Split-phase electric power

Last updated April 21, 2024

Pole-mounted single-phase transformer with three-wire center-tapped "split-phase" secondary. On the three secondary terminals, the center tap is grounded with a short strap to the transformer case. Polemount-singlephase-closeup.jpg — Pole-mounted single-phase transformer with three-wire center-tapped "split-phase" secondary. On the three secondary terminals, the center tap is grounded with a short strap to the transformer case.

A split-phase or single-phase three-wire system is a type of single-phase electric power distribution. It is the alternating current (AC) equivalent of the original Edison Machine Works three-wire direct-current system. It primary advantage is that, for a given capacity of a distribution system, it saves conductor material over a single-ended single-phase system.^[1]

The system is common in North America for residential and light commercial applications. Two 120 V AC lines are supplied to the premises that are out of phase by 180 degrees with each other (when both measured with respect to the neutral), along with a common neutral. The neutral conductor is connected to ground at the transformer center tap. Circuits for lighting and small appliance power outlets use 120 V circuits connected between one line and neutral. High-demand applications, such as ovens, are often powered using 240 V AC circuits—these are connected between the two 120 V AC lines. These 240 V loads are either hard-wired or use outlets which are deliberately non-interchangeable with 120 V outlets.

Other applications of a split-phase power system are used to reduce the electric shock hazard or to reduce electromagnetic noise.

Connections

A transformer supplying a three-wire distribution system has a single-phase input (primary) winding. The output (secondary) winding has a center tap connected to a grounded neutral. As shown in Fig. 1, either end to center has half the voltage of end-to-end. Fig. 2 illustrates the phasor diagram of the output voltages for a split-phase transformer. Since the two phasors do not define a unique direction of rotation for a revolving magnetic field, a split single-phase is not a two-phase system.

In the United States and Canada, the practice originated with the DC distribution system developed by Thomas Edison. By connecting pairs of lamps or groups of lamps on the same circuit in series, and doubling the supply voltage, the size of conductors was reduced substantially. Connection of the junction point of each parallel branch of two series lamps to a common neutral, returned to the center tap of the supply voltage, stabilized the branch circuit voltages from changes when loads were switched on and off. The neutral conductor carried only the imbalance of current flowing from one group of loads to the other.

The line to neutral voltage is half the line-to-line voltage. Lighting and small appliances may be connected between a line wire and the neutral. Higher-power appliances, such as cooking equipment, space heating, water heaters, clothes dryers, air conditioners and electric vehicle charging equipment, are connected to the two line conductors. This means that, for the supply of the same amount of power, the current is halved. Smaller conductors may be used than would be needed if the appliances were designed to be supplied by the lower voltage.^[2]

If the load were guaranteed to be balanced (the same current drawn from each line), then the neutral conductor would not carry any current and the system would be equivalent to a single-ended system of twice the voltage with the line wires taking half the current. This would not need a neutral conductor at all, but would be impractical for varying loads; just connecting the groups in series would result in excessive voltage and brightness variation as lamps are switched on and off.

By connecting the two lamp groups to a neutral, intermediate in potential between the two live legs, any imbalance of the load will be supplied by a current in the neutral, giving substantially constant voltage across both groups. The total current carried in all three wires (including the neutral) will always be twice the supply current of the most heavily loaded half.

For short wiring runs limited by conductor current carrying capacity, this allows three half-sized conductors to be substituted for two full-sized ones, using 75% of the copper of an equivalent single-phase system.

Long wiring runs are limited by the permitted voltage drop limit in the conductors. Because the supply voltage is doubled, a balanced load can tolerate double the voltage drop, allowing quarter-sized conductors to be used; this uses 3/8 the copper of an equivalent single-phase system.

In practice, some intermediate value is chosen. For example, if the imbalance is limited to 25% of the total load (half of one half) rather than the absolute worst-case 50%, then conductors 3/8 of the single-phase size will guarantee the same maximum voltage drop, totalling 9/8 of one single-phase conductor, 56% of the copper of the two single-phase conductors.

Balanced power

In a so-called balanced power system, sometimes called "technical power", an isolation transformer with a center tap is used to create a separate supply with conductors at balanced voltages with respect to ground. The purpose of a balanced power system is to minimize the noise coupled into sensitive equipment from the power supply.

Unlike a three-wire distribution system, the grounded neutral is not distributed to the loads; only line-to-line connections at 120 V are used. A balanced power system is used only for specialized distribution in audio and video production studios, sound and television broadcasting, and installations of sensitive scientific instruments.

The U.S. National Electrical Code provides rules for technical power installations.^[3] The systems are not to be used for general-purpose lighting or other equipment and may use special sockets to ensure that only approved equipment is connected to the system. Additionally, technical power systems pay special attention to the way the distribution system is grounded.

A risk of using a balanced power system, in an installation that also uses "conventional" power in the same rooms, is that a user may inadvertently interconnect the power systems together via an intermediate system of audio or video equipment, elements of which might be connected to different power systems. The chance of this happening may be reduced by appropriate labelling of the balanced power outlets and by the use of a type of power outlet socket for the balanced system that is physically different from that of the "conventional" power system to further differentiate them.

Applications

Europe

In Europe, three-phase 230/400 V is most commonly used. However, 130/220 V, three-wire, single-phase discontinued systems called B1 are used to run old installations in small groups of houses when only two of the three-phase high-voltage conductors are used. A split-phase final step-down transformer is then used, with the centre-tap earthed and the two halves usually supplying different buildings with a single phase supply. Some installations, such as farms (especially those never subsequently upgraded to three-phase) may be supplied with both phases to the same consumer. Whilst usually metered through two chosen phases of a typical three-phase meter, these two phases will only ever be used individually, not, as in the USA, to provide a higher voltage. Nonetheless they help with situations where a single supply cannot provide enough power for an installation.

In the United Kingdom, electric tools and portable lighting at larger construction and demolition sites are governed by BS 7375, and where possible are recommended to be fed from a centre-tapped system with only 55 V between live conductors and the earth (so-called CTE or centre-tap earth, or 55–0–55). This reduced low-voltage system is used with 110 V equipment. No neutral conductor is distributed. In high-hazard locations, additional double-pole RCD protection may be used. The intention is to reduce the shock hazard that may exist when using electrical equipment at a wet or outdoor construction site, and eliminate the requirement for rapid automatic disconnection for prevention of shocks during faults. Portable transformers that transform single-phase 240 V to this 110 V split-phase system are a common piece of construction equipment. Generator sets used for construction sites are equipped to supply it directly. However, a large farm may be given a 230–0–230 (nominal) supply.

An incidental benefit is that the filaments of 110 V incandescent lamps used on such systems are thicker and thus mechanically more rugged than those of 240 V lamps.

North America

This three-wire single-phase system is common in North America for residential and light commercial applications. Circuit breaker panels typically have two live (hot) wires, and a neutral, connected at one point to the grounded center tap of a local transformer. Usually, one of the live wires is black and the other one red; the neutral wire is always white. Single-pole circuit breakers feed 120 V circuits from one of the 120 V buses within the panel, or two-pole circuit breakers feed 240-volt circuits from both buses. 120 V circuits are the most common, and used to power NEMA 1 and NEMA 5 outlets, and most residential and light commercial direct-wired lighting circuits. 240 V circuits are used for high-demand applications, such as air conditioners, space heaters, electric stoves, electric clothes dryers, water heaters, and electric vehicle charge points. These use NEMA 10 or NEMA 14 outlets that will not accept 120 V plugs.

Wiring regulations govern the application of split-phase circuits so that the shared neutral can be protected from excess current. A neutral wire can be shared only by two circuits fed from opposite lines of the supply system, using circuit breakers connected by a bar so that both trip simultaneously (^[4] NEC 210.4); this prevents 120 V from feeding across 240 V circuits.

Railways

In the electric power supply system of railways in Sweden split-phase electric power is also used on some railways. The center tap is grounded and one pole is fed to an overhead wire section, while the other wire is used for another section.

Split-phase distribution is used on Amtrak's 60 Hz traction power system in the Northeast Corridor between New York and Boston. Two separate wires are run along the track, the contact wire for the locomotive and an electrically separate feeder wire. Each wire is fed with 25 kV with respect to ground, with 50 kV between them. Autotransformers along the track balance the loads between the contact and feeder wires, reducing resistive losses.

Related Research Articles

<span class="mw-page-title-main">Three-phase electric power</span> Common electrical power generation, transmission and distribution method for alternating currents

Three-phase electric power is a common type of alternating current (AC) used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires and is the most common method used by electrical grids worldwide to transfer power.

Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. The abbreviations AC and DC are often used to mean simply alternating and direct, respectively, as when they modify current or voltage.

Mains electricity or utility power, grid power, domestic power, and wall power, or, in some parts of Canada, hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to homes and businesses through the electrical grid in many parts of the world. People use this electricity to power everyday items by plugging them into a wall outlet.

<span class="mw-page-title-main">Electric power distribution</span> Final stage of electricity delivery to individual consumers in a power grid

Electric power distribution is the final stage in the delivery of electricity. Electricity is carried from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 33 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment and household appliances. Often several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.

<span class="mw-page-title-main">Single-phase electric power</span> Type of electric power distribution

In electrical engineering, single-phase electric power is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current electric motor does not produce a rotating magnetic field; single-phase motors need additional circuits for starting, and such motors are uncommon above 10 kW in rating.

A polyphase system is a means of distributing alternating-current (AC) electrical power that utilizes more than one AC phase, which refers to the phase offset value between AC in multiple conducting wires; phases may also refer to the corresponding terminals and conductors, as in color codes. Polyphase systems have two or more energized electrical conductors carrying alternating currents with a defined phase between the voltage waves in each conductor. Early systems used 4 wire two-phase with a 90° phase angle, but modern systems almost universally use three-phase voltage, with a phase angle of 120°.

Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power from an electrical grid to remote areas at lowest cost. The earth is used as the return path for the current, to avoid the need for a second wire to act as a return path.

Electrical wiring in North America follows the regulations and standards applicable at the installation location. It is also designed to provide proper function, and is also influenced by history and traditions of the location installation.

Two-phase electrical power was an early 20th-century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by one-quarter of a cycle, 90°. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor. Some early two-phase generators had two complete rotor and field assemblies, with windings physically offset to provide two-phase power. The generators at Niagara Falls installed in 1895 were the largest generators in the world at that time, and were two-phase machines. Three-phase systems eventually replaced the original two-phase power systems for power transmission and utilization. Active two-phase distribution systems remain in Center City Philadelphia, where many commercial buildings are permanently wired for two-phase, and in Hartford, Connecticut.

<span class="mw-page-title-main">Ground and neutral</span> In mains electricity, part of a circuit connected to ground or earth

In electrical engineering, ground and neutral are circuit conductors used in alternating current (AC) electrical systems. The neutral conductor returns current to the supply. To limit the effects of leakage current from higher-voltage systems, the neutral conductor is often connected to earth ground at the point of supply. A ground conductor is not intended to carry current for normal operation of the circuit, but instead connects exposed metallic components to earth ground. A ground conductor only carries significant current if there is a circuit fault that would otherwise energize exposed conductive parts and present a shock hazard. Circuit protection devices may detect a fault to a grounded metal enclosure and automatically de-energize the circuit, or may provide a warning of a ground fault.

In electrical engineering, an autotransformer is an electrical transformer with only one winding. The "auto" prefix refers to the single coil acting alone. In an autotransformer, portions of the same winding act as both the primary winding and secondary winding sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings that are not connected by an electrically conductive path. between them.

Electrical wiring in the United Kingdom is commonly understood to be an electrical installation for operation by end users within domestic, commercial, industrial, and other buildings, and also in special installations and locations, such as marinas or caravan parks. It does not normally cover the transmission or distribution of electricity to them.

<span class="mw-page-title-main">Current transformer</span> Transformer used to scale alternating current, used as sensor for AC power

A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary.

<span class="mw-page-title-main">Distribution transformer</span> Final stage in power distribution to users

A distribution transformer or service transformer is a transformer that provides a final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. The invention of a practical efficient transformer made AC power distribution feasible; a system using distribution transformers was demonstrated as early as 1882.

An earthing system or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary among countries, though most follow the recommendations of the International Electrotechnical Commission (IEC). Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants.

In electric power distribution, a service drop is an overhead electrical line running from a utility pole, to a customer's building or other premises. It is the point where electric utilities provide power to their customers. The customer connection to an underground distribution system is usually called a "service lateral". Conductors of a service drop or lateral are usually owned and maintained by the utility company, but some industrial drops are installed and owned by the customer.

A Scott-T transformer or Scott connection is a type of circuit used to produce two-phase electric power from a three-phase source, or vice versa. The Scott connection evenly distributes a balanced load between the phases of the source. The Scott three-phase transformer was invented by Westinghouse engineer Charles F. Scott in the late 1890s to bypass Thomas Edison's more expensive rotary converter and thereby permit two-phase generator plants to drive three-phase motors.

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 desired to be supplied from a three phase transformer. The three-phase power is connected in the delta configuration, and the center point of one phase is grounded. This creates both a split-phase single-phase supply and three-phase. It is sometimes called orange leg because the L3 wire is required to be color-coded orange in the United States. By convention, the high leg is usually set in the center lug in the involved panel, regardless of the L1–L2–L3 designation at the transformer.

A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.

An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The electrical grid can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centers to the load centers, and the distribution system that feeds the power to nearby homes and industries.

References

↑ Terrell Croft and Wilford Summers (ed), American Electricians' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07-013932-6, chapter 3, pages 3-10, 3-14 to 3-22.
↑ Gonen, Turan. Electric Power Distribution System Engineering, 2nd ed. CRC Press, 2007, p. 284.
↑ NFPA 70, National Electrical Code 2005, National Fire Protection Association, Inc., Quincy, Massachusetts USA, (2005). no ISBN, articles 640 and 647
↑ "Branch Circuits – Part 1 | EC&M".

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[1] Terrell Croft and Wilford Summers (ed), American Electricians' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07-013932-6, chapter 3, pages 3-10, 3-14 to 3-22.

[gonen07-2] Gonen, Turan. Electric Power Distribution System Engineering, 2nd ed. CRC Press, 2007, p. 284.

[3] NFPA 70, National Electrical Code 2005, National Fire Protection Association, Inc., Quincy, Massachusetts USA, (2005). no ISBN, articles 640 and 647

[4] "Branch Circuits – Part 1 | EC&M".

[1]

[2]

[3]

[4]