delta or wye?

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I need to specify a 5kva transformer to get from 480 3 phase to 240v 3 phase. I have been told we will be building 30 of these systems to be installed at various locations around the country. we seed isolation, so a auto-transformer has been ruled out. Would I then want to use a delta-delta transformer?

thank you
Bob
 

GoldDigger

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If you want to minimize the line to ground voltage, you would USD a wye secondary even though you would have no line to neutral loads.
If you use a delta secondary you would have to justify ungrounded and install a ground detector or else corner ground the delta.
If the transformer and the secondary output are all within the equipment with no field wiring on the secondary side the NRC may apply and you will have to look at UL standards for guidance.
 

Jraef

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And...

You do not get 240V on a Wye secondary from a 480V primary with a standard transformer, it's going to be 208Y120V. Often that's OK, occasionally not. You would need to know that first. If not, and you have some very specific need for a custom wound transformer with a 240Y139V secondary, be prepared to pay through the nose for them.

If you describe the reason for the 240V 3 phase requirement, we may be able to help more, otherwise we are guessing.
 
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Jraef

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Electrical Engineer
If you want to minimize the line to ground voltage, you would USE a wye secondary even though you would have no line to neutral loads.
If you use a delta secondary you would have to justify ungrounded and install a ground detector or else corner ground the delta.
If the transformer and the secondary output are all within the equipment with no field wiring on the secondary side the NEC may not apply and you will have to look at UL standards for guidance.

FIFY... I'll bet you are using an iPad.:p
 
And...

You do not get 240V on a Wye secondary from a 480V primary with a standard transformer, it's going to be 208Y120V. Often that's OK, occasionally not. You would need to know that first. If not, and you have some very specific need for a custom wound transformer with a 240Y139V secondary, be prepared to pay through the nose for them.

If you describe the reason for the 240V 3 phase requirement, we may be able to help more, otherwise we are guessing.



I will be running a chiller and some power supplies. I can live with 208v and the 120v l-n may come in handy.

thank you
Bob
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
I will be running a chiller and some power supplies. I can live with 208v and the 120v l-n may come in handy.

thank you
Bob
Sounds like you have your answer, then. :)
There may be a code requirement for a 120V service receptacle within some distance of the equipment (but not on the same disconnect), so that would be a very persuasive argument.
 
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