Split-phase electric power

A split-phase or single-phase three-wire system is a form of single-phase electric power distribution. Such a system is the alternating current (AC) equivalent of the original three-wire direct current (DC) system developed by the Edison Machine Works. The main advantage of split-phase distribution is that, for a given power capacity, it requires less conductor material than a two-wire single-phase system.

Split-phase distribution is widely used in North America for residential and light commercial service. A typical installation supplies two 120 V AC lines that are 180 degrees out of phase with each other (relative to the neutral conductor), along with a shared neutral conductor. The neutral is connected to ground at the transformer's center tap.

In North America, standard household circuits for lighting and small appliances are connected between one line and the neutral, providing 120 V. Higher-demand appliances—such as ovens, dryers, and water heaters—are powered by 240 V circuits, connected between the two 120 V lines. These 240 V loads are either hardwired or use outlets designed not to be interchangeable with 120 V outlets.

Split-phase systems are also used in some specialized applications to reduce the risk of electric shock or to minimize electromagnetic noise.

Connections

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

In the United States and Canada, the practice of using split-phase electric power originated with the DC distribution system developed by the inventor and businessman Thomas Edison.^{[citation needed]} Connecting pairs of lamps or groups of lamps in series on the same circuit and doubling the supply voltage reduced the size of conductors substantially. When there are two circuits, each with unique loads, the common wire for each circuit can be replaced by a single conductor returned to the center tap of the supply voltage, thereby stabilizing the branch circuit voltages from changes when loads are switched on and off. The current carried in the neutral is only the imbalance of current flowing from one group of loads to the other. This arrangement reduces the number of conductors from four to three, and it provides significant cost savings in labor and materials.

The line-to-neutral voltage is half of 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 arrangement means that, to supply 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.

If the load were guaranteed to be balanced (that is, the same current being drawn from each line), then the neutral conductor would carry no current, and the system would be equivalent to a single-ended system with twice the voltage in which the line wires take half the current. This system would not need a neutral conductor, but it would be impractical for varying loads; merely connecting the groups in series would result in excessive voltage and brightness variation as lamps are switched on and off.

Connecting the two lamp groups to a neutral, which is intermediate in potential between the two live legs, means that any imbalance of the load is 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) is always twice the supply current of the most heavily loaded half.

For short wiring runs limited by the current-carrying capacity of a conductor, this arrangement allows three half-sized conductors to be substituted for two full-sized ones, using 3/4 of the copper in 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 approach uses 3/8 of the copper in an equivalent single-phase system.

In practice, some intermediate limit is chosen. For example, if the imbalance is limited to 25% of the total load (that is, half of one half) rather than the worst-case 50% imbalance, then conductors with 3/8 of the single-phase size guarantee the same maximum voltage drop, totaling 9/8 of one single-phase conductor and using 9/16 of the copper in the two single-phase conductors.

Balanced power

In a balanced power system (sometimes called "technical power"), an isolation transformer with a center tap creates a separate supply with conductors at balanced voltages relative to ground. The purpose of such a system is to minimize the noise coupled into sensitive equipment from the power supply.

Unlike in 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. The systems must not be used for general-purpose lighting or other equipment, and they may use special sockets to ensure that only approved equipment is connected to the system. In addition, special attention is paid to how the distribution system is grounded.

There is a risk when using a balanced power system in an installation that also uses "conventional" power in the same rooms: a user may inadvertently interconnect the power systems via an intermediate system of audio or video equipment, elements of which might be connected to different power systems. The chances of this happening may be reduced by appropriately labeling the balanced power outlets, as well as using a type of outlet socket for the balanced system that is physically different from that of the "conventional" power system.

Applications

Railways

In railway electrification, split-phase electric power is used on high-speed lines with 25 kV AC railway electrification. 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 (the center tap is grounded), with 50 kV between the wires. Autotransformers along the track balance the loads between the contact and feeder wires; one pole is fed to an overhead wire section, while the other wire is used for another section, reducing resistive losses. (The arrangement is known as a 2 × 25 kV autotransformer system.)

See also

• Shared neutral

References