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Difference between single phase and-3-phase
Difference between single phase and-3-phase All single-phase circuits require 2 wires to complete circuit.
.... unlike commercial 3-phase used for motors etc that require 3 wires to complete circuit.

With 120 volt single-phase, it requires 1 Hot and 1 Neutral .... and has efficiency of pedaling bike with 1 leg

With 240 volt single-phase, it requires 2 out-of-phase Hot wires, ... and has efficiency of pedaling bike with 2 legs

Commercial 3-phase requires 3 out-of-phase Hot wires, ... and has efficiency of pedaling bike with 3 legs due to higher average power compared with single phase. 
3-phase is not available for residential homes because of cost factors, such as heavier materials, more wires, more transformers, complicated load factors, and imbalance issues etc.

More detail:
"In a balanced 3-phase system, the wires can be about 75% the size of conductors (wires) for a single-phase two-wire system of the same KVA rating. This helps offset cost of supplying the third wire required by 3-phase systems."
Terminology: KVA is Kilo volt amps, or 1000 volt amps. Since volts x amps = watts (power), the KVA rating is referring to kilowatts and how much power is produced by a motor, transformer, heating element etc. KVA is a static number for comparing available power, and not to be confused with kilowatt-hour, which is a measurement of power consumption over 1 hour.

Motor switch
Single phase motor switch
3-phase motor switch

What is 3-phase
3-phase generator
Generator illustration:
Larger image
How are wires out-of-phase from each other?
Out-of-phase means the electrons on one Hot wire are accelerating different directions at any given moment from the electrons on other Hot wire(s). It's important to note that electricity is dynamic and is measured over time.

How it happens: Passing a magnet over a coil of wire causes electrons to move along the wire, creating electricity. This is called electromagnet induction.
Inside a power plant generator, the electromagnet rotates in a 360 circle going past 3 coils of wire located exactly 120 apart.
The magnet has a south and north pole. As the south pole gets close to the coil, the electrons begin to accelerate one direction on the wire. As the south pole moves away and the north pole approaches the coil, the electrons slow down, momentarily stop, before accelerating back the other direction down the wire. Each time the electrons stop, the voltage drops to zero. The generator spins at 60 times per second and the direction change happens so quickly that the end user cannot see their light bulbs are actually flickering with the momentary drop to zero volts, and the average voltage is always above zero so the lights stay on.
Back and forth the electrons oscillate on each wire, producing what is called alternating current.
Since there are 3 coils in the generator, 3 Hot wires emerge from the power plant. Each Hot wire carries electrons that are accelerating different directions at any given moment, thus causing the 3 Hot wires to be out of phase from each other.
DC or direct current, produced by solar panels or batteries, causes electrons to move one direction only, and must be changed into matching phases of alternating current before being transmitted along power company wires.
Household single-phase generator
More about 3-phase generation
How to set up solar array
Why you need a ground wire
power plant to house electricity
Larger image of grid
Single phase is a derivative of 3-phase from the power plant.
Illustration shows 3 phase from power plant to household single-phase

How it happens: 3 phase from power plant transmits 300,000-500,000 volts across groups of 3 Hot wires. Long distance transmission of power requires high voltage to reduce heat loss caused by high amperage.
A Neutral wire arises from a point inside the generator where the 3 coils meet. The Hot wire comes off one end of each coil, and other end of each coil is connected to the Neutral. The result is Hot to Hot has potential voltage, and Hot to Neutral has potential voltage. The Neutral travels with the Hot wires and is bonded to the ground rod array to protect the grid against overvoltages, malfunctions and lightning.

At the distribution level, there are still 3 Hot wires plus Neutral, but voltage has been reduced at substation transformers to 4500-7200 volts.

Single phase is derived by taking one 7200 volt Hot wire and the Neutral off the 3-phase. The Hot and Neutral are then connected to residential transformer.

More detail:
Each time the voltage is raised or lowered, the amperage is affected. Basic formula: Volts x amps = watts shows that volts and amps are inversely proportional. When you raise volts, it lowers amps. Wires with 500,000 volts have low amps and low heat which is good for long-distance transmission. But the switchgear and wiring for 500,000 volts is too large, expensive and dangerous for household use.
Using transformers across the grid, the power company changes volt-amp ratio to accomplish different objects. The result is household receives low volt, high amp power. It works nicely because 120-240 voltage can be safely controlled by small switches, relays, cell phone chargers etc contained within steel and plastic enclosures, while the amperage (heat) is controlled by circuit breakers and then distributed to outlets, switches, dryer etc using correctly sized wire to match amp rating of breaker.

See inside main breaker box
Basic residential electric
Why you need ground wire
Wire and breaker size

Larger image

Voltage per turn x number of turns
Residential transformer
-Transformers work on a basic principle of magnetic induction where applying electricity to one coil of wire creates a magnetic flux that will energize the other coil of wire with electricity.
-'Transformers have no moving parts, ensuring a long trouble-free life under normal conditions.' Inside a transformer are two coils of wire called the primary and secondary coils. Each coil is wrapped around a laminated steel core or more efficient amorphous metal core. The metal core is shared by both coils, but both coils of wire are 'insulated' from each other. They are electrically separated. There is no wire in common between the coils, only the metal core is shared.

-Having different numbers of turns of wire, or varying the turns ratio, on each coil will reduce or raise voltage. Different voltages can be achieved throughout the grid by varying the number of turns on the primary and secondary coils.

-The 7200 volt Hot wire and Neutral are connected to primary coil via 2 taps or connection points on top of transformer.

-Applying 7200 volt to the primary coil will produce 240 volt on the secondary coil because the transformer selected for the job has the correct turns ratio for residential voltage. Connecting the 7200 volt Hot and Neutral across the transformer's primary coil 'completes the circuit' causing electrons to oscillate back and forth 60 times per second. This flow of electrons on the primary coil causes electrons on the secondary coil to oscillate in the same frequency.

-The secondary coil has 3 taps located on side of transformer: 2 outputs for Hot wires and 1 output in center for Neutral wire
-Residential electricity receives 3 wires consisting of 2 out of phase Hots and 1 Neutral.  The Neutral wires on both primary and secondary side of transformer are bonded to the ground wire at the pole. All Neutrals throughout the grid are connected together and bonded to ground wires to create massive array of grounding that provides safety and stability to the grid.

-To get 240 volt, you pull a Hot wire from each end of the secondary coil. These two Hot wires are out of phase from each other because electrons are oscillating back and forth on the secondary coil, and since each Hot wire is connected to a different end of the coil, each Hot wire carries electrons that are accelerating different direction from each other at any given moment of time.
-By pulling a Neutral off the center of the coil, you get 1/2 voltage or 120 volt potential.
As a result, 120 volt is achieved using 1 Hot and a Neutral. While 240 volt is achieved using a Hot wire from both ends the secondary coil.
-As a footnote, drawing 2 Hots from same side of transformer yields no voltage when connected to an appliance since both Hot wires are in phase with each other... each Hot must come from the opposite end of secondary coil.

Why you need ground wire

Larger image
Residential single-phase breaker box
After leaving the transformer, residential wires go to the meter box that records Kwh of electrical usage, and then into the main breaker box. Once inside the main breaker box, the 2 Hots and Neutral are connected to 3 separate busbars.
The 2 Hot wires are connected to the main breaker which is snapped over the top of 2 busbars. The two Hot busbars are electrically separated and never come into contact with each other.
The Neutral is connected directly to the Neutral busbar which is always bonded to the Ground busbar. The ground busbar is connected to a ground wire going out to the ground rod (not shown).
240 volt comes from a double breaker that draws 2 wires, one wire from each Hot busbar.
120 volt is drawn from a single breaker that is snapped over either Hot busbar.
See inside main breaker box
Why you need ground wire
3 phase is used for commercial buildings
Illustration show 3 phase arriving at business

The same 7200 volt distribution wires used for residential power are used to supply 3-phase for businesses.
Each Hot wire connects to a different transformer. The transformers work on same principle as residential transformers, with a primary and secondary coil, except wiring configurations are different.

With residential transformer, voltages are uniform 120-240. But with commercial 3-phase service, there is a choice of voltages ranging from 208-208-208-120V to 480-480-480-277V to 600-600-600-347V, with 600V being the Hot-to-Hot line voltage, and 347 being the Hot-to-Neutral phase voltage calculated using a ratio of √3 or 1.732.
Once a voltage is selected for the service, the power company installs the correct transformers and configures the wiring different ways to achieve voltages specified for the building..

Most businesses also have 120 volt Hot-Neutral single-phase available for outlets, same as a residential home, but usually the lights for commercial stores, buildings, schools etc are connected to higher voltage single-phase, for example 240 volt, 277 volt or 480 volt, to run more efficiently.

What is 3-phase
How to wire 3-phase
Transformer configurations
3-phase service panel3-phase breaker box
Single-phase residential breaker box has 4 busbars, one busbar for Neutral, one for ground, and 2 Hot busbars.
3-phase breaker box is different and has 5 busbars with 3 Hot busbars, a ground busbar, and a Neutral busbar.
Using a 3-pole circuit breaker, and taking 1 wire from each Hot busbar gives 3-phase electricity to run motors and equipment. Each Hot is out of phase with each of the other Hot wires.
Taking 1 Hot wire and a Neutral, or 2 Hot wires instead of three, yields single phase from the 3-phase breaker box.
The difference between commercial single-phase and residential single-phase is that voltage choices vary with 3-phase service. For example, you can get 120, 208, 240, 277, 480 volt single-phase from a 3-phase service, depending on which transformers and which wiring configuration was specified for the service.

3-phase motors are connected with 3 wires, with each wire connected to a different coil inside the motor.

How to wire 3-phase
3 phase loads are controlled by single-phase
How do you switch 3-phase on and off? Using single phase switch, timer, etc.
For example, 3-pole motor starter switch seen in illustration at top of page, or the illustration on left shows T104 timer activating a 3-pole contactor that turns the 3-phase load on-off. Other single-pole switching methods include thermostat, PLC, electronic circuit, etc.

How it works: Pull 2 Hot wires off the 3-wire circuit to get single-phase. In this example, the 3-phase service is 208-208-208-120 WYE or 240-208-208-120 High Leg Delta. The available voltages are determined by type of transformer and transformer wiring installed by the power company.

The coil on the contactor must be rated same voltage on wire, and meet or exceed amp rating of circuit.

3-pole contactors
T104 timer
How to wire 3-phase timer
More detail:
Sine wave shows 4 different aspects of 120-240 volt single-phase electricity.
1) Rise and fall of voltage over time on Hot Line 1 and Hot Line 2 found on 120-240 volt household electricity.
As the generator rotates, and electrons reverse direction back and forth. The electrons race down the wire one direction before slowing to a stop and then race back down the wire the opposite direction.

2) Average Voltage: Each time the electrons stop, voltage drops to zero, as represented by the center line showing 0 volts. As the electrons accelerate one direction on the wire, voltage rises as shown in TOP half of sine wave. As the electrons slow, voltage drops. Electrons stop momentarily and voltage reaches 0 volt again. As the generator keeps spinning, the electrons begin accelerating the other direction and voltage rises as shown in BOTTOM half of sine wave. The voltage rises to more than 120, but using calculus, it can be shown that the average voltage is 120 on each Hot wire. Average voltage is not accidental, and comes from mathematically engineered electric production that delivers standardized voltages across the grid.

3) Time: The illustration shows the length of one cycle which is equal to one full rotation of the generator. This is called cycles, frequency or hertz. The generator in the US and Americas rotates 60 times per second.

4) Phase: Illustration shows how Line 1 and Line 2 are out of phase with each other, with electrons on each line accelerating different direction at any given moment of time.
Sine wave shows 4 aspects of 3-phase electricity as it emerges from generator.
1) Rise and fall of voltage over time on Line 1, Line 2 and Line 3 found on 3-phase electricity.

2) Average Voltage: Power plant generator can produce 30,000 volts. Voltage reaches 0 on each individual line, but at no time does voltage reach 0V on all 3 lines at same time. This translates to higher average voltage per rotation of generator for 3-phase compared with single-phase. When 3-phase is connected to a motor, the average higher voltage means less amperage is needed, which means motor runs cooler. Amperage is the heat on the wire that trips a circuit breaker. Volts x amps = power (watts), so 3-phase can deliver more power with less heat loss, making 3-phase more efficient than single-phase. Lower amps also means that wires can be smaller, thus lowering cost of installation.
"The horsepower rating of three-phase motors is about 150% greater than for single-phase motors with a similar frame size. The power delivered by a single-phase system pulsates, and the power falls to zero three times during each cycle. The power delivered by a three-phase circuit pulsates also, but it never falls to zero. In a three-phase system, the power delivered to the load is the same at any instant. This produces superior operating characteristics for three-phase motors."  3-phase circuits and basic math .pdf

3) Time: The illustration shows the length of one cycle which is equal to one full rotation of the generator.

4) Phase: Illustration shows how Line 1, 2 and 3 are out of phase with each other, with electrons on each line accelerating different direction at any given moment of time.
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