There is typically one power wire (the phase wire) and one neutral wire, with current flowing between the power wire (through the load) and the neutral wire. Three-phase power is a three-wire alternating current power circuit with each phase alternating current signal 120 electrical degrees apart. The third phase is used to offset any imbalance in the first two phases. Three-phase power is commonly available in large buildings where an imbalance of loads on different branches may happen from time to time but which can be corrected by just adding more branch circuits.
The term "three-phase" means that each conductor carries a signal that is 120 degrees apart from the other two conductors. A fourth conductor called the "neutral" is needed to balance currents if there is an imbalance between the three existing lines. Neutral conductors are not normally part of the structure at street level; they must be brought up from below ground or from another floor in the building. They are usually passed along with the hot wires through a conduit or cable tray.
In many countries, including the United States, Canada, and most of Europe, electricity is supplied at 60 cycles per second (Hz). In these regions, three-phase power is simply split into three separate wires: one pair for line voltage and one for neutral. In other countries, such as Japan, Germany, and India, electricity is supplied at 400 Hz or 480 Hz, which requires four-wire operation of the cable plant.
Three-phase power is a way of transmitting electrical power that uses three cables to deliver three separate alternating electrical currents. Each wire's current is separated from the others by one-third of a full cycle, with each current representing one phase. The three phases are transmitted at different times on the same cable, allowing for two different circuits to use the same line without interfering with each other.
Three-phase power is used in many situations where it is useful to have separate circuits for each conductor. This is particularly important in wiring diagrams for machines and equipment that are not designed for three-wire systems. Three-phase power is also useful when dealing with devices that require multiple voltages to function properly. For example, if a motor requires both 120 volts and 240 volts for operation, it can be wired up with three conductors instead of four. Wires carrying different phases of current do not need to go to ground at the same time - they can stay active while the other wires shut down. This method of transmission is called "balanced" because equal amounts of current are flowing in each conductor.
The term "three-phase" comes from the fact that these are the only phases needed to operate some appliances. A simple clock will work with just two conductors - one carrying positive voltage, the other negative. Most clocks have two conductors because it's easy to connect them up in parallel so they can carry more current.
Residential dwellings are typically serviced by a single-phase power supply, but commercial and industrial establishments are often served by a three-phase supply. A special transformer called a "three-phase transformer" or "three-way plug adapter" is required to connect a two-wire residential electric meter to a three-wire power supply.
A three-phase circuit requires that all three wires of the power cord be connected to a wall outlet. If only two wires are needed, then those two wires must be separated from the third conductor. This can be done in several ways: by using a split bus (see below), or with multiple transformers. A three-phase circuit is necessary for some high-power applications such as arc welders and air compressors.
In the United States, most households are supplied with electricity on a single-phase system. In this type of wiring configuration, both the hot and the neutral conductors are carried on the same cable from the utility pole to the house. The third conductor, or "third wire", is also called the "grounding conductor". It is always attached to metal housing around the outside of a building, or it can be attached to the metal water pipe that runs under the floor of the house.
There is a substantial difference between single-phase and two-phase electricity. With three-phase power, there is no net voltage across any conductor; the individual phases are 120 degrees out of phase with one another.
The term "three-phase" refers to the fact that each conductor carries a different electrical potential - one at 0 degrees, one at 120 degrees, and one at 240 degrees from the first. The third phase is present on all three conductors simultaneously and completes the cycle after the other two phases have passed through zero voltage. For this reason, three-phase power can be transmitted over long distances using just three conductors (or cables), while two-phase power requires four conductors (or cables).
In addition, three-phase power is used when large amounts of current need to be drawn from a single source. This is because it is easier to distribute three smaller currents than it is to combine four large currents from a single source. Three-phase power is also useful when you want to avoid having all the voltage come on at once. For example, if you were to connect four wires together to get two phases, they would both have voltage applied to them at the same time.
The distribution of a load is referred to as the phase in electricity. A three-way switch can be used to select between single-phase power, two-phase power (120 volts), or three-phase power (180 volts). A special type of four-way switch called a split-phase switch can also be used to select between all combinations of single-, two-, and three-phase power.
Three-phase power is required for some electric motors to produce unidirectional rotation. Other electric devices that use three-phase power include air conditioners, heat pumps, and some types of power tools. Motors need three-phase power to run continuously without burning out because they will not receive a full voltage waveform if any one of the phases is missing. Also, many appliances that use electric heating elements require three-phase power to operate correctly. In addition, some electric circuits need three separate connections to work properly; for example, a three-way light bulb cannot be replaced by a two-way lamp unless the other terminal is also removed.
One reason three-phase power is used for large installations is that it reduces the amount of wiring needed compared to single-phase power.
Four wires supply three-phase electricity. Three hot wires and one neutral carry 120 volts of power. In a three-phase circuit, the three hot wires are black, blue, and red; a white wire serves as the neutral, and a green wire serves as the ground. The term "third phase" means that any two of the three wires can be used as the positive line.
The shape of the plug is standardized by the International Electrotechnical Commission (IEC). It consists of three parallel pins surrounded by a circular ring. The function of each pin is to receive one of the three conductors from the breaker or fuse panel. The center pin is the third phase, the other two are single phases. A flat blade connector or screw terminal is used to connect the cable to the plug. Third-party plugs and adapters may have different shapes but they all fit into the same hole in a wall outlet.
In America, the standard household electrical system uses 110-volt current, which is split into alternating currents (AC) of 220-240 volts by power companies before being distributed to consumers. Since most appliances are not designed to operate on such high voltage, special outlets are required for use with ungrounded three-wire systems. These outlets are commonly called "split phase" because they divide up the current between the three wires equally.