I am trying to properly size a medium voltage tranformer for a project. My primary voltage is 12.5kv. My secondary voltage is 480v three phase. My load (amps) @ 480v is 1989amps. Can someone help me with the calculation to properly size my MV tranformer?:?
I'm assuming this is this a design project as opposed to a replacement. With that in mind, the 1989A is an NEC calculated number. Second assumption is this is industrial as opposed to commercial.
A 2MVA xfm would 85% loaded. That's heavily loaded for industrial. It would leave me nervous - especially when it hit -40F.
A 2.5MVA would be 68%. No bad if you have a really good handle on the loading and you are pretty sure there isn't going to be another 30% added over the next 10 years. The crystal ball part is difficult for me - even after 40 years in the business.
So looking at a 2.5 MVA, that is ~3000 fla x1.25 = 3750A. Paralleled 500 will require 10 per phase. Short circuit current will be ~45KA to 60KA. However, since the primary is 12,470V there is likely another transformer upstream tied to a 69KV or 138kV utility which will reduce the SSC - unless you are self generated - then that all changes
This is a non-trivial design project. And what I gave you is just a bunch of guesses. Hope it helps.
ice