Umair Gaines
A high leg is a term used by electricians to refer to the unavoidable scenario where you end up with a leg that measures a higher voltage to the ground when you ground the centre tap of one winding of a delta-winding transformer. This happens because there are the most winding turns between L3 and the grounded point, resulting in the highest voltage multiplication. The high leg delta service provides a higher line-to-line voltage than the typical 208 V in three-phase services, and a sufficient line-to-neutral voltage for lighting and appliances.
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What You'll Learn
High-leg delta systems are commonly used in Japan
A high-leg delta system is a type of electrical service connection for three-phase electric power installations. It is used when both single and three-phase power is desired to be 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, resulting in a split-phase single-phase supply and three-phase supply. This system is created in one of two ways. One way is by using a three-phase transformer (or three single-phase transformers) with four wires coming out of the secondary—the three phases, plus a neutral connected as a centre tap on one of the windings. The other method, called the open delta configuration, requires two transformers. One transformer is connected to one phase of the overhead primary distribution circuit to provide the lighting side of the circuit, and the other transformer is connected to another phase on the circuit and its secondary is connected to one side of the lighting transformer secondary. The other side of this transformer is brought out as the high leg.
High-leg delta systems are commonly found in older and rural installations and are supplied using 240 V line-to-line and 120 V line-to-neutral. They provide a higher line-to-line voltage than most three-phase services, and 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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High-leg voltage can vary
In a high-leg delta system, the three-phase power is connected in a delta configuration, and the center point of one phase is grounded. This results in a split-phase single-phase supply (L1 or L2 to neutral) and three-phase supply (L1-L2-L3). The high leg is the leg that measures higher voltage to the ground, typically in the range of 208V to neutral. However, it can vary, as seen in installations in Japan, where the high-leg to neutral voltage is 173V.
The high-leg voltage can also depend on the number of transformers used. In an open-delta high-leg system, two transformers are used instead of three, resulting in a reduced capacity relative to a full delta. In this configuration, the high-leg voltage can be higher than the typical 208V, providing a higher line-to-line voltage. Additionally, the line-to-neutral voltage on two of the phases is sufficient for connecting appliances and lighting without requiring an additional step-down transformer.
It is important to note that the high leg voltage is usually higher than the other legs, and connecting single-phase loads unaware of this higher voltage can cause load failure. Therefore, it is recommended to not exceed 5% of the transformer capacity when using high-leg-to-neutral loading with only two transformers.
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High-leg delta services provide a higher line-to-line voltage
High-leg delta services, also known as wild-leg, stinger leg, bastard leg, orange-leg, red-leg, or dog-leg delta, are 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.
High-leg delta services are provided in one of two ways. The first method involves using a three-phase transformer (or three single-phase transformers) with four wires coming out of the secondary: three phases and a neutral connected as a center-tap on one of the windings. This configuration results in a line-to-line voltage of 240 V and a line-to-neutral voltage of 120 V.
The second method, known as the open delta configuration, requires two transformers. One transformer is connected to one phase of the overhead primary distribution circuit to provide the lighting side of the circuit, and the second transformer is connected to another phase on the circuit, with its secondary connected to the lighting transformer secondary. The other side of this transformer is brought out as the high leg.
The voltages between the three phases are the same in magnitude, but the voltages between a particular phase and the neutral vary. The high leg, usually the "'B' phase", has the highest voltage when measured to neutral. A modification to the 2008 NEC now allows the high leg to be identified as the "C" phase instead of the "B" phase.
High-leg delta services offer a higher line-to-line voltage than other three-phase services, typically providing 240 V line-to-line and 120 V line-to-neutral. This is in contrast to most three-phase services, which provide 208 V line-to-line. This higher line-to-line voltage can be advantageous, as it allows large pieces of equipment to draw less current, requiring smaller wire and breaker sizes.
It is important to note that high-leg delta services can pose a hazard if single-phase loads are connected to the high leg without awareness of the higher voltage. This can lead to excess voltage being supplied to the load, potentially causing its failure. Therefore, it is crucial to observe the high-leg-to-neutral load limit, which is typically recommended not to exceed 5% of the transformer capacity.
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A high-leg delta can be created using two transformers
A high-leg delta system is a configuration in which one of the windings in a delta-connected secondary of a transformer is centre-tapped and grounded. It is also known as a red leg, wild leg, or orange leg connection. This configuration results in a split-phase supply, providing both single-phase and three-phase connections. This type of system is commonly used in North America, particularly in the United States, and in older three-phase electric power installations.
The voltages between the three phases are equal in magnitude, but the voltage magnitudes between a particular phase and the neutral vary. The phase-to-neutral voltage of two of the phases will be half of the phase-to-phase voltage. For example, in a 120/240V system, there will be 120V between each line and neutral, and 240V line-to-line. There will be 240V between any two phases, and any 3-phase load can be supplied, but a load cannot be connected from the high leg to neutral.
The high-leg delta system provides a higher line-to-line voltage than the typical 208V found in most three-phase services, and a sufficient line-to-neutral voltage for connecting appliances and lighting. This allows large equipment to draw less current, requiring smaller wire and breaker sizes. Lights and appliances requiring 120V can be connected to specific phases without needing an additional step-down transformer.
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A high-leg delta can also be created using three transformers
A high-leg delta system is a configuration in which one of the windings in a delta-connected secondary of a transformer is centre-tapped and grounded. This type of system is commonly used in North America, especially in the United States, and also in Japan.
A high-leg delta can be created using three single-phase transformers, with four wires coming out of the secondary—the three phases, plus a neutral connected as a centre-tap on one of the windings. This is also known as a closed delta system.
Alternatively, an open delta configuration can be used with just two transformers. In this setup, one transformer is connected to one phase of the primary distribution circuit to provide the lighting side of the circuit. This will be the larger of the two transformers. A second transformer is then connected to another phase on the circuit, and its secondary is joined to the first transformer to create the third leg, or high leg. This type of system is used by most power companies to save money, but it can be inefficient for large motors.
A high-leg delta system 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.
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Frequently asked questions
A high leg is a term used to refer to the leg that measures a higher voltage to the ground when you ground the center tap of one winding in a delta-winding transformer.
The voltage of a high leg is typically 208V to the ground, while the other two legs are 120V to the ground.
A delta-winding transformer is a type of three-phase transformer with four wires coming out of the secondary, consisting of the three phases and a neutral connected as a center tap on one of the windings.
A high leg 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 thus a higher voltage to the ground compared to the other legs.
The high leg provides a line-to-line voltage higher than the standard 208V found in most three-phase services. It also offers a sufficient line-to-neutral voltage for connecting appliances and lighting, allowing large equipment to draw less current and requiring smaller wire and breaker sizes.
