High‑leg delta (also called a high‑leg three‑phase, wild‑leg, red‑leg, or stinger‑leg system) is a type of three‑phase electric power distribution commonly used in North America for supplying both three‑phase loads and single‑phase loads from the same service. The configuration is a delta (Δ) connected three‑phase winding in which one of the three line conductors is connected to the neutral point of a center‑tapped transformer. This creates a voltage to neutral on two of the phases that is typically 120 V (or 208 V in a 208 V system), while the third phase—the “high leg”—has a higher line‑to‑neutral voltage, usually about 208 V (or 240 V in a 120/240 V system).
Technical description
| Parameter | Typical North American values |
|---|---|
| Phase‑to‑phase voltage | 208 V (for 120/208 V systems) or 240 V (for 120/240 V systems) |
| Two phase‑to‑neutral voltages | 120 V (or 120 V) |
| High leg (phase‑to‑neutral) voltage | ≈208 V (or ≈240 V) |
| Phase sequence | A‑B‑C (any order) |
| Neutral connection | Center tap of one transformer winding, bonded to ground |
The high leg voltage results from the vector relationship of the three phases in a delta: the high leg is the vector sum of the 120 V leg and the 120 V leg that is 120° out of phase, giving a line‑to‑neutral magnitude of √(120² + 120² + 2·120·120·cos 120°) ≈ 208 V.
Typical applications
- Commercial and light‑industrial buildings where both three‑phase equipment (e.g., motors, HVAC) and standard 120 V single‑phase circuits (lighting, receptacles) are needed.
- Older residential constructions that were originally wired for three‑phase service but later required single‑phase convenience outlets.
- Small manufacturing facilities where a single three‑phase transformer can serve both distribution needs, reducing equipment cost compared with separate three‑phase and single‑phase services.
Advantages
- Provides three‑phase power and standard 120 V single‑phase power from a single transformer.
- Simplifies wiring in buildings that need both types of loads.
- Reduces the number of transformers and service entrance equipment compared with separate services.
Limitations
- The high leg cannot be used for standard 120 V loads; appliances designed for 120 V must be connected to the two “low” legs and neutral.
- Loads that mistakenly connect to the high leg may be over‑volted, potentially causing equipment damage.
- Not compatible with many modern three‑phase equipment that expects a true 120/208 V wye system with equal phase‑to‑neutral voltages.
Identification
In the United States, the high leg is conventionally identified by a red‑colored conductor (hence the nickname “red‑leg”). In wiring diagrams it may be labeled “B” (or “C”) with a note indicating the higher voltage to neutral.
Standards and codes
- National Electrical Code (NEC) – Articles 230 and 220 address service entrance equipment and voltage limits for high‑leg delta systems.
- IEEE Std 141 (Red Book) – Provides guidance on transformer connections and voltage calculations for high‑leg delta configurations.
- Canadian Electrical Code (CEC) – Also permits high‑leg delta service in appropriate applications, with similar labeling requirements.
See also
- Wye (Y) connection – The alternative three‑phase transformer configuration that provides equal line‑to‑neutral voltages.
- Phase voltage – The voltage measured between any two of the three phases.
- Line‑to‑neutral voltage – Voltage measured between a phase and the neutral point.
- Center‑tapped transformer – A transformer winding with a tap at its midpoint, used to create the neutral in high‑leg delta systems.
This entry presents established technical information about the high‑leg delta electrical distribution system, summarizing its definition, operation, typical uses, advantages, and constraints as described in standard electrical engineering references and regulatory codes.
