Split-phase electric power
A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. It is the AC equivalent of the former Edison direct current distribution system. Like that system, it has the advantage of saving the weight of conductors for the installation. Since there are two live conductors in the system, it is sometimes incorrectly referred to as "two phase". To avoid confusion with split-phase motor start applications, it is appropriate to call this power distribution system a 3-wire, single-phase, mid-point neutral system.
Connections
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 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.
In the UK and Europe 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 (it is cheaper to run two wires than three). Houses in the UK normally only have single ended single phase power and so (as with three phase) only one active line and the neutral is taken to each house. In the case of a property with a larger demand it is usually split out at the intake point and treated as two totally separate installations since there is no need for 460V final circuits and standard consumer units, meters and distribution boards are designed around either three phase or single phase circuits.
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. 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 volts, 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 where 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.
In the USA 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 cut in half if current carry capacity is determining cable size or by a quarter if cable voltage drop is the size determining factor. Since the load will vary as lamps are switched on and off, just connecting the groups in series would result in excessive voltage and brightness variation. 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.
Split phase systems require less copper for the same voltage drop (voltage drop tends to be the dominant design consideration in the sizing of long mains voltage cable runs), final utilisation voltage and power transmitted than single phase systems. Just how much depends on the situation. However the extra cores may require more insulation material and more complex processing, reducing the cost saving for lower power runs.
If the load was guaranteed to be balanced, then the neutral conductor would not carry any current (and so, would not be needed) and the system would be equivalent to a single ended system of twice the voltage with the live cables taking half the current. Assuming volt drop was the dominant design consideration (as it tends to be for long cable runs at mains voltage) this system would use 25% of the copper of the equivalent single ended single phase system but would be wildly impractical for real varying loads. At the other extreme if the system was designed to provide volt drop in spec when one side was fully loaded and the other completely unloaded the total copper required would be 75% of that required for a single end single phase circuit. In practice a measure between these extremes can be applied (commonly sizing the conductors for correct volt drop with a full balanced load and then making the neutral as big as the other two conductors).
A variation is the 240 V delta 4-wire system. This is a three-phase 240 volt 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 "wild leg" is 208 V to neutral.
Systems split more than two ways are technically 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 utilization voltage and so such systems are not used in normal power distribution.
Construction sites
In the UK, electric tools and portable lighting at construction sites are sometimes fed from a centre-tapped system with only 55 V between live conductors and the earth. This system is used with 110 V equipment and therefore no neutral conductor is needed. The intention is to reduce the electrocution hazard that may exist when using electrical equipment at a wet or outdoor construction site. An incidental benefit is that the filaments of 110 V incandescent lamps are thicker and therefore mechanically more rugged and shock-resistant than 230 V lamps.
Motors
A split phase motor is a type of single-phase electric motor. A split-phase motor runs on a single phase and does not require and has no special relationship to a split-phase (3-wire) distribution system.
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.