transformer sizing

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Coastalguy1

Coastalguy1

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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?:?
 
Coastalguy1 said:
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?:?
Click to expand...
1989 ? 480 ? 1.732 = 1,653,575VA or 1.66MVA as a basic number to start with.
 
Coastalguy1 said:
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?:?
Click to expand...

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
 
to Iceworm

to Iceworm

This is a new project with the calculated 1989amps @ 480v which includes a 25% future factor.
Knowing that 1989A x 480v x 1.723/1000= 1653.58kw. I want to know what the calculation is to derive at the size primary pad mount transformer I would need. This is a commerical project and not an industrial project. In short, I want to know how to work the calculation backwards to get my primary transformer size. Example- 1989a x 480v x 1.732= 1,653,575w/12,500v=??
 
Coastalguy1 said:
This is a new project with the calculated 1989amps @ 480v which includes a 25% future factor.
Knowing that 1989A x 480v x 1.723/1000= 1653.58kw. I want to know what the calculation is to derive at the size primary pad mount transformer I would need. This is a commerical project and not an industrial project. In short, I want to know how to work the calculation backwards to get my primary transformer size. Example- 1989a x 480v x 1.732= 1,653,575w/12,500v=??
Click to expand...
Transformers are sized by their rated kVA/MVA output. You already have the kVA required by the calculated load - 1.66MVA minimum.
 
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