A transformer supplying a 3-wire distribution system has a single-phase input (primary) winding. The output (secondary) winding is center-tapped and the center tap connected to a grounded neutral. This 3-wire system is common in countries with a standard phase-neutral voltage of 120 V. In this case, the transformer voltage is 120 V on either side of the center tap, giving 240 V between the two live conductors, shown as V1 and V2 in Fig. 1. The two outputs are properly called "legs", not "phases".
In North American electrical codes, the split-phase distribution may be carried to the outlet receptacles. Two 120 volt devices may be plugged into a duplex receptacle that connects one neutral wire to both outlets. This saves the cost of one wire back to the panelboard. Such multiwire branch circuits have special rules in the electrical codes to ensure they are safely applied.
In Europe, three-phase 230/400V is most commonly used. However, 230/460 V, 3-wire, single-phase systems are used to run farms and small groups of houses when only two of the three-phase high voltage conductors are available.
In Australia and New Zealand, remote loads are connected to the grid using SWER (Single Wire Earth Return) transmission lines (it is cheaper to run one wire than two). The primary of the transformer is connected between the high voltage line and earth, the secondary is a 3-wire single-phase system as described here, the secondary voltage being 230/460 V. Single phase loads are split between the two circuits. Hot water services use both circuits.
In countries whose standard phase to neutral voltage is 120 V, lighting and small appliances are connected between a live wire and the neutral. Large appliances, such as cooking equipment, space heating, water pumps, clothes dryers, and air conditioners are connected across the two live conductors and operate at 240 V, requiring less current and smaller conductors than would be needed if the appliances were designed for 120 V operation.
No individual conductor will be at more than 120 V potential with respect to ground (earth), reducing the earth fault current when compared to a 240 V, 2-wire system that has one leg (the neutral) earthed.
Split-phase systems require less copper for the same voltage drop, final utilization voltage, and power transmitted than single phase systems. (Voltage drop tends to be the dominant design consideration in the sizing of long power distribution cable runs.) Just how much depends on the situation. However, the extra conductor may require more insulation material and more complex processing, reducing the cost saving for lower power runs.
In the United States, the practice originated with the DC distribution system developed by Thomas Edison. By dividing a lighting load into two equal groups of lamps connected in series, the total supply voltage can be doubled and the size of conductors reduced substantially.
If the load were guaranteed to be balanced, 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 live cables taking half the current. This would not need a neutral conductor at all, but would be wildly 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 ampacity, 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.
Longer wiring runs are more limited by voltage drop 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.
A variation is the 240 V delta 4-wire system, also known as a high-leg or red-leg delta. This is a three-phase 240 V delta connected system, in which one winding of the transformer has a center tap which is connected to ground and used as the system neutral. This allows a single service to supply 120 V for lighting, 240 V single-phase for heating appliances, and 240 V three-phase for motor loads (such as air conditioning compressors). Two of the phases are 120 V to neutral, the third phase or "high leg" is 208 V to neutral.
Multiwire systems split more than two ways are possible with both AC and DC but have the significant disadvantage that no matter which point is tied to ground some of the wires will have a higher earth relative voltage than the utilisation voltage; therefore, such systems are not used in normal power distribution.
Read more about this topic: Split-phase Electric Power
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