step up transformer

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jph320

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I need to turn 3phase 208 into 480. I wired many 480-208 transformer but never the opposite. Is there any difference in calculation..etc.. besides landing wires on opposite terminals. And suggestions on OCP for primary and secondary. need it to feed a 30amp disco. Thanks Jim
 
Is the transformer rated for step up? Or are you back feeding a step down? Backfeeding a trans not rated for it will change your efficiency, and therefore your calculation. It might also cause unforeseen problems depending on what the load is.


If it's designed for step down, the calculation will be by the book.

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When normally used, your transformer primary is a delta and your secondary is a wye.

If you get a proper 208 delta to 480/277 wye step up transformer, then the calculations and connection will be what you are used to; just use the correct numbers.

If you are trying to reverse a step down transformer, then as WarrMann says you are opening a whole 'nother kettle of worms.

30A at 480V is 25kVA.

-Jon
 
WarrMann said:
Is the transformer rated for step up? Or are you back feeding a step down? Backfeeding a trans not rated for it will change your efficiency,
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Not a lot. But the turns ratio is not usually the same as the voltage ratio. Backfeeding it will result that lower voltage out.
 
Also, if the now-primary has a neutral terminal, let it float; don't connect it to the source neutral.
 
jph320 said:
I need to turn 3phase 208 into 480. I wired many 480-208 transformer but never the opposite. Is there any difference in calculation..etc.. besides landing wires on opposite terminals. And suggestions on OCP for primary and secondary. need it to feed a 30amp disco. Thanks Jim
Click to expand...

This is a question that comes up a lot here. I have weighed in on many of these discussions to the point of some probably sick of hearing my comments. So I won't bother with a lot of details of why but just say that using a garden variety delta/Y step down transformer, even if the manufacturer marks it as allowed, in reverse is a bad idea on many levels.
Do as winnie said and just get a 208 delta to 480Y transformer save a lot of heartache and hassle. They are readily available.
 
I asked Schneider Electric about this a few years ago. Their response was " Backfeeding causes very high excitation inrush making coordination to breakers or fuses difficult without exceeding the limits set by the NEC. Avoid backfeeding whenever possible for this reason. "
 
Ill take a stab at providing nice summary of possible negatives:

1. The transformer needs to be marked as suitable for backfeeding which it may not be (relatively new code).
2. there is no center point to ground so you either have to run it ungrounded or corner grounded. This can cause problems with certain equipment, such as VFD's that are expecting 277 to ground. Also Equipment such as panelboards and breakers need to be suitable for a corner grounded systems which often results in increased costs and lead times. Ungrounded and corner grounded systems are strange to many people so personal may not be accustomed to them.
3. Bad things can happen if the wye point is grounded. I had a conductor fail and then a hole blasted through a 2" EMT once that was serving such an arrangement.
4. Inrush will likely be higher (in my experience, this hasnt been a problem, but perhaps with the new energy efficient transformers it is more pronounced).
5. Maybe you loose efficiency? Here is a thread on the topic: https://forums.mikeholt.com/showthread.php?t=181296
 
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