Transformer calc

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ksavoie

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When calculating the primary side of a 480V 3ph - 240/120V 1ph, 15kva transformer do I use the 3 phase calc or do I use the single phase calc? 31.25A or 18A? It is 3 phase on the primary and single phase on the secondary. Just a little confused on which formula I should use.

Thanks
 
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Are you sure it's not single phase primary? I'm not sure why you would have a three phase primary with a single phase secondary. If its 480V 1ph primary, it would be 31.25A. If it is indeed a 3ph primary, then yes, it would be 15kva/(480V)*(1.73), which as you said, is 18A. But again...I'm a bit confused as to why you would have a 3 phase primary and a single phase secondary. Maybe someone else can shed some light on that.
 
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This may be one of those "educational" times for me, but, I'm not aware of a transformer that has a three phase primary and ONLY a single phase secondary. You may put one or more single phase loads on a three phase transformer, but that doesn't mean the secondary is single phase.
I would think you either have a single phase 480/240-120 transformer or a three phase 480/208Y120, or 480/240-120 hi-leg delta transformer.
In you do have the 480/240-120 delta transformer, be careful at they are very limited as to neutral loading.
This thread may be of interest to you:

http://forums.mikeholt.com/showthread.php?t=120364&highlight=transformer
 
OK my fault! Because I was coming from a 3 phase source I was thinking 3 phase all the way. Feel kinda dumb now! Thanks anyway.
 
happens to all of us. I thoght you might have introduced me to a new product.
In your case it's I=P/E for the single phase current.
 
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