Otis Singleton
A high leg is a term used to describe an electrical system configuration with three phases and one of the phases has a higher voltage than the others. This configuration is often found in older and rural installations, as well as in manufacturing facilities with mostly three-phase motor loads. It is also known as a wild-leg, stinger leg, bastard leg, or wild phase service. This type of service provides a higher line-to-line voltage than typical three-phase services and a sufficient line-to-neutral voltage for connecting appliances and lighting. To identify a high leg, you would typically see a higher voltage reading on one of the phases, usually the 'B' phase, but sometimes the 'C' phase.
| Characteristics | Values |
|---|---|
| Type | Three-phase, 4-wire delta-connected system |
| Voltage | 240V phase-to-phase; 120V phase-to-ground; 208V on the "B" phase |
| Load limit | High-leg-to-neutral loading not exceeding 5% of transformer capacity |
| Usage | Older and rural installations; older manufacturing facilities; residential services |
| Marking | Orange outer finish or other effective means |
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What You'll Learn
High-leg delta systems are often found in older and rural installations
High-leg delta systems are electrical service connections for three-phase electric power installations. They are used when both single and three-phase power is supplied from a three-phase transformer or transformer bank. In a high-leg delta system, the three-phase power is connected in a delta configuration, and the centre point of one phase is grounded. This creates a split-phase single-phase supply (L1 or L2 to neutral) and three-phase (L1–L2–L3).
High-leg delta systems can be identified by their B phase (circuits #3 and #4) and every third circuit afterwards, which will be either a three-pole breaker or a blank. The high leg is usually set in the centre (B phase) lug in the involved panel, regardless of the L1–L2–L3 designation at the transformer. The high leg is also typically identified by an orange colour (or sometimes red) and is often labelled as such to warn personnel about the higher voltage present on the equipment.
High-leg delta systems offer some advantages over other three-phase services. They provide 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. This means that large pieces of equipment will draw less current, requiring smaller wire and breaker sizes. Lights and appliances requiring 120 V can be connected to phases A and C without requiring an additional step-down transformer.
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High-leg systems are also used in Japan
High-leg systems, also known as wild-leg, stinger leg, bastard leg, high-leg, orange-leg, red-leg, or dog-leg delta, are a type of electrical service connection for three-phase electric power installations. They are used when both single and three-phase power is supplied from a three-phase transformer or transformer bank.
In a high-leg system, the three-phase power is connected in a delta configuration, with the centre point of one phase grounded. This creates both a split-phase single-phase supply (L1 or L2 to neutral) and three-phase (L1-L2-L3). The high-leg or phase with higher voltage as measured to neutral has traditionally been designated "Phase B". However, a change to the 2008 NEC now allows the high leg of a four-wire three-phase delta service to be labelled as the "C" phase instead.
High-leg systems are commonly used in older and rural installations, as well as older manufacturing facilities with mostly three-phase motor loads and some single-phase lighting and plug loads. They are also used in Japan, with a distribution transformer output of 200 V line-to-line and 100 V line-to-neutral, and a high-leg to neutral voltage of 173 V. This provides 200 V for both three-phase and split-phase appliances.
The high-leg delta service offers a higher line-to-line voltage than most three-phase services, as well as a sufficient line-to-neutral voltage for connecting appliances and lighting. This means that large pieces of equipment will draw less current, requiring smaller wire and breaker sizes.
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The high-leg is also known as the wild-leg, stinger leg, or bastard leg
A high-leg electrical configuration is a type of three-phase delta service that is commonly used in older manufacturing facilities with mostly three-phase motor loads and some 120-volt single-phase lighting and plug loads. It is also known as a wild-leg, stinger leg, or bastard leg.
This configuration is created when a center tap is added to one of the transformer windings, providing a neutral for single-phase loads. The high-leg is the phase with the higher voltage as measured to neutral, typically designated as "Phase B". However, recent changes in the NEC Code now allow the high leg to be labelled as the "C" phase instead.
The high-leg voltage is typically 208V, resulting in a phase-to-ground voltage of 120V. This high-leg voltage is often used to supply 120V loads, requiring grounding. The system acts like a split single-phase system when the high leg is not used, which is a common supply configuration in the United States.
The term "stinger leg" is used because the high-leg can cause issues with loads if misapplied. It has a higher voltage, often over 200V to the neutral, and can cause some controls to continue operating even when they should have stopped. This is why it is important for equipment with a delta-connected, high-leg system to be rated for the supplied voltage and properly labelled.
The high-leg configuration provides a higher line-to-line voltage than typical three-phase services, while also offering a sufficient line-to-neutral voltage for connecting appliances and lighting. This allows for reduced current draw, smaller wire sizes, and lower breaker sizes.
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High-leg systems have specific marking requirements
High-leg systems, also known as wild-leg, stinger leg, or bastard leg systems, have specific marking requirements to ensure safety and compliance with electrical codes. These markings are important to warn personnel about the higher voltage present on the equipment's "B" phase, which can damage or destroy appliances and lighting equipment. Here are the key marking requirements for high-leg systems:
- Durable and Permanent Markings: High-leg systems must be durably and permanently marked to ensure the identification of the high-leg conductor. This marking should be placed at each point in the system where a connection is made if the grounded conductor is also present.
- Color Identification: The national industry norm is to mark the high-leg conductor with an orange outer finish or use orange wire for #6 and smaller. However, other colours like purple or violet may be used based on local requirements, such as in San Francisco (SF). The use of colours like white/grey for grounded conductors and green for grounding conductors should be maintained.
- Textual Markings: In addition to colour identification, textual warnings are required on switchboards, switchgear, or panel boards. The markings should include a cautionary statement, such as "Caution: Phase B Has 208 V to Ground" or "Caution: ____ Phase Has ____ Volts to Ground," indicating the specific voltage.
- Field Marking Requirements: Service equipment, excluding dwelling units, must be legibly field-marked with the maximum available fault current information. This includes the date the fault current calculation was performed, and it should be durable enough to withstand the environment.
- Panelboard Markings: Since 1975, panel boards supplied by a 4-wire, delta-connected, 3-phase system must have the high-leg conductor terminate to the "B" phase. This change in the NEC Code ensures that the high-leg conductor is properly identified.
- Voltage and Phase Configuration Labels: Equipment installed for delta-connected, high-leg systems must be rated for the supplied voltage. Labels should indicate the rated voltage and phase configuration of the equipment.
- Meter Configurations: The NEC Code permits the high leg to be labelled as the "C" phase when metering is part of the switchboard or panel board to accommodate utility meter configurations.
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The high-leg is not used for single-phase loads
A high-leg delta is a type of electrical service connection for three-phase electric power installations. It is used when both single and three-phase power is supplied from a three-phase transformer or transformer bank. The three-phase power is connected in a delta configuration, and the centre point of one phase is grounded. This creates both a split-phase single-phase supply and three-phase power.
The high-leg or phase with the higher voltage as measured to neutral has traditionally been designated "Phase B". However, a change to the 2008 NEC now allows the high leg to be labelled as "Phase C" instead. The high leg is not used for single-phase loads. This is because the high-leg voltage is higher than the other two phases, and if a single-phase load is connected to the high leg, it can cause excess voltage to be supplied to that load, leading to load failure.
A four-wire delta (4WD) electrical service is a common type of high-leg delta service. It is a three-phase delta service with a centre tap on one of the transformer windings to create a neutral for single-phase loads. Motor loads are commonly connected to phases A, B, and C, while single-phase loads are connected to either phase A or C and to neutral. Phase B, the high leg, is not used for single-phase loads. This type of service is common in older manufacturing facilities with mostly three-phase motor loads and some 120-volt single-phase lighting and plug loads.
The high-leg delta service provides a line-to-line voltage higher than the usual 208 V that most three-phase services have, and a line-to-neutral voltage sufficient for connecting appliances and lighting. This means that large pieces of equipment will draw less current than with 208 V, requiring smaller wire and breaker sizes. Lights and appliances requiring 120 V can be connected to phases A and C without requiring an additional step-down transformer.
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Frequently asked questions
A high leg, also known as a wild leg, stinger leg, or bastard leg, is a type of electrical configuration where one phase conductor has a higher voltage than the others. This configuration is typically found in older and rural installations, as well as in manufacturing facilities with three-phase motor loads and single-phase lighting.
A high leg system is created by grounding the center tap of one winding in a delta-winding transformer. This results in one leg having more winding turns and, consequently, a higher voltage to ground. The high leg voltage is typically 208V, while the other two legs measure 120V to ground.
High leg systems can be identified by their voltage configuration. Typically, the high leg will have a voltage of 208V to ground, while the other two legs will have a voltage of 120V to ground. Additionally, as per NEC requirements, high leg systems should be marked with a warning label indicating the higher voltage on the equipment's "B" phase.
