# 225 KVA Transformer



## Zog (Apr 15, 2009)

patrickw1961 said:


> I'm a trainee and was curious what the available amperage a local power company's 225 kva transformer at 480 volts 3 phase is,and also how you figure the answer.
> Also what would be available at 208 volt 3 phase using a customer owned transformer ?


You asking about full load current or available fault current?


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## bobelectric (Feb 24, 2007)

It's all in the book!


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## micromind (Aug 11, 2007)

Here's how to figure current on 3 phase transformers, it doesn't matter if it's oil-filled or dry. Inside or outside is the same as well.

If you know the phase to neutral voltage, multiply that figure by 3. This is the VA per amp. 

If you know only phase to phase voltage, multiply that by 0.577 This will give you phase to neutral voltage.

For the purpose of calculating amperage, it doesn't matter if the transformer is connected wye or delta. 

In your example, the 225KVA unit operating at 480 would be as follows;

480 X 0.577 = 277. 277 X 3 = 831. 225,000 ÷ 831 = 271.

Without getting too technical, this is the maximum continuous current the transformer can handle. 

To figure the fault current (the maximum current the transformer will produce during a dead short; assuming full voltage at the primary), simply divide the full load current by the impedance (expressed as a decimal).

Using the above transformer as an example, if the impedance is 6%, it'd be 271 ÷ 0.06 = 4517. The actual short-circuit current will always be less than this because there will always be some (and maybe a lot) of voltage drop on the primary during a fault. 

Rob


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## Bob Badger (Apr 19, 2009)

Patrick, you should keep in mind that there are often many ways to figure it out. 

I don't know if any one method is best, but we do have to make sure we don't combine a couple of methods.

For example the way micromind does it below works fine but I had never seen it done that way.



micromind said:


> In your example, the 225KVA unit operating at 480 would be as follows;
> 
> 480 X 0.577 = 277. 277 X 3 = 831. 225,000 ÷ 831 = 271.


I do it like this.

225*1000/480/1.73=271

225 KVA x 1000 results in the watts

Watts dived by volts and by 1.73 for 3 phase results in 271 amps.

I bet there are other ways as well.


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## Jlarson (Jun 28, 2009)

Bob Badger said:


> I do it like this.
> 225*1000/480/1.73=271
> 225 KVA x 100 results in the watts
> Watts dived by volts and by 1.73 for 3 phase results in 271 amps.
> I bet there are other ways as well.


You can do math but can't keep track of your service van? :laughing:

I figure it the same way Bob does.


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## micromind (Aug 11, 2007)

I guess I'm a bit different than most because I wasn't taught how, I had to figure it out myself. 

But, as long as you get the right answer, it shouldn't matter how.

Rob


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## Jlarson (Jun 28, 2009)

micromind said:


> I guess I'm a bit different than most because I wasn't taught how, I had to figure it out myself.


Then we are both a little different cause I never was taught that stuff either.


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## brian john (Mar 11, 2007)

I always try to teach Bob's way, but also show the other method as this seems easy for those that seldom do the math, to learn.


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## frenchelectrican (Mar 15, 2007)

The way Bob describing on how to figure out the Kva rating on transfomer that one of most common way and Mircomind is the other way { it will work with other voltage and HZ as well }

I do use both way but Bob's verison is little faster if you know the quick answer.

Merci.
Marc


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