480V transformer

I’m giving an estimate on some 3phase work. There in coming service is 3ph 4w 240V they had a massive bead blast booth installed which they have not run yet. Some one before me installed a 45kva transformer to make the 480V everything looked fine it was bonded had the right size feeders no problem. On my way out I wanted to check the 480V at the blaster disconnect. It’s reading like a ungrounded corner tap delta system
line one 480V to grd line two 480V to grd line 3 0V to ground 480V between all legs is that transformer not wired correctly or does this have something to do with the incoming power. Never came across this before. I want to make sure they did this right before this 50K blaster gets fired up. Transformer reads 208/120 sec 480V primary
 

augie47

Moderator
Staff member
Location
Tennessee
As far as the 480v, it appears to be corner grounded, either accicently or by design. Possibly no problem unless the unit has drives,
(If it is an "accidental" ground, it needs to be corrected; if by design, it needs to be wired accordingly)
If there are any varible speed drives you need tom discuss the install with the manufacturer.
It also sounds as if it was "back-wired".. works that way but is actually a Code violation unless designed for that by the manufacturer
 

hillbilly1

Senior Member
Location
Atlanta,Ga
As far as the 480v, it appears to be corner grounded, either accicently or by design. Possibly no problem unless the unit has drives,
(If it is an "accidental" ground, it needs to be corrected; if by design, it needs to be wired accordingly)
If there are any varible speed drives you need tom discuss the install with the manufacturer.
It also sounds as if it was "back-wired".. works that way but is actually a Code violation unless designed for that by the manufacturer
Since the op said the incoming was 240, I would venture a guess it is the correct transformer, a 208 wye/480 delta back fed will put out around 560 volts.
 

jim dungar

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Location
Wisconsin
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Engineer
It is no longer uncommon to find 480V corner-grounded (usually the B phase) delta transformers because ungrounded delta units would require a ground detection/indication system.
 

infinity

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Staff member
Location
New Jersey
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Journeyman Electrician
How do you get 480 volts out of a 208Y/120 volt-480 volt transformer on a 240 volt system?
 

hillbilly1

Senior Member
Location
Atlanta,Ga
How do you get 480 volts out of a 208Y/120 volt-480 volt transformer on a 240 volt system?
They probably have the correct transformer wound for that voltage, probably a delta/delta. If I was to order one, I would have went with a delta/wye. I back fed a 208Y/480D 75 Kva for a machine shop that had Canadian equipment. They required 560, and with the 240 delta service, it put it almost right on the money. That was about 20 years ago, and they are still using it!
 

MD Automation

Member
Location
Maryland
Occupation
Engineer
Is it possible that the incoming service is 208/120 and not 240? Either that, or as Hillbilly1 notes, maybe they adjusted taps on the Wye side?

It sounds like this is a step-down Delta - Wye and is being reverse fed. If so, take care to ensure that the XO terminal is floating on the Primary Wye side. Op indicated it was "bonded", just wanted to make sure that the typical XO bond is removed for this back fed application.

Lastly, sounds like a corner grounded Delta secondary. As Augie47 points out, make sure that's OK with the manufacturer of the cabinet. If there are any VFDs there, they will either insist on a Wye supply, or have you remove some jumper inside the drive to prevent the failure of MOVs that protect the Mains input.
 
I have a picture of the nameplate on the transformer it’s a Sq D brand new 45KVA I did not read 560V anywhere when checking voltage. The incoming was closer to 220 than 240 but it def was not 208 Here’s the problem they want to run a large dipping furnace that requires 480V. off the same transformer or swap it out for a 75kva that they have sitting on a shelf. It also has 208/120 primary 480V secondary. Why would a brand new bead blaster require corner grounded delta by design. 480V Disconnect feeds a cabinet with a few contractors in it as well as a small 1kva transformer making 120V for some idiot lights on the same panel. Could this all be caused by reverse wiring the transformer and or using the wrong taps. Sounds dangerous to me. I did not open the transformer but did notice the bond wire was inside the transformer not just a lug on the outside it runs up the wall to the building steel. I have a picture of the nameplates of both transformers but can’t seem to up load them.
 
Did a lot of reading on this last night and the code won’t allow for anything else on this kind of grounded phase system. It also states that the grounded phase not be fused and it clearly is. I’m going to suggest a new 75 KVA transformer with a 240V primary and 480V secondary and avoid this whole mess. It’s very doubtful this was done by design and more likely they had this transformer on there shelf and repurposed it for this application. I’m not doing it again just to reuse there own 75kva transformer sitter on there shelf. They want a load center installed for steel dipping furnace and future equipment. I’m not installing a confusing corner grounded system to save 3K on the proper trans. That system will require special breakers or two pole breakers and just seems stupid and a troubleshooting nightmare for the next guy. From what I read the only way to make the transformer they had work was to reverse feed and corner ground which IMO is stupid just get the proper transformer. I will find out why it was done this way and why is the grounded phase fused. Am I thinking clearly is there any possible reason brand new equipment would need to be wired with a grounded phase ?
 

MD Automation

Member
Location
Maryland
Occupation
Engineer
Your problem, as you are finding out, is that the secondary of your existing transformer is Delta wound. Likely because they are reverse feeding a StepDown unit.

When you purchase a new transformer make sure it is a pupose wound StepUp transformer, with a Delta primary and a Wye secondary. Grounding that Wye secondary (using the HO) is easy. It might take a few days more to order the right transformer, because they are not as common as the step down units. But they are available and they are exactly what you want.
 

jim dungar

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Location
Wisconsin
Occupation
Engineer
It also states that the grounded phase not be fused and it clearly is.
Also, the phase wire connected to ground is now a Grounded Conductor, as you noted it cannot be fused, and so it needs to be colored white or grey.

This is a situation that shows all neutrals are grounded conductors but not all grounded conductors are neutrals.
 
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kwired

Electron manager
Location
NE Nebraska
They probably have the correct transformer wound for that voltage, probably a delta/delta. If I was to order one, I would have went with a delta/wye. I back fed a 208Y/480D 75 Kva for a machine shop that had Canadian equipment. They required 560, and with the 240 delta service, it put it almost right on the money. That was about 20 years ago, and they are still using it!
Why would you rewind this and/or why would they mark it 120/208 if it is delta winding? About has to be a typical delta-wye that was reverse fed, but at same time you will have high output on 480 side if you input 240 into the 120/208 rated side.

Some details are not adding up.

One thing that seems certain is the 480 output has grounded phase conductor.
 

Jraef

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San Francisco Bay Area, CA, USA
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Electrical Engineer
That system will require special breakers or two pole breakers and just seems stupid and a troubleshooting nightmare for the next guy
Just a side note on one thing you said here: you do not need "special" or 2 pole breakers just because one phase is grounded. You cannot FUSE the grounded conductor, but you CAN use a 2 or 3 pole breaker on any phase, because the breaker will open all poles simultaneously, grounded or not. The only thing "special" about the breakers would be that they are not "slash" rated, meaning they are not rated as "480/277 only", which at 480V is not that difficult to find.

There may be other valid reasons to not use a corner grounded delta anyway, so your plan to get a more standard arrangement has merit. I just wanted to point out that one issue about breakers if it ever comes up again.
 

jim dungar

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Staff member
Location
Wisconsin
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Engineer
... not need "special" or 2 pole breakers just because one phase is grounded.
According to some manufacturers you might, it depends on their UL Listing.
I believe the current Square D PowerPact is listed as 2 pole only for 240V and 3 pole only for 480V corner grounded systems. Some of their obsolete breakers needed special construction (e.g. 2 poles built using a 3-pole frame).
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Why would you rewind this and/or why would they mark it 120/208 if it is delta winding? About has to be a typical delta-wye that was reverse fed, but at same time you will have high output on 480 side if you input 240 into the 120/208 rated side.

Some details are not adding up.

One thing that seems certain is the 480 output has grounded phase conductor.
Here is what they probably done, on the 480 side of a 120/208 step down transformer, you have input taps to lower or raise the output. Since they are using this transformer as a step up, moving the coil taps to 7, will lower the output voltage in relation to the input voltage. If the taps were left at 3, (480), the output would be around 560, because the input is much higher than 208.

 
Just a side note on one thing you said here: you do not need "special" or 2 pole breakers just because one phase is grounded. You cannot FUSE the grounded conductor, but you CAN use a 2 or 3 pole breaker on any phase, because the breaker will open all poles simultaneously, grounded or not. The only thing "special" about the breakers would be that they are not "slash" rated, meaning they are not rated as "480/277 only", which at 480V is not that difficult to find.

There may be other valid reasons to not use a corner grounded delta anyway, so your plan to get a more standard arrangement has merit. I just wanted to point out that one issue about breakers if it ever comes up again.
I appreciate that and yes I read that as well about no slash breakers. But I also could not feed another machine if I wired there 75 KVA transformer which is the same thing just bigger. That was my original plan but doesn’t look like it will work because of the 240V. So I’m looking for a 75 KVA delta 240V pri and wye sec 480V step up trans. Or can I still reverse feed as long as it’s 240V which may be more common and easier to find.
 
The problem I will have next is what to do with this new transformer wired this way. I’m sure they don’t want to simply eat a recently purchased 45Kva transformer. Maybe I will suggest they leave it as is and get another 45kva with the proper 240/480 and have a nice day. I really don’t want to use there 75K reverse feed and corner ground if it’s even possible on this much older German transformer. Chances are ( and I haven’t looked inside )that the extra taps won’t be there. The previous sparky should fix the code violation with the fuse and marking. Or I will Still seems odd to me to ground a phase and still have 3 phase but that’s school for another day. Real quick am I correct in my thinking that there shelved 75kva transformer with the 208/120 sec 480V pri the only way it will work is to reverse feed corner ground. But can only be a separately derived system I could not install a additional 480V sub panel for more equipment with that setup. Just making sure I understand everything before taking it to my client tomorrow. Thank you all for all your help I’m learning every day and this corner ground transformer I’ve never come across before. So thanks again big huge help that would otherwise take a mountain of research to figure out. Don’t have that kind of time.
 
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