# Corner grounded Delta



## RePhase277 (Feb 5, 2008)

Yeah that sounds like a corner grounded delta. The point of it is to have a grounded system. For big motor loads like saw mills and irrigation pumps, delta transformers are superior to wye connected systems. But the only way to ground a delta transformer is to ground one of the phases, or get a more expensive transformer with one of the phases center-tapped.

If all you are running is a single voltage motor load, corner grounded delta is fine, and maybe even preferable. If you want multiple voltages, you need a 4-wire delta high leg, or wye system.

Your system can be used but you'll need to have a transformer installed that takes your 240 volts in and puts 120/240 volts out.


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## brian john (Mar 11, 2007)

I'll never understand why people do not check what they have and try to figure out why the are seeing the results of their investigation.

He should have read

A-B 240
B-C 240
C-A 240
A-Gnd 240
B-Gnd 0
C-Gnd 240

This assumes they grounded B phase.

No 120 VAC anywhere in that mix.

Case closed. 

And under NO CASE is this a NEUTRAL, it is a GROUNDED CONDUCTOR


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## mpoulton (Jul 17, 2009)

Wasn't there a transformer in there somewhere? Seems unlikely that the lumber mill had absolutely no 120V stuff at all.


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## Corysan (Jan 20, 2017)

Hey guys. I live in an area where 75%+ of small factories have, or at one time had what we refer to as "grounded B". Corner grounded delta systems are good in theory, but only with older types of loads. In my experience, there always seems to be a higher current on the grounded phase. It is also a serious hazard to those not experienced with this system. The accepted color standard here is black, white, red for this system. Square D single phase panels are UL listed for use with 3 ph. 240 V delta systems as long as you use the appropriates "H" type circuit breakers. The predominant equipment in this area is Square D. The H breakers are a bit more, and harder to find. frequently people install regular QO breakers. If a novice opens a panel like this, it looks for all intents and purposes like single phase 120/240 volts. Anyone see a problem with this?

Also, what manufacturer makes a listed single phase buss system? With most of the old buss plugs on these systems, we either find the grounded phase fused (often with a larger fuse than the ungrounded phase), or a piece of copper tube in place of the fuse. Nice, huh?

My understanding is that the use of this system locally started some time around WW2. The reasoning, as Rephase277 stated, was to provide a grounded system. With a floating delta system, you are required to provide ground fault detection. With a corner grounded system, the OCPD should open in the event of a ground fault, thus providing a safer system. With a floating delta system, if a fault occurs, it has to be tracked down. With a corner grounded system, if a fault occurs, the nearest upstream OCPD opens making troubleshooting more efficient. Did I mention safer? Nothing like an active ground fault you can't find. This system supposedly offers the benefit of a grounde system, but saves the cost of having a fourth wire installed.


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## Southeast Power (Jan 18, 2009)

mpoulton said:


> Wasn't there a transformer in there somewhere? Seems unlikely that the lumber mill had absolutely no 120V stuff at all.


I'm sure when they moved out, the took the single phase transformer and panelboard.
If he had to run all new stuff it sounds like the place was stripped.


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## lightman (Oct 14, 2015)

This sounds more like a sawmill than a lumber yard! Brian John is correct on the voltage reading that you should see. This was a common type of bank to build if all it was serving was a motor load. There are a few reasons for this type of bank. You can accurately meter a 3 phase service by metering 2 of the 3 phases. Three wire meters were a little cheaper than 4 wire meters. Smart customers, or smart electricians learned that they could connect a dry transformer to the unmetered phase and get unmetered electricity. Grounding the unmetered phase stops this. Another reason was that some feel that a grounded system is more resistant to lightning. I always found that not grounding anything on a delta system serving only motor loads was more resistant to lightning. But I was just a dumb lineman!


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## JRaef (Mar 23, 2009)

dielectricunion said:


> What is the purpose of a corner grounded delta configuration? It seems pointless.
> 
> I was checking out a friend's new storefront that used to house a lumberyard. His son, who he said is an electrician, had setup a subpanel feeding mostly 120v receps, etc. It wasnt until he ran everything that he realized 240 was the only voltage available on that service.
> 
> ...


Yes it is, corner grounded delta and as was said, you are going to need a separate single phase transformer to give you 120V loads. if you want to do it right, find a 240-208Y120 3 phase transformer so that you can balance the loading across all three phases. 

As suggested, most likely the previous tenant had to supply that themselves when the place was built originally so felt entitled to take it with them when they left. Not too uncommon, although in a lot of leases now the landlords will stipulate that anything hard wired, other than machinery, is considered permanent site improvements and must be left behind. Wasn't always that way though.


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## Corysan (Jan 20, 2017)

Unless your State or local code, or your utility does not allow it, you can have an additional 120 volt service installed. This is much more cost effective depending on the size needed. If you need more than 200A 120/240 volts, the cost becomes prohibitive. The major difference in cost is essentially saving the cost of transformer.


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## lightman (Oct 14, 2015)

After reading the original post again, I think I was wrong about agreeing with the voltage. Its not clear to me if you have a separate ground in the panel or if you are calling B phase ground. If you are reading 240 from A to Ground, 240 from C to ground and 0 from B to ground you should have 480 from A to B, 480 from A to C, 480 from B to C. In this case you will need a 480 to 240-120 transformer.

And I agree, another service for 120-240 would probably be cheaper than a transformer and the associated switch gear.


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## Corysan (Jan 20, 2017)

240V 3ph corner grounded delta will definitely yield:

A-Grd: 240V
B-Grd: 0V (assuming B ph is the grounded phase)
C-Grd: 240V
A-B: 240V
B-C: 240V
A-C: 240V

The phase to phase voltage will not be effected by grounding one phase. Grounding one phase only brings that phase to the same potential as ground. The result is not a divided voltage like with a center tap. Typically, delta secondary provides erratic readings to ground, in this case grounding one phase provides a reference to ground.


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## RePhase277 (Feb 5, 2008)

lightman said:


> After reading the original post again, I think I was wrong about agreeing with the voltage. Its not clear to me if you have a separate ground in the panel or if you are calling B phase ground. If you are reading 240 from A to Ground, 240 from C to ground and 0 from B to ground you should have 480 from A to B, 480 from A to C, 480 from B to C. In this case you will need a 480 to 240-120 transformer.
> 
> And I agree, another service for 120-240 would probably be cheaper than a transformer and the associated switch gear.


That would be the case if you had a single phase center-tapped 240/480 volt service, which I've never seen. This is a 3-phase 240 volt service with one phase grounded, and the readings should be as others have posted.


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## JRaef (Mar 23, 2009)

lightman said:


> After reading the original post again, I think I was wrong about agreeing with the voltage. Its not clear to me if you have a separate ground in the panel or if you are calling B phase ground. If you are reading 240 from A to Ground, 240 from C to ground and 0 from B to ground you should have 480 from A to B, 480 from A to C, 480 from B to C. ...


Think about this a little more. If A to B = 240, B to C = 240, and B is grounded, then A to ground and C to ground is the same potential as A to B or B to C.



Corysan said:


> 240V 3ph corner grounded delta will definitely yield:
> 
> A-Grd: 240V
> B-Grd: 0V (assuming B ph is the grounded phase)
> ...


:thumbsup:


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## lightman (Oct 14, 2015)

Guys, I guess I'm all screwed up and over thinking this! This was not a standard bank for my company although some areas of the company used them on irrigation wells. I scratched my head trying to remember where we might have built one and when I went out to check voltage it had been pulled off at the end of pumping season. So, no joy!

Back to the original post. If this is a 240 bank, the electrician might could get the power company to move the ground from the corner to the mid point, thus having 240-120. Depending on whether any other customers are on this bank or not. He may have to change the meter socket to a 4 wire socket as its probably a 3 wire socket. Just a thought.


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## Southeast Power (Jan 18, 2009)

JRaef said:


> Yes it is, corner grounded delta and as was said, you are going to need a separate single phase transformer to give you 120V loads. if you want to do it right, find a 240-208Y120 3 phase transformer so that you can balance the loading across all three phases.
> 
> As suggested, most likely the previous tenant had to supply that themselves when the place was built originally so felt entitled to take it with them when they left. Not too uncommon, although in a lot of leases now the landlords will stipulate that anything hard wired, other than machinery, is considered permanent site improvements and must be left behind. Wasn't always that way though.


I think it's going to be quite a hunt to find a Delta Wye transformer in that configuration.
I like that first suggestion of using a single phase transformer for those receptacles. I doubt a small office would ever see 10kw of load of 120 volts.
A 10kw single phase transformer is easy to find.
I would keep a that original panel as an Equipment only panel and setup a separate panel for the 120 loads.
It will work quick and easy that way.


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## Corysan (Jan 20, 2017)

The worst transformer I ever had to find was a 400 kVA, 480V delta primary, 600V wye secondary. A local Crescent Electric Supply branch got it from MGM transformer, I think. It had to be custom wound. Other companies offer it standard as a step-down. Shipping was something like maybe two weeks, and the cost was cheaper than a major brand's step down. It was not a big deal at all. I could be wrong, but we have gotten 240V Pri. 120/208V Sec. regularly in the past without hiccups (maybe even off the shelf from a nearby distribution center). Again, though, 75% of small factories in my area are powered with 240V corner grounded delta systems.

I agree that single phase would probably suffice, but the EC should do load calculations first. Some municipalities require this. It is still probably much less expensive to install a new single phase service. If the three phase is unnecessary, it could be removed. It could be also be kept and utilized for air conditioning loads. It is common practice here to have 240V three phase services with 120/240V services provided by the utility to the same customer.

Having a utility re-tap their transformer may be the most difficult route to take. Our local utility provider will not install anything but a wye system now for three phase, unless they need an emergency repair to an existing service. High leg systems are even more irregular and awkward to deal with due to the three different voltages available in the system.The only places I see high leg systems from a utility are for stand alone lift stations or sewage grinder installations at waysides. These are decades old now, so that is no evidence for the utility still providing it. It's good practice to consider all options and think out of the box, though.


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## RePhase277 (Feb 5, 2008)

Some utilities will charge you if they add or change a transformer for a new commercial service.

Acme makes 240 delta - 208/120Y three phase transformers in the standard sizes. Same for single phase. I'd think I'd drop a 15 kVA single-phase in for the receptacle load. All the A/C equipment, water heater, anything 240 could be used on the original service. Even the fluorescent or LED lights could be pulled from the original service. Pretty much the only thing that needs 120 is receptacle loads and a 15 kVA is likely more than enough.


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## Southeast Power (Jan 18, 2009)

We have open Delta everywhere here.
This one gets tapped up on the 28th:


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## zac (May 11, 2009)

Suncoast Power said:


> We have open Delta everywhere here.
> This one gets tapped up on the 28th:


Why are the service drops so low? 


Suncoast Power said:


> We have open Delta everywhere here.
> This one gets tapped up on the 28th:



Sent from my SM-G930T using Tapatalk


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## Corysan (Jan 20, 2017)

Suncoast, what's the grounded conductor for? The open delta I am familiar with is three wire three phase with only two windings providing power. Is one winding center-tapped for 120V like a high leg?


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## brian john (Mar 11, 2007)

3700545]Suncoast, what's the grounded conductor for? The open delta I am familiar with is three wire three phase with only two windings providing power. Is one winding center-tapped for 120V like a high leg?[/QUOTE]


I think this will do you in regards to your question

https://www.google.com/search?q=Schematic+of+grounded+open+delta+transformer&biw=1231&bih=666&tbm=isch&imgil=dIqV9M3vf-RSdM%253A%253BGo0sLWYxD9teSM%253Bhttp%25253A%25252F%25252Fwww.industrial-electronics.com%25252Felecy3_19.html&source=iu&pf=m&fir=dIqV9M3vf-RSdM%253A%252CGo0sLWYxD9teSM%252C_&usg=__lFN0GP_3Fn3j151Zncz30zdsIUY%3D&ved=0ahUKEwiu5MGN-qjSAhXC6iYKHWHEBy4QyjcIJQ&ei=h0SwWO6ELsLVmwHhiJ_wAg#imgrc=SmfsGMNo8QaVEM:


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## RePhase277 (Feb 5, 2008)

Corysan said:


> Suncoast, what's the grounded conductor for? The open delta I am familiar with is three wire three phase with only two windings providing power. Is one winding center-tapped for 120V like a high leg?


It's the same as a delta high leg just one less transformer.


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## Southeast Power (Jan 18, 2009)

brian john said:


> 3700545]Suncoast, what's the grounded conductor for? The open delta I am familiar with is three wire three phase with only two windings providing power. Is one winding center-tapped for 120V like a high leg?


 
I think this will do you in regards to your question

https://www.google.com/search?q=Schematic+of+grounded+open+delta+transformer&biw=1231&bih=666&tbm=isch&imgil=dIqV9M3vf-RSdM%253A%253BGo0sLWYxD9teSM%253Bhttp%25253A%25252F%25252Fwww.industrial-electronics.com%25252Felecy3_19.html&source=iu&pf=m&fir=dIqV9M3vf-RSdM%253A%252CGo0sLWYxD9teSM%252C_&usg=__lFN0GP_3Fn3j151Zncz30zdsIUY%3D&ved=0ahUKEwiu5MGN-qjSAhXC6iYKHWHEBy4QyjcIJQ&ei=h0SwWO6ELsLVmwHhiJ_wAg#imgrc=SmfsGMNo8QaVEM:[/QUOTE]

They set two pots on the pole or set two single phase pad mount transformers. Most of the time one is larger than the other and is center tapped. The power one is usually a bit smaller


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## Southeast Power (Jan 18, 2009)

I would suspect that if we were increasing the service to 900 amps, they would just come out and set a third pot and give us a closed Delta.


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## Forge Boyz (Nov 7, 2014)

We have a job coming I that the poco is giving us a 480/240 open delta service. They originally said they would give a 480/277 wye but when they came out to the site they discovered they only had 2 phases.

Sent from my XT1585 using Tapatalk


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## Ultrafault (Dec 16, 2012)

Forge Boyz said:


> We have a job coming I that the poco is giving us a 480/240 open delta service. They originally said they would give a 480/277 wye but when they came out to the site they discovered they only had 2 phases.
> 
> Sent from my XT1585 using Tapatalk


I did not know this was possible. How does this system work?


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## RePhase277 (Feb 5, 2008)

Ultrafault said:


> I did not know this was possible. How does this system work?


It would be a 480 open delta, with one of the transformers center tapped.


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## Forge Boyz (Nov 7, 2014)

Ultrafault said:


> I did not know this was possible. How does this system work?


I worked on one other one. It has two transformers, one center tapped. Basically it's like a 120/240 high leg, just higher voltage. The one I've worked on has one leg about 416v to ground.

Sent from my XT1585 using Tapatalk


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## micromind (Aug 11, 2007)

Forge Boyz said:


> We have a job coming I that the poco is giving us a 480/240 open delta service. They originally said they would give a 480/277 wye but when they came out to the site they discovered they only had 2 phases.
> 
> Sent from my XT1585 using Tapatalk


This is one of the the main reasons for an open ∆.


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## varmit (Apr 19, 2009)

There seems to be more confusion about corner grounded systems and high leg systems than any other subject on this forum. Neither are complicated, but high leg systems require a little more thought when connecting loads. Especially 120 volt loads on a 240/120 high leg service. 

A corner grounded delta system is very common in older industrial facilities. These were usually equipment power only, with either another service, or on site transformer(s) for general 120 volt lighting and outlets. 

WHEN WORKING AT A LOCATION, ALWAYS CHECK/TEST TO SEE WHAT TYPE SYSTEM YOU ARE DEALING WITH. Wear your PPE when doing this.

Note: Names will vary regionally. Voltages listed are nominal rating. This is a basic description of what you should find.

Delta, open or ungrounded 230 (or 460) volt
A to B 230 (460)
B to C 230 (460)
A to C 230 (460) 
Any phase to ground will read random values. Usually different on each phase.

Corner grounded Delta (One phase conductor is connected to the grounding electrode system. Normally the grounded conductor is A or B phase- but you never know until you test.
A to B 230 (460)
B to C 230 (460)
A to C 230 (460) 
A to ground 230 (460)
B to ground 0 (0) grounded phase conductor in this example
C to ground 230 (460)

High leg system ( Usually 240/120, but I have seen other voltages on rare occasions.) The B phase position is normally the high leg- but always verify. This system has a "neutral" (grounded conductor) for use on 120 volt loads connected to THE NON HIGH LEG PHASES.
A to B 240 
B to C 240 
A to C 240 
A to ground 120
B to ground 208 (high leg phase conductor in this example)
C to ground 120 

Resistance grounded ( A large resistor is connected to the supply transformer center tap and connected to the grounding electrode system. This helps to limit fault current to an acceptable level on larger services.Usually 460 volt or higher.)
A to B 460
B to C 460
A to C 460
All phases to ground will read a random value that will be about the same on all three phases.


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## Corysan (Jan 20, 2017)

Thanks for the links. I found the same ones earlier, too. That's exactly what i was figuring it was. I Never would have imagined a utility company would do something like that. The only time I have installed open delta was for a particularly tight (squeaks when he walks) client. He needed 208V three phase for his machine tools, so my boss had me install a couple of buck transformers to get there. I am curious how it effects the tools electronic controllers.

This is where being an electrician becomes interesting, when we deal with the oddball stuff.


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## brian john (Mar 11, 2007)

Corysan said:


> Thanks for the links. I found the same ones earlier, too. That's exactly what i was figuring it was. I Never would have imagined a utility company would do something like that. The only time I have installed open delta was for a particularly tight (squeaks when he walks) client. He needed 208V three phase for his machine tools, so my boss had me install a couple of buck transformers to get there. I am curious how it effects the tools electronic controllers.
> 
> This is where being an electrician becomes interesting, when we deal with the oddball stuff.


The only time a buck boost can mess up controllers (IN MY EXPERIENCE) is if they need 120 as well as 208


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## Southeast Power (Jan 18, 2009)

brian john said:


> The only time a buck boost can mess up controllers (IN MY EXPERIENCE) is if they need 120 as well as 208


Had a similar problem years ago. I took a chance and tried running a large fan without the transformers and it was just fine. Interestingly enough, the overloads were sized for the lower voltage.
It was like the turtle on top of a fence post. I can see it was there but, wondered why someone would do that.


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