# Reverse wire Transfrmer 480v delta to 120/208 wye



## Amelco (Feb 5, 2012)

I am taking a skid and temp wirirng taking 120/208 landing on the X's and going to try and get 480v out on the H's. So where I really earn my money is what do I do with the XO on the 208 side. I got a Grd off the 208 side and my Grd on my 480 going in the pipe. I think I just bond both Grds to frame of trans, along with the XO? Does anyone know for sure, I am doing this tommorrow. Going to feed the Elevator temp 480 off a historical 120/208 service. Need the XO info?


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## brian john (Mar 11, 2007)

You do not take 4-wire 208/120 to the transformer you take 3-wire 208, you lift the ground on the XO side, You will not connect anything to XO, you can ground the transformer shell with your EGC you carry with the 208 primary feeder. The secondary is where I would ground the system. Treat one phase (I think B) but it really does not matter to the whole operation of the system. Bond B phase to the transformer shell and to the grounding electrode conductor.


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## MDShunk (Jan 7, 2007)

Be advised that when you wire a transformer "backwards", the inrush will be great, and the style of circuit breaker should be selected with care, or use fuses.


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## ColoradoMaster3768 (Jan 16, 2010)

brian john said:


> You do not take 4-wire 208/120 to the transformer you take 3-wire 208, you lift the ground on the XO side, You will not connect anything to XO, you can ground the transformer shell with your EGC you carry with the 208 primary feeder. The secondary is where I would ground the system. Treat one phase (I think B) but it really does not matter to the whole operation of the system. Bond B phase to the transformer shell and to the grounding electrode conductor.


 
Brian, if he doesn't solidly ground the center-point on the Wye-side (the X-0) the voltage will not be stable—it will float.


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## MDShunk (Jan 7, 2007)

ColoradoMaster3768 said:


> Brian, if he doesn't solidly ground the center-point on the Wye-side (the X-0) the voltage will not be stable—it will float.


Not true. It will just be like a delta-delta transformer.


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## micromind (Aug 11, 2007)

ColoradoMaster3768 said:


> Brian, if he doesn't solidly ground the center-point on the Wye-side (the X-0) the voltage will not be stable—it will float.


The X side is already grounded at the source. 

The fact that the transformer is a wye is of no consequence. The voltage across any coil will not exceed 208, and the voltage from any point in any coil will not exceed 120 to ground. 

Think of a wye wound 3ø motor. The center of the wye is isolated from ground, just as it should be with a wye wound transformer. 

If the H side is not grounded, the voltage will indeed float. If there are no ground faults (none at all, even small ones) capacitive coupling will keep the voltages around 277 to ground, but even then it'll vary somewhat.


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## ColoradoMaster3768 (Jan 16, 2010)

MDShunk said:


> Not true. It will just be like a delta-delta transformer.





micromind said:


> The X side is already grounded at the source.





micromind said:


> The fact that the transformer is a wye is of no consequence. The voltage across any coil will not exceed 208, and the voltage from any point in any coil will not exceed 120 to ground.
> Think of a wye wound 3ø motor. The center of the wye is isolated from ground, just as it should be with a wye wound transformer.
> If the H side is not grounded, the voltage will indeed float. If there are no ground faults (none at all, even small ones) capacitive coupling will keep the voltages around 277 to ground, but even then it'll vary somewhat.


Perhaps I misunderstood the Amelco's intended connection. I understood Amelco was intending to make a 3-phase transformer connection where Amelco wanted to step 208Y/120 volts up to 480 volts Delta. If that is a correct understanding, then the Wye side of the transformer must be solidly grounded, otherwise the voltage will float. The float will occur on the primary (Wye) side and be reflected accordingly in the secondary (Delta). 

I have encountered and dealt with this phenomenon on ungrounded-Wye—Delta configurations in 12470Y/7200 volt to 480 systems. Even though the primary (source) sides were derived from solidly grounded 12470Y/7200 volt Wye systems, the fact that the primary side of the transformers (Wye side) were not solidly grounded caused the voltage across the Wye-side windings to become unstable. This instability was reflected into the Delta windings, which caused problems on the secondary (Delta) side. Since these transformers were 3-phase oil-filled cans that were manufactured as ungrounded-Wye—Delta units, the only practical solution was to replace them with Wye-Wye units. (Also, it is unknown to me and still unclear as to why that particular winding configuration was selected.) 

An ungrounded Wye connection does not act like a Delta connection. An ungrounded Wye has two windings in series from point-to-point (A – B, B – C, or C – A) whereas, a Delta connection has only have one winding from point-to-point (A – B, B – C, or C – A). And, regardless as to how solidly-grounded the source is, if a Wye transformer is not solidly grounded, the voltage within the transformer windings will become unstable.


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## brian john (Mar 11, 2007)

ColoradoMaster3768 said:


> Perhaps I misunderstood the Amelco's intended connection. I understood Amelco was intending to make a 3-phase transformer connection where Amelco wanted to step 208Y/120 volts up to 480 volts Delta. If that is a correct understanding, then the Wye side of the transformer must be solidly grounded, otherwise the voltage will float. The float will occur on the primary (Wye) side and be reflected accordingly in the secondary (Delta).


You are way off base on this, the "WYE" side is grounded at the service, how can a load (the transformer) take a solidly grounded wye system and un-ground it?

NO and let me make sure the OP understands a BIG *NO*, DO NOT GROUND THE XO, and LIFT ANY FACTORY BONDS.


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## Podagrower (Mar 16, 2008)

Let me just ask this because I do not know. Does the secondary side of this need to be delta? Wouldn't it be easier to go from 120/208y to 277/480y? 

If you had an elevator or other motor that was running 480 delta and you connected it to 480 wye what would the difference be? (I have not yet run into a 480 delta, so excuse my lack of knowledge)


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## Mshea (Jan 17, 2011)

the incoming 208 can connect XO to an identified conductor but definitely no ground since the Neutral is already connected to ground at the service. any fault in the supply side would be carried to source via the equipment ground (bond wire)
the secondary at 480 volts is an ungrounded service so a ground detector will be required. The voltage on the secondary with respect to ground is irrelevant as long as the voltage leg to leg is 480 it does not matter to the load if the voltage floats relative to ground except when there is a fault. A single ground isn't a problem but a second one causes havoc if on a different phase.
So does the load need a ground? I don't know the NEC allowances for ungrounded systems but very few systems here get approved as ungrounded. The utility won't generally deliver ungrounded services here any more. Some resistance grounded services are done where ungrounded was used.
Are there people working at the building that know what a ground detection light is? Frankly unless there is a staff electrician at this location I would not allow a transformer to be connected this way. Pretty much must ground the secondary and this transformer does not have an H0 to connect to.


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## brian john (Mar 11, 2007)

Podagrower said:


> Let me just ask this because I do not know. Does the secondary side of this need to be delta? Wouldn't it be easier to go from 120/208y to 277/480y?
> 
> If you had an elevator or other motor that was running 480 delta and you connected it to 480 wye what would the difference be? (I have not yet run into a 480 delta, so excuse my lack of knowledge)


I would utilize a 208 Delta to a 480/277 wye.

Wye Wye is unnecessary and in smaller sizes I would assume very uncommon.


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## brian john (Mar 11, 2007)

ColoradoMaster3768 said:


> An ungrounded Wye connection does not act like a Delta connection. An ungrounded Wye has two windings in series from point-to-point (A – B, B – C, or C – A) whereas, a Delta connection has only have one winding from point-to-point (A – B, B – C, or C – A). And, regardless as to how solidly-grounded the source is, if a Wye transformer is not solidly grounded, the voltage within the transformer windings will become unstable.


In regards to this I would say SO. You can (not necessarily by NEC) operate both systems ungrounded, what does the Earth care if there is a giant transformer sitting on it?


Here read this, this is regarding a wye wye


http://forums.mikeholt.com/showthread.php?t=143029

Answer this,

What difference does it make if I ground the wye primary at the source or at the XO in a transformer. 

How do you deal with the fact that the installation your are discussing is an NEC violation.


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## electricalperson (Jan 11, 2008)

We have a transformer like this in the panel shop. its a general purpose 480v too 120/208v wye 3 phase. We wired it with straight 208 and have 480v for testing. Its ungrounded. I wired transformers like this for customers that have a 208v service but have one 480v machine. If the breaker is too small it will trip when you try to turn it on.


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## BBQ (Nov 16, 2010)

electricalperson said:


> We have a transformer like this in the panel shop. its a general purpose 480v too 120/208v wye 3 phase. We wired it with straight 208 and have 480v for testing. Its ungrounded. I wired transformers like this for customers that have a 208v service but have one 480v machine. If the breaker is too small it will trip when you try to turn it on.


Did you install ground detectors on these ungrounded systems?



> *250.21 Alternating-Current Systems of 50 Volts to Less
> Than 1000 Volts Not Required to Be Grounded.
> 
> (B) Ground Detectors.* Ground detectors shall be installed
> ...


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## electricalperson (Jan 11, 2008)

BBQ said:


> Did you install ground detectors on these ungrounded systems?


I don't know. Last I did this I was a apprentice with a company that had hacks working for it. I was just wondering if we ever grounded them I don't remember


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## Vintage Sounds (Oct 23, 2009)

You're in the US where it's legal to ground one corner of the delta. Why don't you just do that instead of leaving it floating without a ground fault detector?


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## ColoradoMaster3768 (Jan 16, 2010)

brian john said:


> You are way off base on this, the "WYE" side is grounded at the service, how can a load (the transformer) take a solidly grounded wye system and un-ground it?





brian john said:


> NO and let me make sure the OP understands a BIG *NO*, DO NOT GROUND THE XO, and LIFT ANY FACTORY BONDS.


I'm not off base here, my responses were in regard to your direction to Amelco to not bring a neutral to the transformer or to connect anything to it: 




brian john said:


> You do not take 4-wire 208/120 to the transformer you take 3-wire 208, you lift the ground on the XO side, You will not connect anything to XO...


 

This caused me to subsequently post this comment, "...regardless as to how solidly-grounded the source is, if a Wye transformer is not solidly grounded, the voltage within the transformer windings will become unstable." Here I am talking about establishing a solid grounding-point with respect to the supply (120/208 primary) at the X0 for voltage stability. This can be done via the neutral from the 3-phase, 4-wire 208Y/120 source feeding transformer—which I presume to be solidly grounded. 




brian john said:


> In regards to this I would say SO. You can (not necessarily by NEC) operate both systems ungrounded, what does the Earth care if there is a giant transformer sitting on it?





brian john said:


> Here read this, this is regarding a wye wye
> http://forums.mikeholt.com/showthread.php?t=143029
> Answer this,
> What difference does it make if I ground the wye primary at the source or at the XO in a transformer...


My point exactly. Hence, I reiterate: This can be done via the neutral from the 3-phase, 4-wire 208Y/120 source feeding transformer—which I presume to be solidly grounded. 




brian john said:


> In regards to this I would say SO. You can (not necessarily by NEC) operate both systems ungrounded, what does the Earth care if there is a giant transformer sitting on it?





brian john said:


> Here read this, this is regarding a wye wye
> http://forums.mikeholt.com/showthread.php?t=143029
> ...
> How do you deal with the fact that the installation your are discussing is an NEC violation.


I'm not suggesting anything contrary to the _Code._ I'm simply standing on the ground that the X0 must be grounded for voltage stability, again this can be done via the neutral on the source side. I am not suggesting that Amelco parallel the neutral and ECG downstream of the source.

By the way, thanks for the link. It was an interesting read. However, I will subscribe to the following: 

In his book entitled, "Electric Power Distribution System Engineering," McGraw-Hill publications, first edition, copyright 1986, Mr. Turan Gonen states:
_...In the case of the wye-delta connection, if the neutral is spared on the primary side the voltage waveform tends to deform, but this deformation causes circulating currents in the delta, and these currents act as magnetizing currents to correct the deformation. Thus there is no objection to neglecting the neutral. _[So far, so good] _However, If the transformer supplies a motor load, a damaging overcurrent is produced in each three-phase motor circuit, causing an equal amount of current to flow in two wires of the motor branch circuit and the total of the two currents to flow in the third. If the highest of the three currents occurs in the unprotected circuit, motor burnout will probably happen. This applies to ungrounded wye-delta and delta-wye banks._

This is caused by the fact that voltage "floats" from the ungrounded neutral point. (This sort of reminds me of an open-neutral situation on a single-phase service.)


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## micromind (Aug 11, 2007)

If the voltage will float in a wye connected transformer with the center of the wye not connected, then it will also float in a wye connected motor with the center of the wye unconnected. 

Yet millions of these motors are presently in operation, many of them have run for decades without issue. 

The voltage cannot possibly float with reference to ground, because the source is grounded. It also cannot possibly float phase to phase because it is held to a specific value by the source. 

I've reverse connected several transformers like the one described. Every time, I made sure that X0 was completely isolated. Also, H2 was grounded. 

I've yet to have any sort of trouble from any of them.


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## Amelco (Feb 5, 2012)

So I hooked up the Transformer last night with much debate and I admit my heart did race when I flipped the breaker. First let me make this clear. This is a temporary install, for 3 months max. I am in a historical part of San Francisco hence the 120/208 service on the building the size of a city block. ( lots of challenges on a complete new 6 story building). 
I took the ground out of the solidly grounded 208/120 amp board which is an 800 amp board and tied it to a tripple lug on the Transformer frame. I took the XO and tied it to the tripple Lug. I alzo took the ground going up the pipe to the 480 side and tied it to the tripple Lug. 
To recap. Ground out of 120/208, XO , and 480 ground all on Tripple Lug to Frame. 208 3 phase off 100 amp breaker on the X's and 480 on the H. Their is no HO. 
I turned it on at the end of the day when the jobsite was closed down. I got 490 on each leg, and 250ish to ground. Tommorrow ,I wil adjust the taps.and try to get close to 480. I have not put load on it yet. And since I am pumping 480 to the elevator, The elevator guy is then taking my feed an d puttimg it in a 480 in and 480 out transformer ( purifyer). Posting this because I want to get it right, and pass it along to the next guy.
L 
On groundng a Delta leg. I just don't feel good about it. I know their is no mechanical connection on te 480 delta side, but I am only going to do it if tommorrow does not workout. I did not go to building steel when I set the Trans, rember I am latching on to the 800 amp panels ground. Thank you for everyones input.


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## Amelco (Feb 5, 2012)




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## nez (Jan 16, 2010)

Amelco, you should not ground xo to the frame. You are asking for problems.
You should remove that jumper. Install a grounding electrode and land it on h2 along with your b phase of your 480 side. Connect the other end of your grounding electrode to building steel or a ground rod. Do not hook anything at all to xo.


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## BBQ (Nov 16, 2010)

nez said:


> Amelco, you should not ground xo to the frame. You are asking for problems.
> You should remove that jumper. Install a grounding electrode and land it on h2 along with your b phase of your 480 side. Connect the other end of your grounding electrode to building steel or a ground rod. Do not hook anything at all to xo.


I agree 100% and if you look at the Square D FAQ section you will find they say the same thing when backfeeding a why delta transformer.


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## brian john (Mar 11, 2007)

> I'm not off base here, my responses were in regard to your direction to Amelco to not bring a neutral to the transformer or to connect anything to it:


You NEVER said word one about a neutral you said GROUND IT, there is a major difference.

What you said



> Perhaps I misunderstood the Amelco's intended connection. I understood Amelco was intending to make a 3-phase transformer connection where Amelco wanted to step 208Y/120 volts up to 480 volts Delta. If that is a correct understanding, then the Wye side of the transformer must be solidly grounded, otherwise the voltage will float. The float will occur on the primary (Wye) side and be reflected accordingly in the secondary (Delta).


That is a total MIS-STATEMENT, no how, no way.



> I'm not suggesting anything contrary to the _Code._ I'm simply standing on the ground that the X0 must be grounded for voltage stability, again this can be done via the neutral on the source side. I am not suggesting that Amelco parallel the neutral and ECG downstream of the source.


You implied that XO should be solidly grounded.

Some additional reading.

http://forums.mikeholt.com/showthread.php?t=134233

http://www.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White%20Papers%7CGET-3388B%7Cgeneric


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## Vintage Sounds (Oct 23, 2009)

Amelco said:


> On groundng a Delta leg. I just don't feel good about it. I know their is no mechanical connection on te 480 delta side, but I am only going to do it if tommorrow does not workout. I did not go to building steel when I set the Trans, rember I am latching on to the 800 amp panels ground. Thank you for everyones input.


What happens if there is a ground fault on the 480 side?


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## Amelco (Feb 5, 2012)

If any one has ran a skid like this please reply. Everyone I have talked to says " Ya, we can do it". But then when it comes to grounding it they shuffle their feet and defer to anyone. Evry story is ya I did something like it, I heard of it. I know a lot of ship yard electrcians have done this, I just hope one uses the internet.


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## mattsilkwood (Sep 21, 2008)

Vintage Sounds said:


> What happens if there is a ground fault on the 480 side?


 That is why you ground one leg of the 480 side, or you can install ground fault detectors.


Amelco said:


> If any one has ran a skid like this please reply. Everyone I have talked to says " Ya, we can do it". But then when it comes to grounding it they shuffle their feet and defer to anyone. Evry story is ya I did something like it, I heard of it. I know a lot of ship yard electrcians have done this, I just hope one uses the internet.


 Listen to what Brian and Micromind are telling you. Ground one leg of the 480, but keep in mind by doing this you will have 480 volts to ground on two of the legs and 0 volts to ground on the grounded leg. I know that should go without saying but a lot of guys cant grasp that concept.


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## frenchelectrican (Mar 15, 2007)

Amelco.,

Belive or not we do the same methold as Brian John or Micromind mention and I have done this all the time in either state side or In France it is the same rules always lift the XO end of discussion for your safety.

I am pretty sure I have a delta corner grounded diagram it will show it clear so let moi post it.










This will give you a idea.

Btw it will work either 240 or 480 volts in North Americane side and in France it will be either stright 240 volt delta or 415 volt delta 

Merci,
Marc


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## brian john (Mar 11, 2007)

Vintage Sounds said:


> What happens if there is a ground fault on the 480 side?


 
Then your system continues to work, no harm except you have no idea where the ground fault is and there is current on the EGC.

Additionally (outside my realm of understanding) there are issues with impulses on ungrounded systems that can cause insulation damage.
http://www.csanyigroup.com/grounded-or-ungrounded-systems


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## Amelco (Feb 5, 2012)

So I did it my way and grounded the XO to the frame, I put my meter on the fuse box on the secondary, and I got 480. From each phase. Then..............................................wait for it.
Then I took my meter to the other side of the fuses (load side ) and the voltage was wacky.
Debated. Made some calls. And then I took the XO off and grounded H1 to the frame of the trans. Got 480 on all sides of the fuses and at the Elevator room disconnect 6 floors up. I forgot to mention I suited up fully on the breacker flip on the H1. So this system is called a corner grounded Delta and is legal. I will post the pics to show I really did it in a day or so. Thanks, for all the help.


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## power (Feb 27, 2012)

*120Y/208 VAC - 480vac transformer*

Standard dry-type distribution transformers can be connected in either direction with no consequence. Other than the expected inruch currents on any load, there are no significant increases in inrush currents when a transfomer is used backwards.....or what is perceived as "backwards". Connect the X0, X1, X2, and X3 lugs the same way you always would......the XO needs to be grounded. The H side now becomes your load side, that is, the three-wire Delta. I would NOT ground the "B" phase, but rather, use ground indicating lights where they would be most likely noticed by the end-user of your installation. Remember too, a manually operable fusible disconnect switch is required at the transformer, that is, switching it's primary conductors.


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## brian john (Mar 11, 2007)

power said:


> Standard dry-type distribution transformers can be connected in either direction with no consequence. Other than the expected inruch currents on any load, there are no significant increases in inrush currents when a transfomer is used backwards.....or what is perceived as "backwards". Connect the X0, X1, X2, and X3 lugs the same way you always would.....*.the XO needs to be grounded.* The H side now becomes your load side, that is, the three-wire Delta. I would NOT ground the "B" phase, but rather, use ground indicating lights where they would be most likely noticed by the end-user of your installation. Remember too, a manually operable fusible disconnect switch is required at the transformer, that is, switching it's primary conductors.



No you are 100% wrong, unless electricity behave differently in Canada?:blink:


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## power (Feb 27, 2012)

*Transformer Connection*

Actually, last night I was thinking about this post......and I do agree with you brian john. My error.....I was thinking that the star-point (or wye) will float if it's not "nailed down", that is, grounded.....and I still believe that. However, in this case, it does not matter if it floats as it's not being used anyway! So, my error, sorry! I guess a three-wire (X1, X2, and X3) supply, or line-in, connection would work fine!


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## BBQ (Nov 16, 2010)

power said:


> Standard dry-type distribution transformers can be connected in either direction with no consequence. Other than the expected inruch currents on any load, there are no significant increases in inrush currents when a transfomer is used backwards.....or what is perceived as "backwards". Connect the X0, X1, X2, and X3 lugs the same way you always would....*..the XO needs to be grounded. *


Absolutely not.

You do not ground XO when using a delta / Wye transformer in the reverse direction.

As a mater of fact if you do bond XO to ground you can cause serious problems under fault collections.


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## BBQ (Nov 16, 2010)

Here is what Square D has to say about it



> *Can Transformers be backfed (used in reverse)?*
> 
> 
> 1. .The practice of backfeeding a general purpose transformer is NOT recommended, especially in transformers smaller that 3Kva. Backfeeding is not allowed for any Industrial Control Transformers of any size, because windings are compensated and backfeeding will result in lower than expected output voltage.
> ...


http://www.schneider-electric.us/si...&country=US&lang=EN&id=FA101888&redirect=true


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## power (Feb 27, 2012)

*Transformer Connection*

Yes....I see this now....my bad. You guys are right. Short circuit conditions would pose a significant problem....I agree. 

Of course, electrical circuitry is the same in Canada as in the US, however, there are some things we do differently than our US neighbors. I am going to try to start a threid about another question I have....however, it is off topic to this forum's discussion. While I've owned an electrical company in Alberta for 15 years now, I do realize that our American neighbors do many things better than we do, and visa versa, there are some things we do the excel the US. In any case, my question is involving a delta hi-leg, which I believe is colored purple.....anyway, I'll try to start a topic on this, and I'll see where I get. Thanks for your input!


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## brian john (Mar 11, 2007)

> 3. If a Delta-Wye transformer is to be backfed so that the Wye side is the input, do not connect the neutral terminal to the primary system neutral, nor should the neutral terminal be connected to ground.


Do you have any other information like the issues involved when this is done?

I have seen two jobs and the neutral bond jumpers were burnt off.


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