How Does Delta High-Leg get 208 to Neutral?

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teufelhounden91

Senior Member
Location
Austin, TX, USA
I'm new to the forum, but am a Journeyman Electrician in Austin, TX. I'm starting to learn about transformers and I've searched the hell out of this site for an explanation that I understand but to no avail...

My Question:

How does a Delta High Leg get 240V from leg to leg, but get 208V from the High Leg to Ground, when the other Legs get 120V to Ground?

I know there's a calculation - 120V x 1.73 = 208...but that equation doesn't explain the HOW I guess. Every diagram I see of Delta transformers make visual sense to me...but how does it happen? If there's 240V from A to B, and B to C...shouldn't all 3 legs have the same voltage to ground?

I still don't really know why Wye transformers have a ground and Delta's do not so maybe that's where my lack of understanding lies.

Thanks for any info you have!
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
How good are you with the Pythagorean formula?

Hypotenuse of triangle = 240V L-L
Short leg of triangle = 120V L-N
Missing Leg = ??V High L-N
 

broadgage

Senior Member
Location
London, England
And BTW, some delta transformer secondaries are grounded, look up "corner grounded delta"
The advantage of such a system is that three phase power is available for motors, but because one wire is grounded, only 2 pole circuit breakers and switches are needed, rather than 3 pole.
 

kwired

Electron manager
Location
NE Nebraska
And BTW, some delta transformer secondaries are grounded, look up "corner grounded delta"
The advantage of such a system is that three phase power is available for motors, but because one wire is grounded, only 2 pole circuit breakers and switches are needed, rather than 3 pole.
The "delta high leg" system is grounded also otherwise there would be no "high leg" it is just grounded at a different point in the system.
 

teufelhounden91

Senior Member
Location
Austin, TX, USA
And BTW, some delta transformer secondaries are grounded, look up "corner grounded delta"
The advantage of such a system is that three phase power is available for motors, but because one wire is grounded, only 2 pole circuit breakers and switches are needed, rather than 3 pole.
Is it always the "high" leg that gets corner grounded?

Thanks for the info
 
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jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Is it always the "high" leg that gets corner grounded?
The high-leg is never grounded, if it was it wouldn't be high.

The NEC requires that the B leg be the high leg in new construction (and it hass for many decades), utilities also tend to use the B leg as the high one, except in their meter sockets, where it occupies the "c" position.

For corner grounded system, the most common location is the B phase. In fact many peopleand a lot of literature call these systems "Gounded-B Phase" instead of the more accurate 'corner grounded'
 

teufelhounden91

Senior Member
Location
Austin, TX, USA
No. You won't have a high leg on a corner grounded delta.
Now I'm confused. So what is the benefit/difference/reason for corner grounding a delta transformer. Is it just about generating a Neutral? How does the high leg disappear?

Again, sorry for the ignorance but I appreciate you hanging with me on this...
 

teufelhounden91

Senior Member
Location
Austin, TX, USA
The high-leg is never grounded, if it was it wouldn't be high.

The NEC requires that the B leg be the high leg in new construction (and it hass for many decades), utilities also tend to use the B leg as the high one, except in their meter sockets, where it occupies the "c" position.

For corner grounded system, the most common location is the B phase. In fact many peopleand a lot of literature call these systems "Gounded-B Phase" instead of the more accurate 'corner grounded'
Thank you...so under normal circumstances is the Delta configuration is ungrounded? Is this corner grounding deal an exception to the standard Delta? And if the B leg is the high leg, and the B leg is the "Grounded B Phase" then isn't that contradictory if you're saying the High Leg is never grounded?

Also, does the corner grounding have anything to do with classifying it an "open-delta" vs "closed-delta" ?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Now I'm confused. So what is the benefit/difference/reason for corner grounding a delta transformer. Is it just about generating a Neutral? How does the high leg disappear?

Again, sorry for the ignorance but I appreciate you hanging with me on this...
It's either/or. You don't have both. If it is corner grounded, you don't have a neutral. If you have mid point of one phase grounded, you have a neutral.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Thank you...so under normal circumstances is the Delta configuration is ungrounded? Is this corner grounding deal an exception to the standard Delta? And if the B leg is the high leg, and the B leg is the "Grounded B Phase" then isn't that contradictory if you're saying the High Leg is never grounded?

Also, does the corner grounding have anything to do with classifying it an "open-delta" vs "closed-delta" ?
A delta is simply a delta - regardless what many people want to think, based on their limited exposure to these, there is no single normal connection.

Open delta and closed delta are simply variations in how many windings are used to create the system, just like an open-wye is a variation on a 'complete' wye

Not that many years ago that many delta systems were installed as ungrounded systems. However, there are problems with ungrounded systems so the industry starting moving towards grounded systems. The easiest way to ground a delta is at a corner, which also happens to be convenient in converting/upgrading ungrounded systems.

A high-leg system is a hybrid between a standard 3-wire single phase and a 3-phase system. They were very popular on rural farms where the predominant building load was single phase, but the milking parlor required 3-phase for the pumps and in the well pumping industry where the major load was 3-phase but some 120V power was required.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
utilities also tend to use the B leg as the high one, except in their meter sockets, where it occupies the "c" position.
How do we know which is the b phase? Utilities use any phase as the high leg. It is on the c phase position in the meterbase, thus that phase becomes the "C" phase. That is the arbitrary way we know (or call it) as c phase. It is considered grounded b phase because it is placed in the b phase position in the breaker panel. A B and C are arbitrary designations, as are x y and x. we can ground "A" phase, when it gets in the meter base, it is "C" phase, and when it gets into the breaker panel, it is "B" phase.

Of course, some panels are single phase panels utilizing double pole breakers. The leg on the neutral bar is grounded leg of the delta system.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Thank you...so under normal circumstances is the Delta configuration is ungrounded? Is this corner grounding deal an exception to the standard Delta? And if the B leg is the high leg, and the B leg is the "Grounded B Phase" then isn't that contradictory if you're saying the High Leg is never grounded?

Also, does the corner grounding have anything to do with classifying it an "open-delta" vs "closed-delta" ?
Open delta is two transformers, closed delta is three transformers. although a high leg can be on a bank with three transformers with a grounded center. We won't get into the ratio relationships...
 
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