What is a Delta High Leg?

The Dirty Delta

If you were to think of a Delta configuration, you would imagine the triangle of windings connected from end to end (or in series): A-B, B-C, C-A. We know that in a 120/240V system, we have 240V between two points in this configuration. When we have a center tap, a point in which the Neutral is tapped from the center of a winding, we can measure two points, say A and C, and we will still have our 240V; however, if we measure A or C to N, or neutral, we read 120V. In a Delta High Leg system, we can measure 240V between any two points in our system, but when we take the B phase to neutral, we get 208V instead of 240V. Why do we get this measurement? Why does this system exist? In what scenario is this useful? Today, we will be answering these questions and more, so sit back with a tall glass of your favorite beverage, stretch your legs in the Porta-John, or add another pillow under your head as we dive deep into our understanding of the Delta 4-Wire High Leg system!

Secondary Wiring of a High-Leg Delta Transformer

What Is The Purpose?

Why do we have this system in which we can derive 120V, 240V, and 208V? The simplest answer we can offer is that it allows us to have access to both 120V Single-Phase and 240V Three-Phase, all in the same system. This is helpful in smaller buildings that require both Single and Three-Phase but don’t have the capacity or financial situation to have more than one panel and transformers. With this system, we can operate 120V receptacles, lighting circuits, 240V receptacles, and other circuits such as hot water heaters or air handlers, and anything requiring a three-phase voltage. We don’t typically use the single-phase 208V for anything, rather it is just a byproduct of the wiring that occurs. As we stated earlier, this is a center-tapped system, meaning the grounded conductor, or neutral, is tapped from the center of the winding. This is how we can obtain the three different voltages for this system; however, we can also have a corner-tapped Delta configuration. In this other system, we typically take the B Phase and ground it, so when we measure A-C, C-B, or B-A, the result is 240V; A and C to ground will yield a measurement of 240V, as well, but B to ground will be 0V. In this instance, the B Phase is not considered a Neutral conductor, but a Grounded conductor. 

How Do I Tell?

No one likes to troubleshoot just to find out what should be visually given. What I mean by this is that when you open a panel and expose the feeders coming in, you should automatically know what kind of system you’re looking at. The requirements of conductor identification are as follows in no particular order:

200.6 Means of Identifying Grounded Conductors

(A) Sizes 6 AWG or Smaller. An insulated grounded conductor of 6 AWG or smaller shall be identified by one of the following means:

  1. The insulated conductor shall have a continuous white outer finish
  2. The insulated conductor shall have a continuous grey outer finish
  3. The insulated conductor shall have three white or grey stripes along the conductor’s entire length on other than green insulation.

(B) Size 4 AWG or Larger. An insulated grounded conductor 4 AWG or larger shall be identified by one of the following:

  1. A continuous white outer finish
  2. A continuous grey outer finish
  3. Three continuous white or grey stripes along the conductor’s entire length on other than green insulation
  4. At the time of installation, by a distinctive white or grey marking at its terminations. This marking shall encircle the entire conductor or insulation.

This excerpt is referencing the grounded conductor, more commonly referred to as the Neutral conductor.

250.119 Identification of Equipment Grounding Conductors.

Unless required elsewhere in this “Code”, equipment grounding conductors shall be permitted to be bare, covered, or insulated. Individually cover or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes except as permitted in this section. Conductors with insulation or individual covering that is green, green with one or more yellow stripes, or otherwise identified as permitted by this section shall not be used for ungrounded or grounded circuit conductors.

(A) Conductors 4 AWG or Larger. Equipment grounding conductors 4 AWG and larger shall comply with 250.119(A)(1) and (A)(2).

  1. An insulated or covered conductor 4AWG or larger shall be permitted, at time of installation, to be permanently identified as an equipment grounding conductor at each end and at every point where the conductor is accessible. Exception: Conductors 4 AWG and larger shall not be required to be marked in conduit bodies that contain no splices or unused hubs.
  2. Identification shall encircle the conductor and shall be accomplished by one of the following:
    1. Stripping the insulation or covering for the entire exposed length
    2. Coloring the insulation or covering green at the termination
    3. Marking the insulation or covering with green tape or green adhesive labels at the termination.

This section is referencing the Equipment Grounding Conductor (EGC) or, as we commonly refer to it in the field, the “ground”.

310.6 (C) Ungrounded Conductors. Conductors that are intended for use as ungrounded conductors, whether used as a single conductor or in multiconductor cables, shall be finished to be distinguishable from, grounded or equipment grounding conductors. Distinguishing markings shall not conflict in any manner with the surface markings required by 310.8 (B)(1). Branch-circuit ungrounded conductors shall be identified in accordance with 210.5(C), Feeders shall be identified in accordance with 215.12.

Both of the above references are similar in nature, so we will paraphrase them together. If a premises wiring system contains more than one nominal voltage system, each ungrounded conductor shall be identified by phase or line and by system voltage class at all terminations, connections, and splice points in compliance with either 210.5(C) for Branch-Circuits or 215.12 for Feeders. Different systems within the same premises that have the same system voltage class shall be permitted to use the same identification. 

  1. Means of Identification. The means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved methods.
  2. Posting of Identification Means. The method utilized for conductors originating within each panelboard or distribution equipment shall be documented in a manner that is readily available or shall be permanently posted at each panelboard or distribution equipment.

What does all that mean? The NEC doesn’t specifically call out colors to be used in a coding system other than the grounding and grounded conductors. Here in the United States, most electricians understand a standard for Three Phase is 120/208V is Black-Red-Blue, 277/480V is Brown-Orange-Yellow. Delta High-Leg is, however, called out to be identified differently in 110.15.

110.15 High-Leg Marking.

On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded, only the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point on the system where a connection is made if the grounded (neutral) conductor is also present.

Some ways this can show up is as Black-Orange-Blue conductors or in some jurisdictions, Black-Orange-Red. In the field, we often refer to this orange conductor as the “stinger”, “high leg”, or “wild leg” of the system. According to the NEC, though, we could use Teal-Pink-Yellow for anything other than a High Leg system. 

Example of Conductor Identification of a High Leg Delta System

Fun Fact: The IBEW is credited with not only standardizing the color code system we know today but also with the color-coding of NM-B sheathing for quick identification of sizes. An example would be white sheath is 14-2 or 14-3, yellow is 12-2 or 12-3. 

The Mathematics

When talking about a three-phase system, most of us know we use the square root of three (√3) to determine the voltage between two conductors. Where does the √3 come from? For this answer, let’s dig into some trigonometry.

Phinding The Phasor

First, we need to understand Kirshoff’s Current Law which states: “The sum of the currents entering a node must equal the sum of the current leaving the same node”. Imagine the A-phase terminal, we can picture the current flowing to it as IBA and the current leaving it as IAC. According to Kirshoff’s Current Law, the A-Phase line current (IA) leaving the delta transformer secondary must be equal to the A-Phase delta phase current (IBA) minus the C-Phase delta phase current (IAC), or IA=IBA-IAC. With that in mind, let’s set our phases onto a phasor diagram. Let’s make IBA set at 0º, making IAC 120º and ICB -120º. By thinking of these phasors (or vectors) into magnitudes, we can flip these by -180º without changing the magnitudes of the vectors. With that rule, we take 120º + -180º and result in -60º. To make the math simple, let’s let the current of A-Phase be one ampere, or IA=1A. Using the same idea that all three-phase currents are equal, so IBA = IAC = ICB = 1A. So we can imagine these magnitudes as a triangle.

 a = -IACcos(-60º)

a = 1cos(-60º)

a = 0.5

We then can take -IAC through this formula.

b = -IACsin(-60º)

b = 1sin(-60º)

b = √3/2 ≈ -0.866

With two sides of our right angle triangle found, we can then apply the Pythagorean Theorem. 

A² + B² = C²

1.5² + -0.866² = 1.732

In Conclusion

I wouldn’t recommend going out of the way to special order all these unique tape colors and tags to mark wires, but you do you. We have structure and standards to level the fields here. Most electricians, and some non-electricians, understand how this color coding works and why it’s important. Another fun fact for your conversation starters: when you’re driving along where distribution lines are run close to the road, you notice transformer sets on landings. If you notice a set of two, that is most likely an Open-Delta configuration to reduce the cost of transmitting power in three conductors. Yet, they are still able to get all three phases at the other end by connecting two of the transformers together to form a third point or leg on the system. Something to ponder and look for in your travels!

Open Delta wiring achieved by connecting two transformers together in the middle – Source: quora.com

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