High Leg Delta Transformer Circuit – Connection and Calculation

High-voltage power is transferred over vast distances. It must be stepped down to 120V or 230V single phase to provide domestic and residential applications.

This transformation takes place within a transformer. The center of any one of the phases is center-tapped and grounded if the secondary of this transformer is delta.

This permits a single transformer to supply 120V single phase for single-phase loads and 240V three-phase for three-phase loads.

What is High Leg Delta

The high-leg delta (also known as the wild-leg, stinger leg, bastard leg, high-leg, orange-leg, red-leg, and dog-leg delta) is a three-phase electrical service connection.

It’s employed when a three phase transformer needs to supply both single and three-phase power (or transformer bank).

Three-phase power is linked in a delta configuration, with one phase’s center point grounded. This results in a split-phase single-phase supply (L1 or L2 to neutral in the diagram above) as well as a three-phase supply (L1-L2-L3 at right).

The cable is color-coded orange, hence the name “orange leg.”

Regardless of the transformer’s L1-L2-L3 classification, the high leg is normally installed in the center (B phase) lug in the associated panel.

Later with NEC 2008 revision, the high leg delta is assigned to C phase.

high leg delta 1

Furthermore, if you simply need a 208V system, High leg delta is not advised because you may get it with a Wye-Wye, three phase four wire system that offers 120V and 208V, single and three phase power.

The electric power supply business builds three transformers to deliver 120V, 208V, and 240V (1 & 3 Phase) supply voltage levels dependent on end user requirements for high Leg delta supply.

The 4.5k-7.2kV supply is connected to the primary side of these three step-down transformers. The transformers then decrease the voltage to the desired 240V, 208V, or 120V for commercial and industrial buildings and installations.

What Phase is High Leg Delta

“Phase B” has typically been the high-leg or phase with the highest voltage measured to neutral.

The upper leg of a four-wire three-phase delta service can now be identified as the “C” phase instead of the “B” phase, thanks to a modification to the 2008 NEC.

A = 0 degrees, B = 90 degrees, and C = 180 degrees are the phase angles (referenced from neutral). This is unlike a standard 3Y-208 or 3D-208 circuit, which has phase angles of 0, 120, and 240 degrees.

What is High Leg Delta Used for

It’s commonly seen in older, rural facilities. 240 V line-to-line and 120 V line-to-neutral are commonly used to provide this type of service.

In some aspects, the high leg delta service offers the best of both worlds: a greater line-to-line voltage than other three-phase services, as well as a sufficient line-to-neutral voltage (on two of the phases) for connecting appliances and lighting.

As a result, huge items of equipment will use less current than when using 208 V, necessitating the use of lower cable and breaker sizes.

Lights and appliances that require 120 V can be connected directly to phases A and C without the use of a step-down transformer.

Even if the system isn’t labeled, it’s easy to spot because the “B” phase (circuits #3 and #4) and every third circuit after that is either a three-pole breaker or a blank.

Currently, single-phase and three-phase loads are served separately, for example, 120 V split-phase (lighting, etc.) and 240 V to 600 V three-phase (for large motors).

Many jurisdictions, however, prohibit the use of more than one class for a single premises’ service, thus the options may be 120/240 split-phase, 208 single-phase or three-phase (delta), 120/208 three-phase (wye), or 277/480 three-phase (wye) (or 347/600 three-phase (wye) in Canada).

High Leg Delta Connection

There are two ways to get high-leg delta service.

First method is to use a three-phase transformer (or three single-phase transformers), which has four wires coming out of the secondary, three phases, and a neutral attached as a center-tap on one of the windings.

Second method is open delta configuration, and needs two transformers.

One transformer is connected to one phase of the overhead primary distribution circuit to provide the ‘lighting’ side of the circuit (this will be the larger of the two transformers).

The second transformer is connected to another phase of the circuit, with its secondary connected to one side of the ‘lighting’ transformer secondary, and the other side of this transformer is brought out as the ‘high leg.’

The magnitudes of the voltages between the three phases are the identical, while the magnitudes of the voltages between each phase and the neutral differ.

Two of the phases’ phase-to-neutral voltages will be half of the phase-to-phase voltage. The phase-to-neutral voltage will be 1/3 of the phase-to-phase voltage remaining. If A-B, B-C, and C-A are all 240 volts, A-N and C-N will be 120 volts, but B-N will be 208 volts.

Wye connections, ungrounded delta connections, and corner-grounded delta (“ghost”) leg configuration connections are all examples of three-phase supply. These connections do not have a high leg and do not produce divided single-phase electricity.

High Leg Delta Transformer Connection

A high leg delta connection is when one of the windings of a transformer’s delta connected secondary is center-tapped and grounded. Red leg connection, wild leg connection, and orange leg connection are all terms used to describe this condition.

By doing so, we are able to create both a split phase and a three-phase delta linked supply.

This is a typical connection in North America, particularly in the United States. We can get two 120V supplies, three 240V supplies, and a 208V supply by employing a high leg delta connection.

High-Leg or Orange Leg refers to the phase having a 208V supply to the ground.

Wherever a connection is made, this conductor must be permanently marked with an orange exterior finish.

WYE or star-connected secondary with the center point grounded is a good substitute for high leg delta. This connection is capable of providing both three-phase and single-phase connections.

High leg delta voltage

Observe the schematic of three-phase four-wire delta connection below:

high leg delta 2

The difference in voltage between the phases,

    \begin{align*}V_{AB}=V_{BC}=V_{CA}\end{align*}

The following formula can be used to calculate the phase to neutral voltage:

    \begin{align*}V_{AN}&=\frac{V_{AB}}{2}\\V_{BN}&=\frac{V_{AB}}{2}\\V_{CN}&=1.743V_{AN}=1.743V_{BN}\end{align*}

If the VAB is set to 240V,

    \begin{align*}V_{AN}&=\frac{240}{2}=120V\\V_{BN}&=\frac{240}{2}=120V\\V_{CN}&=1.743\times120=208V\end{align*}

Typically, the 208V connector is not used.

High Leg Delta Calculations

Consider the low-voltage side of a 120/240 V high leg delta linked transformer, with the ‘high’ leg being the ‘c’ phase.

high leg delta 3

The magnitudes of the line-to-line voltages are all the same:

    \begin{align*}V_{ab}&=V_{bc}=V_{ca}=240V\end{align*}

The line-to-neutral voltages for the ‘a’ and ‘b’ phases are as follows since the winding between them is center-tapped:

    \begin{align*}V_{an}&=V_{bn}=\frac{V_{ab}}{2}=120V\end{align*}

The phase-neutral voltage for the ‘c’ phase, on the other hand, is different:

    \begin{align*}V_{cn}=\sqrt{V_{ac}^2-V_{an}^2}\approx208V\end{align*}

This can be demonstrated by using angle notation to write a KVL equation starting with the grounded neutral:

    \begin{align*}&0+120\angle0^o+240\angle0^o\\=&0+120\angle0^o+240(-5)\angle0^o+240\frac{\sqrt{3}}2{}\angle90^o\\=&0+120\angle0^o-120\angle0^o+240\frac{\sqrt{3}}2{}\angle90^o\\=&240\frac{\sqrt{3}}2{}\angle90^o\\=&120\sqrt{3}\angle90^o\end{align*}

Or:

    \begin{align*}&0+120\sin(0^o)+240\sin(120^o)\\=&0+0+240\frac{\sqrt{3}}{2}\approx207.8\end{align*}

High Leg Delta Color Code

The high leg, which is usually the “B” phase, is required by the NEC Code to be identified by an orange hue (it was sometimes referred to as a red-leg delta) or other effective means.

However, where metering is part of the switchboard or panel board, the high leg can be the “C” phase to fit utility meter designs.

The switchboard or panel board must be permanently marked with readable, permanent field markings, according to the Code modification in this section.

Please adhere to the National Electric Codes, such as NEC wiring color codes or other regional color codes.

We used the NEC + general practice wiring color codes for 120V, 208V, 240V, 1-phase, and 3-phase circuits.

  • Hot 1 or Line 1 = blue
  • Hot 2 or Line 2 = Orange (HIGH LEG DELTA)
  • Hot 3 or Line 3 = black
  • Neutral Wire = White
  • Green = Ground cable with no conductor ( as grounding & earthing)

For three hot wires in a high leg delta three phase system, the suggested colors are Black, Orange, and Blue, with the Orange color indicating High Leg Delta. In other words, among other hot wires, High Leg delta must be the same.

For voltage conductor cable, do not use green, green with a yellow stripe, or a bare conductor. Copper wire, not aluminum wire, should be used in the main panel box wiring to reduce resistance and heat.

High Leg Delta vs Open Delta

To deliver a three-phase supply to the load, an open delta connection transformer uses two single-phase transformers. A V-V system is another name for an open delta connection system.

Because their efficiency is low as compared to delta-delta (closed delta) systems, open delta connection systems are normally only employed in emergency situations (which are used during standard operations).

We shall describe this system with the help of certain numerical values in the following discussion.

Assume you have three 10 kVA single-phase transformers. They are connected in a closed delta system if they are connected in a delta connection (both primary and secondary sides).

How much 3-phase balanced load can this combination provide?

The answer is that this combination can provide a three-phase balanced load of 30 kVA. Each transformer will be loaded to 10 kVA, which means it will be working at full capacity.

Let’s pretend that one of the transformers is damaged and needs to be repaired. The remaining system will now operate in an open delta mode (i.e. in the open delta, we have two single-phase transformers).

Now, how much 3-phase balanced load can this configuration provide?

The answer is that we currently have two 10 kVA single-phase transformers, but we are unable to offer a 20 kVA, three-phase balanced load.

This combination can provide a three-phase balanced load of up to 17.32 kVA. Each transformer will be loaded to 10 kVA, which means it will be working at full capacity.

When compared to a closed delta system, this open delta system will be less efficient. Because both transformers are working at full load (i.e. 10 kVA), their losses will be full load, but the output will be halved (output is 17.32 kVA instead of 20 KVA).

If the output of an open delta system can reach 20 kVA, the efficiency of closed delta and open delta systems will be equal, and instead of three single phase transformers, two single phase transformers may be adequate to provide three phase loads around the world.

As a result, you can continue to supply three phases as an open delta system, but at a lower efficiency.

High Leg Delta Advantages

The system behaves as a split single-phase system if the “high leg” is not used, which is a frequent supply arrangement in the United States.

A single transformer bank may supply both three-phase and single split-phase power.

When the three-phase load is minor in comparison to the overall load, two separate transformers can be utilized instead of three for a “full delta” or three-phase transformer, resulting in a lower cost and a wider range of voltages.

This is known as a “open-delta high-leg,” because it has a lower capacity than a full delta.

High Leg Delta Disadvantages

Load balancing will be poor if the single-phase load is substantially more than the three-phase load.

Three transformers supply the service in most circumstances, two of which are substantially smaller than the third, and the third larger transformer will be center tap grounded.

One of the phase-to-neutral voltages is higher than the other two (typically phase “B”). If single phase loads are connected to the high leg (when the connecting individual is uninformed that that leg has higher voltage), the load will get extra voltage.

This can easily result in the load failing.

When only two transformers are employed, there is usually a high-leg to neutral load restriction. According to one transformer manufacturer’s website, high-leg to neutral loading should not exceed 5% of transformer capacity.

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