Transformer reverse connected...

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chevyx92

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Never had to reverse connect a transformer so I was wondering if there are anythings in particular I need to know. I have a 3 phase delta- wye transformer 480 to 208/120volt. I need to reverse connect it for a 480v 25 hp motor. Do I need to run a neutral in my feed to the transformer as I won't have any line to neutral loads?
 
Feed it delta, leave the XO floating.

You should consider corner grounding the secondaries.

Go as large as possible with your primary overcurrent device as in rush can be a problem.
 
iwire said:
Feed it delta, leave the XO floating.

You should consider corner grounding the secondaries.

Go as large as possible with your primary overcurrent device as in rush can be a problem.
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Exactly how do I need to do this? Just one corner of it or what???
 
chevyx92 said:
Exactly how do I need to do this? Just one corner of it or what???
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Pick a secondary phase and bond it as we normally do with XO, that will now be treated as a grounded conductor.

Mark it white, don't put a fuse on it.

Hopefully someone else will jump in as I have never had to do this and I do not have my code book with me.
 
iwire said:
Pick a secondary phase and bond it as we normally do with XO, that will now be treated as a grounded conductor.

Mark it white, don't put a fuse on it.

Hopefully someone else will jump in as I have never had to do this and I do not have my code book with me.
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Ok so let me get this straight. I will run 3 phase 208v, no neutral to feed the WYE side of transformer. Then on the DELTA side going out I will have 3 phase 480v to my disconnect. And your saying to ground one phase of the delta with my EGC and building steel ground that I will have in there and also bond to frame, plus tape the corner ground white. Am I correct?
 
chevyx92 said:
Ok so let me get this straight. I will run 3 phase 208v, no neutral to feed the WYE side of transformer.
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The wye side or call it the 'X' side, in your application the primary side.

Then on the DELTA side going out I will have 3 phase 480v to my disconnect.
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Yes the delta side or 'H' side in your application the secondary side.


And your saying to ground one phase of the delta with my EGC and building steel ground that I will have in there and also bond to frame, plus tape the corner ground white. Am I correct?
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As messed up as that sounds....yes.

If you do not bond one of the 480 phases to ground you will have an ungrounded system. This can be a good thing sometimes but will require ground fault indicating system.

Please wait for some others to back me up as I have not had to personally do this before.
 
That sounds bizarre. Then again, I've never seen or encountered a corner grounded delta in real life.

What is the voltage from phase to ground on a CG delta? 480?
 
iwire said:
The wye side or call it the 'X' side, in your application the primary side.



Yes the delta side or 'H' side in your application the secondary side.




As messed up as that sounds....yes.

If you do not bond one of the 480 phases to ground you will have an ungrounded system. This can be a good thing sometimes but will require ground fault indicating system.

Please wait for some others to back me up as I have not had to personally do this before.
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Ok and even though you do ground a corner of it I will still have a 3 phase 480 output to my disconnect correct?
 
peter d said:
What is the voltage from phase to ground on a CG delta? 480?
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Could be 480 or 230 or etc.

But it will be 0 volts to ground on one phase and the line to line voltage to ground on the others.

A to ground 0

B to ground 480

C to Ground 480 assuming A is the grounded phase.
 
chevyx92 said:
Ok and even though you do ground a corner of it I will still have a 3 phase 480 output to my disconnect correct?
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That is correct.

What is even odder is some singe phase panel boards can be used with corner grounded delta as the grounded phase can use the neutral bar and the breakers only have to be two pole.

You motor starter will still require an overload unit on each phase.
 
iwire said:
That is correct.

What is even odder is some singe phase panel boards can be used with corner grounded delta as the grounded phase can use the neutral bar and the breakers only have to be two pole.

You motor starter will still require an overload unit on each phase.
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I understand the overloads are required. You also said not to put a fuse on the grounded phase. I am trying to understand this correctly here so lets say from the "H" side of the trasformer to my disconnecting means I will run 3 hots and a ground correct? Now your saying not to fuse the one thats grounded?
 
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chevyx92 said:
I understand the overloads are required. You also said not to put a fuse on the grounded phase. I am trying to understand this correctly here so lets say from the "H" side of the trasformer to my disconnecting means I will run 3 hots and a ground correct? Now your saying not to fuse the one thats grounded?
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First, the correct phase to connect to ground is B (unless the facility already uses a different phase).

For all intents, treat the grounded phase conductor the same as you would any neutral.

Particularly, beware of using circuit breakers or distribution panels on grounded-phase systems. UL has specific listings for any equipment (including fusible switches) used on these systems.
 
iwire said:
Feed it delta, leave the XO floating.
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I've never understood this. It seems like connecting X-0 to the system neutral would be required to keep the Delta side voltages equal. Without the connection, wouldn't the voltages vary with load as X-0 swings away from 0 volts?
 
chevyx92 said:
I understand the overloads are required. You also said not to put a fuse on the grounded phase.
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I figured you knew you need motor OLs what I was trying to point out that while you will not put a fuse on this grounded phase at a fused disconnect you still have to place a motor OL on this grounded phase.

I am trying to understand this correctly here so lets say from the "H" side of the transformer to my disconnecting means I will run 3 hots and a ground correct? Now your saying not to fuse the one thats grounded?
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Thats correct, just like you would not place a fuse on the grounded neutral of a Wye system.

If this grounded phase faults to 'ground' really nothing happens.

If the grounded phase faults to either or both ungrounded phases the fuses in those conductors will protect both the grounded and ungrounded conductors.
 
LarryFine said:
I've never understood this. It seems like connecting X-0 to the system neutral would be required to keep the Delta side voltages equal. Without the connection, wouldn't the voltages vary with load as X-0 swings away from 0 volts?
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Larry this is really beyond my depth here.

I do know XO will not remain at O volts to ground, I also know that will not mater. That XO has absolutely no electrical connection to what ends up being the secondary side in this back fed installation. In other words this secondary side will not be effected by changes in XOs potential to ground.

I believe there is a mode of failure that if the XO is grounded can overload that conductor on XO. Seen that before on a service call.

If you go to Square D's FAQs they also say to leave XO floating on reverse fed transformers.

I believe if you lose one of the supply phases that the transformer will try to continue operating using XO to ground.
 
iwire said:
I figured you knew you need motor OLs what I was trying to point out that while you will not put a fuse on this grounded phase at a fused disconnect you still have to place a motor OL on this grounded phase.



Thats correct, just like you would not place a fuse on the grounded neutral of a Wye system.

If this grounded phase faults to 'ground' really nothing happens.

If the grounded phase faults to either or both ungrounded phases the fuses in those conductors will protect both the grounded and ungrounded conductors.
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So then I theoretically I would need a single phase disconnect because only 2 of the phases would have fused protection and the 3rd phase (grounded phase) with white tape would pass through my disconnect? Correct???
 
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So then I theoretically I would need a single phase disconnect because only 2 of the phases would have fused protection and the 3rd phase (grounded phase) with white tape would pass through my disconnect? Correct???
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Maybe...it becomes a listing issue...some single phase equipment is marked for this use and other equipment is not.
Don
 
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