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Joined: Feb 2002
Posts: 36
Locating my source, I only noticed 2 transformers, yet I wanted to verify that the drop was (as I was told correctly) indeed 3-phase 240v. I measured 240v.between phases A,B&C, but when I measured between V-lines and neutral, A=120v. B=120v. and C=neighborhood of 218v. A couple of days later, a TXU field operator told me that was an open loop Delta configuration. Tonite, I cracked open an old text of mine and indeed found out that 3 wire 3 phase service can be derived from 2 transformers. Of course I'm only running A,B, & neutral for 240/120v. service. But the textbook drawing only shows 3 wire service. Can anyone tell me or hopefully show me a diagram how a grounded neutral can be added to obtain the voltages I encountered?

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Joined: Nov 2000
Posts: 2,148
Take a look at the drawing that Scott provide for this setup:

There are drawings in this forum for all of the transformer hook-up that you will ever see and some for ones that you will probably never see.


Joined: Jul 2001
Posts: 599
JBD Offline
There are several other names for your 120/240V 3 phase 4 wire distribution system, they include: wild-leg, high-leg, and christmas tree. The most common construction is two single phase transfomers connected in an open-delta configuration.

One transformer is siginificantly larger than the other. The large unit is sized to provide 3 phase and 1 phase loads, and has a center tap, so you can get 120/240 single phase from it. The other, smaller, unit is there to provide 3phse loads only.

Your strange voltages come from the delta configuration. On the utility side of the meter these are A-B 240V, B-C 240V, C-A, 240V, A-N 120V, B-N 120V, and C-N 208V. On the customer side of the meter (the area covered by the NEC) the B and C phases are swapped, so B-N is 208V and A-N-C is 120/240V single phase.

Never connect single phase loads to the high-leg.
Always use 240V breakers (not the standard 120/240V ones) when connecting any 2 pole loads to the high leg.

My personal preference is to install (1) 3-phase panel (without a neutral bar) for the 240V loads and a seperate 1-phase panel for the 120/240V loads. This prevents a lot confusion when adding breakers, because you don't have to skip the high-leg bus bars.

Joined: Feb 2002
Posts: 36
Thank you, Resqcap & JBD for the info...I already understood the common three secondary Delta windings set-up with a grounded neutral tapped on one of the winding's midpoints, but 3 phase derived from only two xfmr's was new to me. Luckily, a TXU field engineer lent me two good transformer manuals. Both are dated but are still excellent. One is the "Transformer Study Manual" by Westinghouse,1952 and the other is "Distribution Transformer Manual" by GE's Power Delivery and Control group. I'm now going to go check out Scott's diagram as you suggested. Thanks again, Burns M.

Joined: Feb 2002
Posts: 4
Junior Member
Is this the standard Delta with a "High Leg" or "booger leg"? The diagrams would appear to confirm that. While you are on this, in the oil fields of Oklahoma we have a "grounded" 3-phase that the third phase is directly connected to the ground rod, either here or somewhere else please explain what and how this works? We worked the oil field occasionally but never researched the detail differences.

Joined: Oct 2000
Posts: 2,723
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Broom Pusher and
3rd degree,

The Open Delta will have the same output Voltage as the Closed Delta - in either 3 or 4 wire arrangements.

If you look at the Open Delta's schematic, you will notice that it's just a Closed Delta with one winding removed. Therefore the High Voltage Line is still at the top of the Triangle, and still equals apx. 208 VAC to ground [Grounded Conductor or EGC].

Did you check out the posted Transformer Schematics? Best to select a search for all items posted [over 1 year] to see the assortments.


Ya, it's a common type Delta. Instead of using 3 Transformers, it uses 2 Transformers. It is still a 120/240 VAC 3 phase 4 wire Delta system - just an Open Delta.
Being an Open Delta, it has a lower overall KVA rating as compared to the Closed Delta.
Capacity is roughly 83% of the Closed Delta.

The Closed Delta system can run as an Open Delta system if needed - due to one Transformer's failure.
With one Transformer out, it can continue to supply 3 phase loads - but at a reduction of 58% the original KVA rating.

As to the Corner Grounded Delta 3 wire system you mentioned, that's a common type Animal!

It functions as would any other type of Grounded AC system. Ground one -AND ONLY ONE- Line Conductor from the Transformer's Secondary, plus bond it to the Equipment Grounding Conductors [EGC]. This creates a stabilized Voltage to ground, plus a low Impedance path for ground fault currents to flow back to the source [Transformer].

A system can have any conductor grounded, but normally the conductor that allows the system to establish the lower voltage to ground will be used. That one is the center tapped "Neutral" on 1 phase 3 wire setups, or the common connection point of three Secondary coils on a Wye system.

Best if you viewed the before mentioned Transformer Schematics in order to see what I am getting at.

Anyhow, let's use a simple 1 Transformer - 1 phase 3 wire setup - where the Secondary coil is wound for 240 VAC and the center point of that winding is physically tapped - which gives us a 120/240 3 wire output.

Typically the center tapped conductor [AKA the "Neutral"] becomes the Grounded Conductor, since the highest Voltage it has is 120 VAC. The Voltage on the center Tap is limited to 120 VAC because it can only reference from the mid point of the winding to either end - hence it splits the coil's voltage by 1/2, so the Voltage on the Center Tap is 120 VAC.
Grounding it makes the highest Voltage to Ground 120 VAC.

If I was to remove all Ground Bonding from the Center Tapped Neutral conductor [at the Transformer and at each service fed from the Transformer], the system is now Ungrounded.

There's still a Voltage To Ground, which is derived from Capacitive Coupling [this has been covered in other areas of ECN]. At various points along the system, the voltage from any of the 3 Secondary conductors can rise to levels between 40 VAC upto 1000 VAC!

If you Ground Faulted one conductor, there would be no sparks, no flames, nothing as would be the typical ground fault. The level of current that can flow depends on the "Loop Impedance", and consequently, the derived Voltage to ground depends on the "Loop Impedance" between the Capacitively Coupled points.
Currents less than 1 ampere would flow.

Now if you place something in series with the loop, having a high Impedance level - such as a person, the Voltage built up across that Impedance can rise very high. A person could easilly get killed from a shock to ground on an ungrounded system.

Now back to the Transformer example!

We have an ungrounded system, 1 phase 3 wire 120/240 VAC.
Let's connect all the ground bonding to one end of the secondary coil - for example X2.

The system will function normally and all connected loads will operate properly - just as if nothing has been changed.
The problem is now we have a maximum Voltage to ground of 240 VAC.

If someone contacts the center tapped neutral, the Voltage to ground will still be 120 VAC.
However, if someone makes contact with the other end of the coil [X1], the voltage to ground is 240 VAC!

On your 3 wire Delta, there's only one available voltage, so grounding one line is proper grounding technique.
The voltage to ground will be 240 VAC from either one of the ungrounded conductors, and nearly zero from the grounded conductor.

Hope this makes sense!

Scott SET

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Apr 2002
Posts: 2,527
"Open Loop Delta" may be a local colloquilaism.

Ref: distribution-transformer configurations—

p.7 Delta-Delta for Light and Power
p.9 Open Delta for Light and Power
p.11 Y-Delta for Light and Power
p.12 Y-Delta With One Unit Missing

[primary voltage 0.48-34.5kV]

Also discussed at:

[This message has been edited by Bjarney (edited 04-13-2002).]

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