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need instructions to calculate what kva transformer i need for new power center

1455 Views 2 Replies 3 Participants Last post by  woodchuck2
Hey guys. New to the forum and need some help. I am an underground electrician in the coal industry and i need to purchase a transformer to go in a power center underground. It has to be three phase dry type with incoming voltage of 12,470 vac. It needs secondary voltages of 1000 vac and 480 vac to operate various pieces of mining equipment. It needs four 1000 vac breakers with 800amp rating each, it needs fourteen 480 vac breakers with 225 amp rating each and two 480 vac breakers with 400 amp rating each. My question is can anyone here tell me how to calculate the kva rating for the transformer i need to run all these breakers? You should also note that this equipment will pull more amperage at startup due to large horsepower motors starting. I know this is a factor to be aware of and can get the specific hp rating of each motor if needed for the equation. Any help you can give is appreciated. Thanks. Also keep in mind that this power center does not fall under the NEC. It falls under mining law because it is considered "face equipment". If that makes any difference.
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Since nobody has answered, a few things to think about. Yep mining transformers with multi tap voltages are available. Autotransformers are often used. So says my pal Napo who worked in New Mexico mining ops. With a 12 KV primary makes it a little more difficult. Just a quick calc says you will need one big arse transformer. Just the 480 is more than 3000 kva.
Do it yourself KVA= volts X amperes X 1.732 /1000
Way out of my league on this one but i am sure you will need a load calc to even begin with. Oversizing sure wont hurt either. Best place to ask this question is at this sister site: . Bit of a tough crowd over there but you wont find more knowledgeable folks for this field.
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