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#128636 - 08/17/03 09:08 AM Open Delta
sudsy4 Offline

Registered: 04/24/02
Posts: 32
Loc: Elizabeth,PA USA
Could someone please explain an open delta secondary?

I would think you would get 220/220/440?

How does this connection create a balanced three phase source?

Bill Somerfield

#128637 - 08/18/03 07:01 AM Re: Open Delta
JBD Offline

Registered: 07/12/01
Posts: 599
Loc: WI, USA
First, let me make a statement to ECN members.
We all need to make an effort to use our country's nominal voltages when discussing general circuits (i.e. US -> 120, 208, 240, 480, 600). The NEC requires the use of these voltages when performing general calculations.

Now to answer your question.

The Open-delta transformer arrangement works exactly the same as a closed delta system, as far as creating three-phase voltages that are spaced 120 degrees apart. This spacing provides the standard 240-240-240 voltages you are familiar with. Open-delta transformers can only supply 57.7% of the current, and therefore the KVA, of closed systems.
The only advantage of an open-delta is the cost saving of supplying or the availability only two single phase transformers.

#128638 - 08/18/03 08:52 AM Re: Open Delta
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Voltages for 4-wire delta and 4-wire open delta are the same, where Bø is the {Orange} “high leg”

There are three "phase-to-phase" voltages

  • Aø-Bø 240V
    Bø-Cø 240V
    Cø-Aø 240V

There are three "phase-to-neutral" voltages

  • Aø-N 120V
    Bø-N 208V
    Cø-N 120V

Reference 99 NEC Article 384 — Switchboards and Panelboards §384-3 Support and Arrangement of Busbars and Conductors (e) High-Leg Marking (f) Phase Arrangement Note that Bø is the standard “high leg” per NEC.

Some older utility standards used [and may still require] Cø as the “high leg” in their meter sockets and related equipment.

480V 4-wire delta systems are rare and not generally used on new equipment, or offered by utilities for new service.

ANSI Standard C84.1-1995 Electrical Power Systems and Equipment--Voltage Ratings (60Hz) Table 1 — Standard Nominal Systems Voltage and Voltages Ranges lists a nominal system voltage as 240/120 for 3ø 4-wire delta systems.
The first two diagrams show delta and open-delta secondary connections. Voltage relationships are the same. Open delta is useful for smaller loads.
Compare figures 11 using three 1ø transformers and figure 15 using two 1ø transformers. Both produce the same 240/120V 4-wire ∆ 3ø voltage.

[This message has been edited by Bjarney (edited 08-18-2003).]

#128639 - 08/20/03 12:23 PM Re: Open Delta
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
A 4-wire delta transformer-secondary interconnection using three 1ø transformers is

A 4-wire open-delta transformer-secondary interconnection using two transformers is

Illustration links at

Phase-to-phase voltages and phase-to-neutral voltages are effectively the same for the two transformer arrangements. Both are 3ø systems, but are generally intended to supply smaller 3ø loads and a combination of 1ø loads.

One way of explaining the difference is by the change in voltage from no-load to loaded conditions. Comparing the two systems, if a 1ø 2-wire 240V load is connected to Bø and Cø, there will be less voltage drop with the three-transformer arrangement than in the two-transformer arrangement.

Three-phase induction motors operate most efficiently {meaning with minimal excess heating} when voltage at their terminals is closest to that on their nameplates. In 3ø circuits, there two parts to line voltage—besides the gross quantity of, say, 230V, there is also balance of the three 1ø voltages that must be considered. At 3% unbalance, an induction-motor horsepower load should be derated about 10%. At 5% voltage unbalance, the motor output should be derated by about 25%.

[This message has been edited by Bjarney (edited 08-20-2003).]

#128640 - 08/20/03 12:41 PM Re: Open Delta
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
This is a simplified application—a case where a gas station has a combination of motor loads. There might be a 3ø 7½-hp 230V motor for an air compressor, and four 1ø ½-hp 115V motors in fuel dispensers. The smallest distribution transformers that utilities typically use are around 10kVA. If three transformers are used, the bank capacity is 30kVA, and is able to serve a load of about 30hp. For a single customer that may be a little wasteful, so instead two 1ø transformers could be used in the open-delta arrangement, and serve a load of 17kVA. Considering that the load for the gas station may be around 12kVA—the utility can adequately serve the gas station while not having to maintain a third transformer over its life of ~25 years.

There is a caveat in all this. In motors intended for dedicated applications like submersible water pumps and hermetic refrigeration compressors, manufacturers recommend not using the open-delta transformer configuration to serve their products. These two types of motors operate at higher current densities that most plain-vanilla NEMA T-frame motors. Consequently, at their rated mechanical load, {output} they are designed ‘closer to the edge’ in terms of heating and overall durability, so are generally more likely to run hotter at a particular degree of voltage unbalance compared to otherwise similar T-frame motors.


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