Transformer Secondary

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GFH

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I have an installation that appears by code to be okay but does not appear right.

45KVA XFMR 3ph, 480V -> 120/208V

Primary protection is 70amp FRS-R Fuses.

3/0 secondary conductors (less than 10') to a 200amp main breaker panel.

Somehow this installation feels wrong but I can't figure out why.
 
Unless I am missing something about the math then I come up with this:

Primary: 45000/831 = 54 Amps primary current looking at table 450.3(B)
54 x 2.50 = 135 Amps so the 70 amp fuse looks good, a little low IMO. But when I look at the secondary I get:
45000/ 360 = 125A x 1.25 = 156A. So IMO the secondary is to large according to note 1 (240.6 = 175A). I am only an apprentice so lets see what others say about my math, I'm sure I'm missing something :lol:

Kevin
 
A 45KVA transformer is properly sized for a 125A 120/208V panel. You have a 200A panel and panel feeders. This is not necessarily a problem if the connected load does not exceed the transformers capacity and the transformer is protected according to the requirements of article 450.
 
Kevin
You type faster than me. :D To supplement your excellent math, I will also say that since the primary calculates to 54 amps, if you utilize the upper half of the table (primary protection only) and multiply by 125% you get 67.5 amps and note 1 allows you to go to the next higher standard of 70A. So the secondary can be considered protectected by the primary in this case. Then only the conductors require protection. So since they are 3/0 and feed a 200A MCB it should be okay. I would check the actual load just to be sure the transformer will not be overloaded by this oversized panel. The weak link here is the Primary OCP (we hope anyway).
 
New Info

New Info

Thanks guys there is a new twist however -

The load on the transformer (as calculated by the engineer):

48.2 KW Connected & 44.5 KW Demand

I performed my own load calculation due to the demand factors not being shown and the loads not being accurate for many of the items shown & came up with the following loads:

46.2 KW connected & 44.6 KW Demand

The transformer will be installed in Colorado and due to the high altitude it should be derated approximately 2% - I do not believe that this installation will work - at least not for very long.

I have two choices install the 45KVA and hope it lasts & when it doesn't replace it with a 75KVA making additional $$$ or install a 75KVA transformer now for minimal $ and informing the engineer that he is high (in many more words).
 
GFH,

Like I have said before I am just an apprentice and every day I am amazed at the knowledge that exist on this site so if I am out of line I am sure I will be corrected. IMO if you install the 45KVA transformer as described in your earlier post it will be a mistake. Please understand I have no ideal who you are doing this work for or who the engineer was who designed the setup, and I am NOT saying the engineer was wrong. I am going to say your customer won't care who designed it either. All they will see is you were the electrician who installed it so if it fails to soon down the line (in the customer's mind) you might get the blame. What is your reputation worth? I would inform the customer of what you have found and let them decide what size they want to pay for. Even try and talk to the engineer and make sure the 45KVA wasn't a type-O, maybe he meant for it to be a 75KVA transformer. The math works out better that way. Again I am not trying to tell you how to run your show nor am I bad mouthing your engineer I am just saying CYA. I hope I wasn't out of line. This is just an opinion of this apprentice.
 
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