Split phase
From Biocrawler, the free encyclopedia.
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".
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Connections
A transformer connected to a 3-wire distribution system has a single phase input (primary) winding. The output (secondary) winding is centre-tapped with a conductor called the neutral on the centre tap, which is normally connected to earth ground. Split phase is most common in countries with a standard phase-neutral voltage of 120V (or thereabouts). In this case, the transformer is rated 120 volts on either side of the centre tap, giving 240 volts between the two ungrounded terminals. It is also feasible to have a 240V/480V split phase system and this is sometimes used in the UK for running large farms or groups of a few houses off a single transformer where only two of the 3 phase cores of the 11KV systems are availible (its a lot cheaper to run two wires than 3). However with such systems power is usually split out to single ended single phase before entering the houses.
The neutral conductor ensures that the voltages on the two legs do not get (far) out of balance. Lighting and small appliances operating at 120 V can be directly supplied, by connection between a live wire and the neutral. By distributing the load between the two live conductors, the system provides better voltage regulation and allows for the use of smaller conductors than a single-ended single phase system would allow. Large applicances, 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 they would need at 120 V. No individual conductor will be at more than 120 V potential with respect to ground (earth) possibly reducing the insulation requirements compared to a 240-volt single-ended system.
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 (and cost) of conductors can be cut in half. Since the load will vary as lamps are switched on and off, just connecting the lamps in series would result in excessive voltage variations and brightness variations. By connected 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 mass of conductors required to supply a given load is reduced, compared to a system operating only at the utilization voltage.
Similar systems with more wires 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 utilisation voltage.
Voltage drop is generally the determining factor in cable sizing for long runs (whereas for short runs it's the cable's current-carrying capacity rating). Generally split phase systems require less copper for the same volt drop and power transmitted than single ended systems (assuming the voltage of the end usage equipment is fixed). Just how much depends on the situation. The following discussion assumes 100% to be the amount needed to supply the load with a single ended single phase system.
If the load was guaranteed to be balanced, then the neutral conductor wouldn't carry any current (and so, wouldn't be needed) and the system would be equivilent to a single ended system of twice the voltage, therefore using 25% of the copper. In practice the neutral is usually the same size as the phase conductors so that the total copper required becomes 37.5%.
On the other hand, if one side of the system must be able to take half the full load power whilst the other side is completely turned off, then the conductors could only be 50% of the size of the single ended system, making the total copper 75%: still a saving but not a huge one.
Furthermore, since smaller wires have higher costs per unit of area for insulation and installation labour, the costs won't go down by as much as the copper use, but the cost savings can still be significant.
A variation is a 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 centre tap which is connected to ground and also 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 will always have 120 V to neutral, but the third phase or "wild leg" will have approximately 208 V to the neutral.
Construction sites
In the U.K., electric tools and portable lighting at construction sites are sometimes fed from a center tapped system with only 55 volts between live conductors and the earth. This system is used with 110V 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 110-volt incandescent lamps are slightly more rugged and shock-resistant than 240 volt 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 Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0070139326, chapter 3, pages 3-10, 3-14 to 3-22.
