# open delta system



## sparkn890 (Sep 18, 2012)

I am a third year apprentice out of Janesville WI and I am trying to get answers on how an open delta system operates. Why wouldn't the utility always supply you with an open delta? One less xfmr can cuts your cost by 33 1/3% wouldn't upsizing your kva rating a bit still be cheaper than a whole other can? Thanks for any input


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## wildleg (Apr 12, 2009)

perhaps this will help to answer your question (the common answer of which is - you don't get something for nuthin)

http://www.engineeringradio.us/blog/2012/02/the-open-delta-three-phase-service/


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## John (Jan 22, 2007)

sparkn890 said:


> I am a third year apprentice out of Janesville WI and I am trying to get answers on how an open delta system operates. Why wouldn't the utility always supply you with an open delta? One less xfmr can cuts your cost by 33 1/3% wouldn't upsizing your kva rating a bit still be cheaper than a whole other can? Thanks for any input


The two transformers have to be larger.....do your math.


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## Going_Commando (Oct 1, 2011)

Open delta provides 57% (iirc) of the power that straight delta does with the same size transformers.


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## frenchelectrican (Mar 15, 2007)

Going_Commando said:


> Open delta provides 57% (iirc) of the power that straight delta does with the same size transformers.


 
That is the key answer with open delta system and I know it is common used on some resdentail and a bit of small commercal location.

But the key issue with any delta sytem is grounding that will get ya due there is a corner grounded delta in open delta set up as well that will really get ya hard if you don't read the voltage carefull. ( of course wild leg if you do bring netural along ) 

Merci,
Marc


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## Going_Commando (Oct 1, 2011)

The neatest system I have seen was open Wye, open Delta for a muni well site. Poco fed 2 phase conductors, and a neutral to 2 transformers, and wired it open wye primary, and an open delta secondary, and got 3 phase out of it to run a couple pumps. Really blew my mind when I first looked at it. When it was installed, the Poco didn't want to pay for 3 phase conductors the distance to the well house.


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## etb (Sep 8, 2010)

Going_Commando said:


> The neatest system I have seen was open Wye, open Delta for a muni well site. Poco fed 2 phase conductors, and a neutral to 2 transformers, and wired it open wye primary, and an open delta secondary, and got 3 phase out of it to run a couple pumps. Really blew my mind when I first looked at it. When it was installed, the Poco didn't want to pay for 3 phase conductors the distance to the well house.


That's pretty common here, and it's the only way it can be done with two phases. If you do have all three you can wire primary in delta to save that third xfmr in low demand situations.



frenchelectrican said:


> But the key issue with any delta sytem is grounding that will get ya due there is a corner grounded delta in open delta set up as well that will really get ya hard if you don't read the voltage carefull. ( of course wild leg if you do bring netural along )
> 
> Merci,
> Marc


Most of the ones I've seen were corner grounded.


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## MWayne (Nov 8, 2010)

I was told that an open delta can have an ongoing ground fault and not be detected unless it is large enough to blow a fuse or open a breaker. And the proper way to use open delta would include a ground fault relay with CTs etc. Maybe someone here can confirm this?


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## frenchelectrican (Mar 15, 2007)

MWayne said:


> I was told that an open delta can have an ongoing ground fault and not be detected unless it is large enough to blow a fuse or open a breaker. And the proper way to use open delta would include a ground fault relay with CTs etc. Maybe someone here can confirm this?


It the same as closed delta format however with open delta they can be come in ungrounded or grounded ( more common ) format and with grounded corner ground delta system ya don't need all the bell et whistle on this one. Again it the same way with closed corner ground delta so there is no differnce on that.

Merci,
Marc


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## MWayne (Nov 8, 2010)

frenchelectrican said:


> It the same as closed delta format however with open delta they can be come in ungrounded or grounded ( more common ) format and with grounded corner ground delta system ya don't need all the bell et whistle on this one. Again it the same way with closed corner ground delta so there is no differnce on that.
> 
> Merci,
> Marc


We had an ancient open delta system that had only three wires comming down from the pole, it was decided to replace the transformers and when doing so I ran a fourth wire for grounding to the disconnect. The disconnect had been rebuilt years earlier and had the fourth wire already. So this is now a grounded system, oui?


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## Arc'n'Spark (Jul 21, 2011)

MWayne said:


> We had an ancient open delta system that had only three wires comming down from the pole, it was decided to replace the transformers and when doing so I ran a fourth wire for grounding to the disconnect. The disconnect had been rebuilt years earlier and had the fourth wire already. So this is now a grounded system, oui?


Did you connect one of the legs coming off the transformer to ground?


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## MWayne (Nov 8, 2010)

Arc'n'Spark said:


> Did you connect one of the legs coming off the transformer to ground?


Arc,
The short answer is no.
I didn't want to change anything else without approval from higher ups and the boss was happy that power was restored and functional. The ground wire I installed lands the same place as the transformer cans.


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## frenchelectrican (Mar 15, 2007)

MWayne said:


> We had an ancient open delta system that had only three wires comming down from the pole, it was decided to replace the transformers and when doing so I ran a fourth wire for grounding to the disconnect. The disconnect had been rebuilt years earlier and had the fourth wire already. So this is now a grounded system, oui?





MWayne said:


> Arc,
> The short answer is no.
> I didn't want to change anything else without approval from higher ups and the boss was happy that power was restored and functional. The ground wire I installed lands the same place as the transformer cans.


Was this ancient delta grounded at the main transfomer or POCO transfomer ? if latter then it is grounded properly but there were alot of ungrounded delta system and if you are not too carefull with reading the voltage it will be allover the place you will may have heckva of time to get correct reading espcally without load on it.

So therefore if you did not ground one of the phase on delta then it is floating ungrounded system.

Merci,
Marc


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## MWayne (Nov 8, 2010)

frenchelectrican said:


> Was this ancient delta grounded at the main transfomer or POCO transfomer ? if latter then it is grounded properly but there were alot of ungrounded delta system and if you are not too carefull with reading the voltage it will be allover the place you will may have heckva of time to get correct reading espcally without load on it.
> 
> So therefore if you did not ground one of the phase on delta then it is floating ungrounded system.
> 
> ...


It is grounded at the pole mount transformers. Originally I thought this was corner ground delta because of only 2 transformers on the pole and we have a corner grounded delta system a few kilometers away. But the meter didn't show 480V between phases and like you said the voltage on one phase is erratic.


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## Zog (Apr 15, 2009)

sparkn890 said:


> I am a third year apprentice out of Janesville WI and I am trying to get answers on how an open delta system operates. Why wouldn't the utility always supply you with an open delta? One less xfmr can cuts your cost by 33 1/3% wouldn't upsizing your kva rating a bit still be cheaper than a whole other can? Thanks for any input


Welcome to the forum, I used to spend a lot fo time in Janesville before the GM plant closed. About 20 weeks/year for 5 years.


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