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Split-phase electric power
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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 original Edison 3-wire
direct current system. Its primary advantage is that it saves conductor material over a
single ended single phase system while only requiring single phase on the supply side of
the distribution transformer.[1] 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.
In North America, the high leg delta system allows single-phase 120 V loads and 240 V
three-phase loads both to be served by the same three-phase, four-wire distribution
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 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 singleended 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 460 V final circuits, and standard consumer units (circuit breaker boxes),
meters and distribution boards are designed around either three-phase or single-phase
circuits.
Pole-mounted single-phase transformer with 3-wire center-tapped "split-phase"
secondary. Note use of ground conductor as one leg of primary feeder.
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.
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, 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 cores may require more
insulation material and more complex processing, reducing the cost saving for lower
power runs.
If the load were 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 to be 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 were 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, 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.
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 utilisation voltage; therefore, 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.
Technical power (balanced power)
In a so-called technical power system, an isolation transformer with a center tap is used
to create a separate supply with conductors at a balanced 60 volts with respect to ground.
Unlike a three-wire distribution system, the grounded neutral is not distributed to the
loads; only line-to-line connections at 120 volts are used. A balanced power system is
only used for specialized distribution in audio and video production studios, sound and
television broadcasting, and installations of sensitive scientific instruments. The purpose
of a balanced power system is to minimize the noise coupled into sensitive equipment
from the power supply. In the United States the National Electrical Code provides rules
for such installations.[2] Technical power systems are not to be used for general-purpose
lighting or other equipment, and may use special sockets to ensure only approved
equipment is connected to the system.
Motors
A split-phase motor is a type of single-phase electric motor. A split-phase motor runs on
a single phase and has no special relationship to a split-phase (3-wire) distribution
system.
References
1. ^ 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.
2. ^ NFPA 70, National Electrical Code 2005, National Fire Protection Association,
Inc., Quincy, Massachusetts USA, (2005). no ISBN , article 640
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