need. 45, 50, 60 KW?
 
read the code

read the code

I'm sorry if my request seemed vague. I am not looking for any " Engineering ". I did look in the code before I posted, I always follow the rules, and I did include my calculations in my post.
I was reading in another thread entitled "transformer sizing" where various members were discussing whether a transformer should be ran "loaded and hot" or at a lower percentage.
I thought this forum was a place where I could get some positve feedback,for a simple question.
 
Sizing

Sizing

The transformer must be protected based on NEC 450.3. That means you are limited to 125% of the transformer nameplate for the OCPD. If you opt for a 45kVA transformer to match the load, you could use a 70A 480V OCPD. However, will your 100A panel accept a 70A branch or subfeed breaker?

Also, the 45kVA transformer will have a 2-5% impedance. This will lead to voltage drop. With the known load and conductor sizes and distances, can you live with this? You may need to use a larger transformer to keep the voltage drop acceptable.
 
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