480V transformer

ActionDave

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Durango, CO, 10 h 20 min without traffic from wing
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wire pulling grunt
.... 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 agree that a delta/wye step up transformer is far better but a corner ground system is not as bad as all that, it's just something most electricians never see. I've only worked on one in my life. It's something new to get your head around, but once you understand that any conductor can be a grounded conductor things make more sense.
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 ?
Safety. There are issues with ungrounded systems having to do with voltage transients and other wacky things. The main reason for a grounded system is to make it possible to rapidly clear a fault to ground.
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.
You can have only line to line loads with a corner ground so that would limit the use of the new panel.
 

jim dungar

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Location
Wisconsin
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Retired Engineer
... can only be a separately derived system I could not install a additional 480V sub panel for more equipment with that setup.
Separately derived does not mean dedicated. Separately derived deals with NEC installation requirements.

You can feed as many loads, and panels, as you want within the loading limits of the transformer and the protective devices you choose.
 

jim dungar

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Location
Wisconsin
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Retired Engineer
Almost certainly not on the transformer nameplate. Look first at catalog cut sheet for the transformer, and if no help there contact the manufacturer's product support directly.
Some Square D transformers now list HV and LV on their nameplates with no reference to primary and secondary.
 

kwired

Electron manager
Location
NE Nebraska
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.
You still have same phase angles and voltages between the three points regardless of where you ground the system, all grounding does is create a ground reference to that point. With three wire delta there is no neutral so there is no code requirement to ground a specific point on the system and you can ground any point you want to.

Ignoring for the moment whether it is rated for reverse feed, the problem with your 480 delta x 208/120 wye transformer is your input voltage is 240 and not 208. It will likely work but you will have high output voltage, may be able to use the original input taps to compensate, but still going to have excess voltage on the 208 volt windings and likely will shorten it's life compared to if you had applied something closer to 208 nominal range, may get lucky if your 240 volts is running at the low end of acceptable tolerances though as that would be near the upper end of 208 tolerances.
 
You still have same phase angles and voltages between the three points regardless of where you ground the system, all grounding does is create a ground reference to that point. With three wire delta there is no neutral so there is no code requirement to ground a specific point on the system and you can ground any point you want to.

Ignoring for the moment whether it is rated for reverse feed, the problem with your 480 delta x 208/120 wye transformer is your input voltage is 240 and not 208. It will likely work but you will have high output voltage, may be able to use the original input taps to compensate, but still going to have excess voltage on the 208 volt windings and likely will shorten it's life compared to if you had applied something closer to 208 nominal range, may get lucky if your 240 volts is running at the low end of acceptable tolerances though as that would be near the upper end of 208 tolerances.
Is that why its corner grounded to help compensate for the higher delta 240V besides also being reverse fed. In this case the transformers involved would need to be wired this way to get the 480V
So he could actually swap out the 45 for the 75kva transformer wire the one machine as is and a sub panel wired the same way. Follow code eliminate the fused grounded leg and mark accordingly throughout. Use a 480V non slash type main breaker as my sub panel main and non slash type breakers for my loads. This is all ok and safe ?
 

kwired

Electron manager
Location
NE Nebraska
Is that why its corner grounded to help compensate for the higher delta 240V besides also being reverse fed. In this case the transformers involved would need to be wired this way to get the 480V
So he could actually swap out the 45 for the 75kva transformer wire the one machine as is and a sub panel wired the same way. Follow code eliminate the fused grounded leg and mark accordingly throughout. Use a 480V non slash type main breaker as my sub panel main and non slash type breakers for my loads. This is all ok and safe ?
Corner grounding has nothing to do with whatever is on the primary side. The 480 volt three wire output is a separately derived system and is no direct connection to the input side other than via whatever conductor on either side may be grounded. If ungrounded then they are entirely separate other than the magnetic coupling that drives the whole concept of what a transformer does.

Yes if you have a three wire delta supply you can ground any one of the three leads, it is then a grounded conductor and follows all the rules any other grounded conductor must follow, just happens to not also be a neutral conductor. Those rules include no overcurrent protection in that conductor(exception being breaker that interrupts all poles simultaneously, but definitely no fuses), installing a grounding electrode conductor, and yes can not have slash rated breaker, or at least not one with lower rating less than the line to line volts which just isn't happening with breaker that can be use with 480 volts anyway, so you are either ending up with one that is rated straight 480, but even more likely are using one rated 600 volts.

It can be two or three pole breaker, the grounded phase can land on a grounded bus just like typical grounded neutral does in a system with a grounded neutral. A fused switch can be used but if grounded phase is run through the switch it must have a non fused link instead of a fuse installed.

You must still have a separate EGC that is only bonded at either the source or first disconnecting means and remains separated from the grounded conductor everywhere else.

And that grounded conductor is a "grounded conductor" and needs to be identified with white or gray finishes/markings in accordance with art 200.
 

kwired

Electron manager
Location
NE Nebraska
Is that why its corner grounded to help compensate for the higher delta 240V besides also being reverse fed. In this case the transformers involved would need to be wired this way to get the 480V
So he could actually swap out the 45 for the 75kva transformer wire the one machine as is and a sub panel wired the same way. Follow code eliminate the fused grounded leg and mark accordingly throughout. Use a 480V non slash type main breaker as my sub panel main and non slash type breakers for my loads. This is all ok and safe ?
My other issue is if you have 240 volts supply but are backfeeding a 208/120 transformer, you have an over voltage problem before you even get to the "secondary" side of the transformer. This is a big reason why about the only solution for this is a transformer made for 240/480 and most common thing to order would be delta-delta whether it has neutral on either side or not.
 

Jraef

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Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
Just keep in mind that current Code only allows reverse feeding a transformer if the manufacturer recognizes that as an allowed use.
Yes, an important point. I used to think that meant it had to be marked that way on the nameplate, but someone in here showed me that it doesn't necessarilly mean that, it could be in the instructions or even just the catalog data. But it does mean you need to be ready to show that information to an AHJ in whatever format it exists. If it doesn't exist, as is sometimes the case whit older equipment, then you can get in trouble. Older german transformer? Probably not there.
 
So back to my original question why would the previous electrician 1) Install a brand new transformer that needed to be back fed. 2) why would they want a corner grounded system feeding this brand new machine. And I’m also pretty sure the egc goes from transformer to IBeam to disconnect and then ties to all other grounds. The grounded phase is phase B in the fused disconnect then is on phase C inside the machine cabinet marked with yellow tape. looking at the name plates on both these transformers they both have different taps to raise and lower (I thought) primary voltage. In this backfed case I’m not sure how they got 480V with a 208/120 primary. Can’t be to good for the transformer you wouldn’t think.
 

ActionDave

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Location
Durango, CO, 10 h 20 min without traffic from wing
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wire pulling grunt
So back to my original question why would the previous electrician 1) Install a brand new transformer that needed to be back fed.
Because they are easier to get and more familiar to some folks
2) why would they want a corner grounded system feeding this brand new machine. And I’m also pretty sure the egc goes from transformer to IBeam to disconnect and then ties to all other grounds. The grounded phase is phase B in the fused disconnect then is on phase C inside the machine cabinet marked with yellow tape. looking at the name plates on both these transformers they both have different taps to raise and lower (I thought) primary voltage. In this backfed case I’m not sure how they got 480V with a 208/120 primary. Can’t be to good for the transformer you wouldn’t think.
Sounds like poor install.
 
Corner grounding has nothing to do with whatever is on the primary side. The 480 volt three wire output is a separately derived system and is no direct connection to the input side other than via whatever conductor on either side may be grounded. If ungrounded then they are entirely separate other than the magnetic coupling that drives the whole concept of what a transformer does.

Yes if you have a three wire delta supply you can ground any one of the three leads, it is then a grounded conductor and follows all the rules any other grounded conductor must follow, just happens to not also be a neutral conductor. Those rules include no overcurrent protection in that conductor(exception being breaker that interrupts all poles simultaneously, but definitely no fuses), installing a grounding electrode conductor, and yes can not have slash rated breaker, or at least not one with lower rating less than the line to line volts which just isn't happening with breaker that can be use with 480 volts anyway, so you are either ending up with one that is rated straight 480, but even more likely are using one rated 600 volts.

It can be two or three pole breaker, the grounded phase can land on a grounded bus just like typical grounded neutral does in a system with a grounded neutral. A fused switch can be used but if grounded phase is run through the switch it must have a non fused link instead of a fuse installed.

You must still have a separate EGC that is only bonded at either the source or first disconnecting means and remains separated from the grounded conductor everywhere else.

And that grounded conductor is a "grounded conductor" and needs to be identified with white or gray finishes/markings in accordance with art 200.
Big huge help I might even be starting to understand. Maybe ? I think ? This is what happens to your brain when your stuck troubleshooting residential for the last 10yrs.
 

kwired

Electron manager
Location
NE Nebraska
So back to my original question why would the previous electrician 1) Install a brand new transformer that needed to be back fed. 2) why would they want a corner grounded system feeding this brand new machine. And I’m also pretty sure the egc goes from transformer to IBeam to disconnect and then ties to all other grounds. The grounded phase is phase B in the fused disconnect then is on phase C inside the machine cabinet marked with yellow tape. looking at the name plates on both these transformers they both have different taps to raise and lower (I thought) primary voltage. In this backfed case I’m not sure how they got 480V with a 208/120 primary. Can’t be to good for the transformer you wouldn’t think.
480 delta by 208/120 is most common stocked units, and if your application would have had 208/120 instead of 120/240 for input voltage would have been somewhat common to back feed such a unit to obtain 480 volts as long as no 277 was needed. Code that says the unit must be designed for reverse feed is a more recent requirement but don't remember which year it was added. Biggest problem disregarding the designed for backfeed issue is you do not have 208 volt for the input and was bad decision on whoever did this initially. Chances are what you really need is not as commonly stocked, may have longer lead times and probably somewhat higher priced.

Other thing with doing this is you only have the three phase conductors and no neutral so there is no feeding any 277 volt loads or grounding of a neutral conductor, you have no choice but to corner ground it or install as an ungrounded system- but ungrounded system needs ground monitoring system many choose corner ground. Ungrounded is more desired in a process that needs an orderly shutdown, you can indicate a fault and have time to shut down instead of having sudden shut down like you will have on a grounded system. Other problem with that is if you ignore the fault indication and eventually a second fault occurs, you will open the overcurrent device and have sudden shut down anyway.

The taps on your unit should be on the 480 volt side, are probably shipped connected to the middle tap. You would leave it as is if input volts is at or very near 480. I believe taps are usually about 2.5% difference each step either direction, so if input volts is consistently 2.5% low (about 468) then you would move the tap one position toward the low side and this would get your secondary volts close to 120/208 even though the 480 input is a little low. If incoming volts is high you move taps the other direction to lower secondary voltage.
 

kwired

Electron manager
Location
NE Nebraska
If you buy a new one, I would go with the delta/wye step up, then you don't have to worry about getting special breakers.
Special breakers are a bigger factor if you want to supply a panelboard and multiple circuits. If this is supplying one machine all you need is a single breaker and enclosure or even a fused safety switch.
 

MD Automation

Member
Location
Maryland
Occupation
Engineer
Hammond Power Solutions (HPS) also makes them...

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I will ask a dumb question again - are you sure the incoming service is 240 VAC and not 208Y/120 ? I know you said you measured something closer to 220. With the previous electrician selecting a 208 transformer and you measuring 480 on the other side of it, having 208 as the supply would answer a lot of questions. I know we have talked about maybe they used taps on the 480 side to compensate, but to adjust 240 to 208, that's a big tap (15%). Have you opened up the existing transformer to look at the tap connections?

Just wanted you to make double sure of the incoming service before shelling out money for a 240 -> 480 transformer.
 

kwired

Electron manager
Location
NE Nebraska
This company has them in stock.

Transformer suppliers probably have stock of 240D to 480Y 277.

Regular electrical supply houses may have lesser chance of stocking them as they wouldn't be as high in demand as 480 delta to 208/120 wye.
 
Hammond Power Solutions (HPS) also makes them...

View attachment 2552420

I will ask a dumb question again - are you sure the incoming service is 240 VAC and not 208Y/120 ? I know you said you measured something closer to 220. With the previous electrician selecting a 208 transformer and you measuring 480 on the other side of it, having 208 as the supply would answer a lot of questions. I know we have talked about maybe they used taps on the 480 side to compensate, but to adjust 240 to 208, that's a big tap (15%). Have you opened up the existing transformer to look at the tap connections?

Just wanted you to make double sure of the incoming service before shelling out money for a 240 -> 480 transformer.
I will be there again this Wed. This job is two hours from me or I would have went back. I have not opened the transformer but I will. I will also double check the incoming. I took readings off the main breaker feeding the transformer. 243 240 242 I was told by an employee that they have had low voltage at time as low as 220 but with the current Covid situation the development is not even at half capacity. So the voltage has remained in the 240 range. There are three loads currently that need to be hooked up. The one that was done bead blaster of which a 45 KVA 480/208/120 transformer was used. A overhead 5 ton crane (12.5amps) and this large steel dipping furnace I’m still waiting on the specs on this. Someone said it needs 60amps but somehow I dont think that’s right. It was moved from another location and the conduit feeding it was half inch with 3 # 12s but will find out this week. They have another transformer a spare which is a 75Kva same voltages originally I simply wanted to swap out the 45 for the 75 and have a nice day but now that might be undersized and wasn’t crazy about the voltages. Will have answers hopefully on Wed. This job is far but it’s a good in and I’m the new guy. I’ve already done one job for them which worked out nice and saved them a few bucks. I really want to get this one right, make it safe, within code, and hopefully with some power to spare for something down the road. Most likely I will leave the bead blaster alone, correct the code problems if needed. carry my 240 V some 130’ to a disconnect feed a 75 KVA step up 240/480 to a load center with crane breaker and furnace breaker.
 
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