75 KVA transformer wire size

[bgggg]

Hi All,

I am installing a thermoforming machine in my shop which runs on 460V 3phase with a load rating of 40kw. My building has 208V service, so I intend to use a 75KVA 480V - 208Y transformer run in reverse (I know this is not the best thing to do but I will take that risk). According to my limited experience, 40000w/460v = 87 Amps is what I need on the secondary side. In order to know the primary current, if I understand correctly, due to the inrush current, it is based on the capacity of the transformer not the actual draw, ie: 75 KVA/(208*1.732) = 208A. Basically, what I would like to know is what size wire and breaker should I use on the primary 208V side? There will be about 10' between the main panel and the transformer, and about 50' between the transformer and the thermoformer. I suspect 75 KVA might be overkill but my only other option is 45 kVA which would be inadequate.


As a corollary to this question, since 208A 3ph is distributed over 3 wires, why can one not size the wire for 70A each? Why must each wire be capable of carrying the entire current?

thanks

 

[augie47]

With all due respect to your vocation, under Forum guidelines this becomes a DIY question and thus not acceptable.
The many variables and particulars involved in any transformer installation require
considerable Code knowledge and this is a case where providing a little information could be worse than providing none.

[FONT=&quot]I am closing this thread, in accordance with the Forum rules. This Forum is intended to assist professional electricians, inspectors, engineers, and other members of the electrical industry in the performance of their job-related tasks. However, if you are not an electrician or an electrical contractor, then we are not permitted to help you perform your own electrical installation work.


If I have misjudged the situation, if for example this project is related to your work, then send me a Private Message. If you can show me that I am wrong, and that you are a licensed electrician (or at least a licensed apprentice), then I will reopen your post, and offer an apology for the delay and inconvenience.

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