Three-wire transmission is used on balanced three-phase systems when the current is known to be equal in all three phases, for example when supplying a three-phase motor. A fourth neutral wire is used when the system is liable to have unequal currents or when multiple single-phase supplies are connected to it.
If the motor is operated from a three phase three wire distribution system the motor will not need a neutral wire.
A ground, or earth, conductor is never included in the conductor count. So, a three-phase, three-wire, system has three line conductors, whereas a three-phase, four-wire system, has three line conductors and a neutral conductor.
There are usually 3 or 4 terminals. If only 3, then each wire is hot with respect to ground. If there are 4, then one wire is considered neutral.
A smaller neutral wire in a three phase system can be used because it does not carry the full line current. It carries the unbalanced current of all three leg loads. This is one reason that three phase loads on a distribution panel should be equalized as much as possible to reduce the current on the neutral.
In a balanced three-phase system, the current in the neutral wire should ideally be minimal as the three phase conductors carry equal and opposite currents that cancel out in the neutral wire. However, if the loads are unbalanced, the neutral wire may have higher current due to the uneven distribution of power among the phases. This can happen when loads on each phase are different or when single-phase loads are connected between a phase and neutral, leading to increased neutral current.
A two-phase system is archaic and you are unlikely to find it in use anywhere these days, so it is mainly of historical interest. A two-phase, three-wire system, consists of two phase voltages, displaced from each other by 90 electrical degrees, and a phase voltage which is 1.414 x phase voltage.A three-phase system consists of three phase voltages which are displaced from each other by 120 electrical degrees. In the case of a three-phase, three-wire, system, the line voltages are numerically equal to the phase voltages; in the case of a three-phase, four-wire, system, the line voltages are 1.732 x phase voltage.
If the motor is operated from a three phase three wire distribution system the motor will not need a neutral wire.
Polyphase systems are used because they allow the efficient transmission of electric power, in terms of power transmitted per kilogram of wire. Polyphase systems with more than three phases are very uncommon.
A ground, or earth, conductor is never included in the conductor count. So, a three-phase, three-wire, system has three line conductors, whereas a three-phase, four-wire system, has three line conductors and a neutral conductor.
If you are referring to a.c. three-phase systems, then a three-wire system is associated with a delta-connected system, while a four-wire system is associated with a star, or wye, system. The conductors being three line conductors and a neutral conductor.
480 Volt
Because a three-phase system needs half the amount of wire to transmit power compared to three single-phase systems. With a balanced three-phase system the three live wires are phased so that the sum of the currents is always zero, which means that if the three neutral wires are combined, the current in the neutral is zero. Therefore the original three neutral wires can be removed, leaving only the three live wires. In practice a neutral wire is still included in three-phase four-wire systems when the load is potentially unbalanced, as for example in neighbouring houses which use different phases of a three-phase system for their individual single-phase supplies.
A three phase four wire system allows for two voltages from a single three phase transformer. The transformer configuration is a wye or star connection. It is used in commercial and industrial applications. Apartment buildings are now starting to use a three phase four wire service distribution system. Some wye connection voltages are 120/208 volts, 480/277, and 600/347. The lower voltage is derived from the sq root of 3 divided into the three phase voltage.
A delta-connected system is described as being a three-phase, three-wire, system, and doesn't have a neutral. But a balanced star (wye) connected load (e.g. a three-phase induction motor) doesn't actually require a neutral.
I think you mean 'single-phase supply', rather than 'phase supply'. All high-voltage a.c. transmission and distribution systems are three-phase systems. This is because, for a given load, a three-phase system uses less copper than a single-phase system. Three-phase generators produce three 'phase voltages', each displaced, by 120 electrical degrees. These voltages are produced in three windings which are electrically connected in what is called a 'delta' configuration, with each 'corner' of the delta connected to the transmission system by 'line' conductors. Three-phase systems are either 'three-wire' or 'four-wire' systems. Generally, three-phase, three-wire, systems are used for high-voltage transmission and distribution, whereas three-phase, four-wire, systems are typically (but not always) used for low-voltage distribution. Three-wire systems comprise three conductors called 'line conductors'; four-wire systems comprise three 'line conductors' and a 'neutral conductor'. Depending the voltage standards used in the country in which you live, a single-phase supply is obtained either by connecting single-phase load between any two line conductors, or between any one line conductor and the neutral conductor.
A two-phase a.c. system is an archaic system, in which two phase voltages are generated 90 electrical degrees apart. It has nothing whatsoever to do with the US 'split phase' system that supplies North American homes. A two-phase system can be a four-wire system, or a three-wire system, and was useful because, unlike a single-phase system, it could create naturally rotating magnetic fields in induction motors. It has long been superseded by the three-phase system.
The formula you are looking for is - phase voltage/1.73 = phase to neutral voltage.