Sizing a Transformer and Determining Wire Size

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horrorsix

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I'm looking to install a 400A 120/208v 3 phase sub panel in a commercial facility. 480V primary, I have two questions. The electrical room is 245' from the sub panel location. Should the transformer be at the electrical room or closer to the sub panel? My second question is obviously wire and transformer size. I was looking at installing a 112.5Kva.


Existing equipment to be hooked up is
2 machines rated at 78A 3 phase each= 156A
2 machines rated at 70A 3 phase each= 140A
total of 296A

400A panel gives some room for expansion. Does this not look right to anyone?

Thanks
Shawn
 
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What size wire would you use to feed a 400 amp load? What size for a 200? What is the cost difference for the installation of these? Now where do you think the transformer should be located?

Not knowing the details of your install, you may want to revisit your load calculations for the transformer size.

You added some details.
 
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Sizing a Transformer and Determining Wire Size

Wire size and transformer size is really not an issue to figure out. I do that everyday. I think the transformer should be near the electrical room but am I wrong to think that installation will cost more since my secondary wire size will normally be larger then the primary. Now due to voltage drop it will be even bigger. I would think if the transformer is near the pub-panel cost on wire will be less.
 
As noted in Post 2 & 3, details make a difference, but generally speaking, a step-down transformer will be located close to the secondary location since the primary conductors are much smaller and less expensive. (would you rather run 245' of 3/o or 245' of 600 kcmil ? )
Your transformer seems to be sized correctly.
Next step is to look at 450.3(b) and determine how you wish to address transformer protection.
Once you decide on the size of your transformer overcurrent protection, select the conductor accordingly.
Then check 240.21(C) for your secondary conductor rules, 408.36 for rules on your panel rules, and 250.30 for grounding requirements.
There are some pitfalls. You might want to follow the process above and tell us what over-currrent/wire size/GEC size you have selected and someone here will be glad to critique it.
 
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