480v step down xfmr

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joshfromoshoto

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Wyoming
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Electrician
I have a question regarding a step down transformer. At a power plant I do contracting work for there'd a 480v-207v transformer fed from a 200a disconnect. The secondary side of the transformer currently feeds two 208 panels, and they want to add a third. How do I calculate to see if the transformer would be rated to feed another panel? I apologize for my ignorance. I'm a first year journeyman and I've been trying to find anything I can online.

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As Dave stated you'll first need to know the amount of existing load on the transformer and then you'll need to know the kva rating of the transformer.
 
Just to expand on the existing loads part of what the other guys said:. You either have to do a calculation, or you can put a logger on it for 30 days and measure it that way. NEC calcs are very conservative so you typically find you can load stuff up a lot more if you measure the actual power with a logger.
 
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At a power plant, the panel loads should be very consistent. Remember to take a look at not only connected load, but actual load profile for running load.
Real simple, if you have 2 x 200A panelboards, and you are adding a third, and the transformer is a 200KVA or larger, you really don't have to do much figuring. Not likely though, but would be nice.
 
Depending on the actual details it's possible that won't be necessary..........................................What size is the transformer (kva) and what size mains are in the existing and proposed panel (mains) ?
 
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