High Leg Delta

[tortuga]

• Three-phase: four-wire 240/120 open delta* volts nominal.

*The service size for this three-phase four-wire 240/120 volt open delta connection shall be
limited to a total of 200 amps per transformer bank. An open delta connection can create
current and voltage unbalances that could potentially damage the Customer’s equipment.
Certain types of equipment, such as motors and refrigeration compressors, are more suscep-
tible to damage with this type of connection. Upgrades of existing delta connections beyond
200 amps are classified as a non-standard voltage connection.

ITs interesting, Seattle city light says the same thing where they only list "open delta" specifically, with apparently no option for a closed delta

We don't do new 'open' delta unless we have to avoid a primary extension.
And I think that is the source of the confusion 'full' delta is still done.

The ABB 'MTP Mini' line is a excellent all in one three phase padmounted transformer that offers self contained units for:
208Y/120, 216Y/125, 460Y/265, 480Y/277, 480∆,
240∆ and 240∆ with 120 volt mid-tap in one phase

 

[ActionDave]

Corner Grounded Delta has all but disappeared.

I've only seen one in my life. It was on service for an irrigation pump. It was a head scratcher for a few minutes.

 

[ggunn]

208 between the high leg and the neutral, that's what I said in my comment.

He was responding to this in post #1: "the reasoning behind it was to get 208v from two hots and not from a hot and a neutral!!"

 

[hillbilly1]

I've only seen one in my life. It was on service for an irrigation pump. It was a head scratcher for a few minutes.

I’ve seen a couple, but they are very rare outside industrial. One was a very old ice plant, the other was an old water treatment plant. Another wasn’t corner grounded, but had a ground detector.

 

[electrofelon]

I've only seen one in my life. It was on service for an irrigation pump. It was a head scratcher for a few minutes.

I thought I saw one once, it was a 600V service in an old mill. Turned out it was an ungrounded with a first fault that I found a little while later while doing some work there.
Later when we got rid of the 600 volt service and set up a new 120/208, I did set up a corner grounded 600 V SDS to refeed the elevator.

 

[kwired]

Standard 120/240V circuit breakers are not rated for 208 L-N circuit interruption, you would need a 277V single pole breaker.

Which should not fit into a 120/240 rated panelboard, so the panelboard also would need to be 277/480 panelboard to be able to accept that breaker.

 

[LarryFine]

Which should not fit into a 120/240 rated panelboard, so the panelboard also would need to be 277/480 panelboard to be able to accept that breaker.

There's nothing wrong with using a higher-rated-voltage breaker. It should fit.

 

[kwired]

I don’t have the experience with this that some of the guys here have, but I remember hearing that, aside from the difficulty with finding a suitable breaker, there is the possibility of transformer overload, if there is an open delta configuration, and there was discussion last time this came up, and I can’t find it now, that the voltage will very often not be stable.

Im not exactly sure why, but hopefully one of the members who knows more about this than me can chime in.

As far as pure function, yes, you can run, say, a 208V single phase motor with a high leg and a grounded conductor. The motor doesn’t care if it’s hot and grounded or hot and hot.

Whether it will work in a facility with other loads on the transformer is what I don’t have the experience to really dive into.

Voltage will be stable. What the risks are is overloading portions of the transformer, especially with an open delta. A circuit connected high leg to neutral, will flow from the souce high leg terminal, through the "B" conductor, through the load, through "N" conductor to the N terminal at the source, then through half the A-C winding plus whichever side of the delta is closed back to B terminal. The current from the high leg doesn't balance out like straight 120/240 single phase or like it does on a Wye connected supply A small amount of load like this can be tolerated but if you already have a fair amount of load on this system you throw the balance off the more load you put B to N. These systems generally were only intended to supply 120/240 single phase and a limited amount of three phase anyway as the stinger pot is usually lesser capacity then the 120/240 pot, so that can increase overloading on the stinger pot as well if you start putting more load on it, whether it be B-N, straight 240 single phase loads or actual three phase loads.

If you have a full delta your B-N current will split both ways through the delta, but depending on other loads may not split evenly and that kind of becomes impossible to predict.

 

[kwired]

There's nothing wrong with using a higher-rated-voltage breaker. It should fit.

Maybe, I mostly install and/or run into existing Square D - particularly for 480/277 volts. None I have ever seen will physically fit into a 120/240 rated panel. Maybe other Manufacturers do?

 

[LarryFine]

Mayhaps I should have said "if it fits."

 

[electrofelon]

Maybe, I mostly install and/or run into existing Square D - particularly for 480/277 volts. None I have ever seen will physically fit into a 120/240 rated panel. Maybe other Manufacturers do?

for Siemens their BQD line, which is their "common" 277/480 line fits and is interchangeable with their BL (240V) breakers. However i believe their GB line (600V) and the newer 3VA line take a different finger despite still being 1" wide per pole. Not sure how eaton or GE does it.

 

[Seven-Delta-FortyOne]

Voltage will be stable. What the risks are is overloading portions of the transformer, especially with an open delta. A circuit connected high leg to neutral, will flow from the souce high leg terminal, through the "B" conductor, through the load, through "N" conductor to the N terminal at the source, then through half the A-C winding plus whichever side of the delta is closed back to B terminal. The current from the high leg doesn't balance out like straight 120/240 single phase or like it does on a Wye connected supply A small amount of load like this can be tolerated but if you already have a fair amount of load on this system you throw the balance off the more load you put B to N. These systems generally were only intended to supply 120/240 single phase and a limited amount of three phase anyway as the stinger pot is usually lesser capacity then the 120/240 pot, so that can increase overloading on the stinger pot as well if you start putting more load on it, whether it be B-N, straight 240 single phase loads or actual three phase loads.

If you have a full delta your B-N current will split both ways through the delta, but depending on other loads may not split evenly and that kind of becomes impossible to predict.

Appreciate you explaining that.

 

[electrofelon]

Voltage will be stable. What the risks are is overloading portions of the transformer, especially with an open delta. A circuit connected high leg to neutral, will flow from the souce high leg terminal, through the "B" conductor, through the load, through "N" conductor to the N terminal at the source, then through half the A-C winding plus whichever side of the delta is closed back to B terminal. The current from the high leg doesn't balance out like straight 120/240 single phase or like it does on a Wye connected supply A small amount of load like this can be tolerated but if you already have a fair amount of load on this system you throw the balance off the more load you put B to N. These systems generally were only intended to supply 120/240 single phase and a limited amount of three phase anyway as the stinger pot is usually lesser capacity then the 120/240 pot, so that can increase overloading on the stinger pot as well if you start putting more load on it, whether it be B-N, straight 240 single phase loads or actual three phase loads.

If you have a full delta your B-N current will split both ways through the delta, but depending on other loads may not split evenly and that kind of becomes impossible to predict.

Ok but we add load to electrical systems all the time without letting the utility know or doing any calcs or analysis, so why does no one blink an eye adding 30 amps to a wye or even two phases of a open delta, but we have to be super cautious adding load to the high leg and neutral of an open delta?

 

[kwired]

I’ve seen a couple, but they are very rare outside industrial. One was a very old ice plant, the other was an old water treatment plant. Another wasn’t corner grounded, but had a ground detector.

That would have been an ungrounded system.

I've only seen one in my life. It was on service for an irrigation pump. It was a head scratcher for a few minutes.

I can think of maybe up to half a dozen utility supplied corner grounded systems that are still in use in this area. None of which would be supplied by that today if they were new installs though.

 

[ggunn]

Ok but we add load to electrical systems all the time without letting the utility know or doing any calcs or analysis, so why does no one blink an eye adding 30 amps to a wye or even two phases of a open delta, but we have to be super cautious adding load to the high leg and neutral of an open delta?

Perhaps it's because when we add a three phase load the VA is shared by all the transformers, and when we add a split phase load to A and C the VA is handled by a much larger center grounded (dual coil) transformer. When we add a B to N load to an open delta service all the VA is furnished by a single transformer that is usually much smaller and more likely to be overloaded.

 

[tortuga]

Ok but we add load to electrical systems all the time without letting the utility know or doing any calcs or analysis, so why does no one blink an eye adding 30 amps to a wye or even two phases of a open delta, but we have to be super cautious adding load to the high leg and neutral of an open delta?

I think we covered the topic of adding loads across the open jaw in this thread:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

and Syncro summed up the math here:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

using the Hileg to - neutral in a open delta would be pretty similar to using the open phase.

 

[hillbilly1]

That would have been an ungrounded system.

I can think of maybe up to half a dozen utility supplied corner grounded systems that are still in use in this area. None of which would be supplied by that today if they were new installs though.

That’s what I said. Delta 480 systems are not that common anymore.

 

[ggunn]

That’s what I said. Delta 480 systems are not that common anymore.

The only one I have encountered was on a farm; it was a corner grounded 480V service for a well pump, and it was years ago when I was very new to the electrician side of electrical engineering. I tested the phase voltages to ground and thought that one of the phases was dead.

 

[electrofelon]

I think we covered the topic of adding loads across the open jaw in this thread:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

and Syncro summed up the math here:

Trouble finding straight 240v breaker vs 120/240 breaker

There is no real derating necessary for using the 'open' phase. Typical formula for sizing the unequal transformers are: Power pot = .58T Lighting pot = .58T + S Where T is the total three phase load and S is the total single phase load. Note how the transformer sizing takes into account the...

forums.mikeholt.com

using the Hileg to - neutral in a open delta would be pretty similar to using the open phase.

Right and like I said in that thread, we usually don't even know which is the open phase, I've never worried about it, never seen a problem. Just to me these issues seem like something we are concerned about on forums, but in practice it's not an issue and no one ever worries about.

 

[ggunn]

Right and like I said in that thread, we usually don't even know which is the open phase, I've never worried about it, never seen a problem. Just to me these issues seem like something we are concerned about on forums, but in practice it's not an issue and no one ever worries about.

Wouldn't the open phase be the one that is 208V to neutral? Several times I have encountered a service like this when applying with the POCO to install a PV system where they wanted me to install it single phase on A and C.