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Understanding how 240Volt circuit works.

Also see difference between single-phase and 3-phase
Electricity arrives from the pole
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Power poles carry high-volt, low-amp electricity.
The lets power company transmit electricity long-distance without heat loss caused by high amperage.

Before entering house, electricity is converted to lower volts and higher amps at transformer.

Homes with underground utilities also have as transformer, and same wiring configurations as above-ground service. Underground wires arrive in conduit pipe that connects to electric meter.

Above-ground or underground, 3 wires enter home ... 2 hot wires and 1 neutral wire. The Neutral is also bonded to the ground wire at different points in the electric system. The ground wire allows overvoltage and short circuits to be routed to earth.
Resources:
Read more detail
Where household electric comes from
Meter and meter box
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Two hot wires and 1 neutral wire enter the meter box.

The meter 'stabs' into meter box using 4 prongs, similar to extension cord plugging into a wall socket.

The 3 wires exit meter box and go to Main Breaker Box

The meter completes the circuit and lets electricity to flow from the transformer to the main breaker box.

Resources:
See illustration of meter wiring
3-phase meters
Inside Main Breaker Box
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2 Hot wires and 1 neutral wire arrive from meter box at the main breaker panel.

Blue color added for illustration <> the 2 hot wires inside typical breaker box are black

The 2 hot wires are connected to the Main Breaker <> a lug is tightened very tight to hold wire in place.
Each hot wire connects to different busbar through the main breaker > here the busbars are named A and B > the 'blue' hot wire provides power to blue busbar, and black to black busbar > Busbars never touch each other > they are suspended away from the main box by insulators.

Neutral is connect to neutral busbar.
The ground busbar and Neutral busbar are usually located on opposite sides of the breaker box... this illustration shows the ground and Neutral busbars on same side of panel for illustrations purposes only.
The Neutral and ground busbars are bonded (connected) together.
Never use Neutral for ground
Never use ground for Neutral. Doing so causes potential hazard. Ground wire should offer no resistance to outdoor soil or earth. Using ground as a Neutral can cause the short circuit and potential fire to detour into the device that is improperly connected.

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Image shows 120Volt line and 240volt line.
Both circuits have ground wire.
Each 120 and 240 require two wires to complete circuit.
120V requires 1 hot and 1 neutral
240V requires 2 hots

Some 240Volt lines have additional neutral wire. Neutral not necessary to complete 240Volt circuit.
Some 240Volt lines have white and black instead of red and black

Resources:
Wire sizes
Basic 120 volt and 240 volt water heater circuits
How to wire 240 volt outlets
How to wire 3-phase outlets and timers
How to convert 240Volt  water heater to 120volts
Main breaker warning

Main breaker warning

Screws on main breaker are HOT at all times
Even when main breaker is OFF, both screws are HOT
Only if meter is PULLED are these screws safe to touch
Each hot busbar carries 120 Volt potential when tested to Neutral or ground.
240 Volt potential exists between busbar A and busbar B.

The reason each busbar carries 120 volt potential when tested to Neutral, but 240 volt potential when tested to other busbar, has to do with way transformer and the grid are structured.

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Voltage per turn x number of turns
Residential transformer
-Transformers work on the principle of magnetic induction where applying electricity to one coil of wire creates a magnetic flux that energizes 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 iron 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. How transformers work pdf

-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 from power company are connected to primary coil via 2 taps H1 and H2 located 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: X1 X2 outputs for Hot wires and X3 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 that connect to ground rods 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 endsthe 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.

Resource
Why you need ground wire
Fig T1 shows residential transformer that converts high voltage distribution power into lower voltage residential power.

Electricity from the grid is alternating current.
Electrons on the wire move one direction, then come to a stop, before moving the other direction. This is caused by the rotation of the power plant generator, where N and S poles of a magnet pass over coils of wire.
In a process called magnetic induction, the electrons move one direction on the wire when the N pole of magnet passes the coil, then momentarily come to a stop and reverse direction when the S pole of magnet passes the coil of wire.
In a simplified version of events, one end of the coil is connected to the Hot wire, and the other end of coil is connected to the Neutral wire.
Read about power generation

Connecting the 7200 volt Hot and Neutral to the Primary coil (H1 H2) on a transformer causes the electrons to oscillate back and forth on the Primary coil,
Illustration shows residential transformer that converts high voltage distribution power into lower voltage residential power.
 
Electricity from the grid is alternating current.
Electrons on the wire move one direction, then come to a stop, before moving the other direction. This is caused by the rotation of the power plant generator, where N and S poles of a magnet pass over coils of wire.
In a process called magnetic induction, the electrons move one direction on the wire when the N pole of magnet passes the coil, then momentarily come to a stop and reverse direction when the S pole of magnet passes the coil of wire.

In a simplified version of events, one end of the coil is connected to the Hot wire, and the other end of coil is connected to the Neutral wire.
Read more detail
Read about power generation

Fig-T1: When the power company connects the 7200 volt Hot and Neutral distribution line to the Primary side of a transformer, it energizes the Primary coil with electrons that are oscillating back and forth in an endless cycle of reversing N S polarity that matches rotation of generator.
The coil creates a magnetic field that magnetizes the iron core of transformer.

Due to the shared iron core, the Secondary side of the core is also magnetized, causing electrons on the Secondary coil to start oscillating in the same endless cycle of reversing polarity.
Electrons on the Secondary coil are oscillating with AC electricity in the same frequency (cycles or Hertz or Hz) as the primary side, except at lower voltage and higher amperage.
It works nicely because 120-240 voltage (the force that pushes electrons, also called Volts, or abbreviated as V or E) can be safely controlled by small switches, relays, cell phone chargers etc contained within steel and plastic enclosures, while the amperage (the flow of electrons, also called amps or current, or abbreviated as A or I) is controlled by circuit breakers and then distributed to Loads (outlets, switches, motor etc) using correctly sized wire to match amp rating of breaker.

In residential service, each Hot wire X1 and X2 in Fig-T1 tests 120 volt to Neutral and also 120 volt to Ground.
So each Hot wire carries 120 volt potential, but when testing across both Hot wires, the test shows 240 volts because each leg (X1 X2) is 'out of phase' with the other.

Out of phase means the electrons on each wire are oscillating different direction from electrons on other wire.
Since both hots wires in a 240 volt circuit come from opposite ends of the Secondary coil, then each hot wire is 'out of phase' from the other wire because electrons are always accelerating different directions on each wire.
This means that each leg of a 240 volt circuit is delivering the same amount of voltage.
When you use two legs for a circuit, it doubles the voltage, like pedaling a bicycle with two legs instead of one.

Footnote 1, taking 2 Hots from same side of coil yields no voltage when connected to an appliance since both Hots are in phase with each other... each Hot must come from the opposite end of Secondary coil.
Footnote 2: The voltage is unchanged no matter how many Loads are being operated. The amps, or flow of electrons vary depending on the number of Loads being operated, with more electrons when Load is ON.
Resources:
Figure volts amps watts
Formulas for ohm's law
What is electricity
How a generator works
Difference between single-phase and 3-phase
Electricity from power plant to end user
240 Volt circuit breakerQuestion: Are both sides of double breaker 30 amp or 15 amp ?
Answer: Both are 30 amp


You can use a double breaker as two single breakers.
If you have a 30 amp double breaker, you can connect a single 10 gauge hot wire and it will work. You would NOT connect a 12 or 14 gauge wire to the 30 amp breaker or it creates a fire hazard.

You can create a 240 breaker by combining 2 single-pole breakers, but code requires a 'common bar' between the breakers so if one trips, then the other breaker also trips.
You would NOT use two 15 amp breakers. You would use two 30 amp breakers and 10 gauge wire.

Volts x amps = watts (power)

-Let's calculate watts for 120 volt single breaker
-15 amp single breaker. 120 volts x 15 amps = 1800 watts of power
-30 amp single breaker. 120 volts x 30 amps = 3600 watts of power.

-Calculate watts for 240 volt double breaker.
-15 amp double breaker. 240 volts x 15 amps = 3600 watts of power
-30 amp double breaker. 240 volts  x 30 amps = 7200 watts of power.

The power is doubled when you use 240 volts compared with 120 volts.
Since each leg of 240 volt circuit offers 120 volt potential to ground, then you're getting two legs of out-of-phase 120 volt when you use double breaker, and doing so doubles the watts, it does not halve the amps.
This shows that each leg of 30 amp double breaker delivers 30 amps, and not 15 amps. .

See basic 120V and 240V water heater circuits
See basic 120V and 240V water heater circuits
All residential circuits require Two wires to complete circuit.
**120Volt circuit requires 1 Neutral wire and 1 Hot wire.
Hot wire can come from either Hot busbar.
By Code, these wires are Black-Hot and White-Neutral.
If either wire is interrupted, the circuit turns OFF. By code the Black Hot wire is switched ON-and-OFF.

**240Volt circuit also requires 2 wires to complete circuit.
In the case of 240V, the circuit requires 2 Hot wires.
One Hot wire from each 'out of phase' busbar is required to complete 240Volt circuit.
These wires can be Black and White or Black and Red, and with heavier gauge wires, like 6 gauge and 4 gauge, the wires are Black and Black.
If either wire in 240V circuit is interrupted, the circuit turns OFF.
Resources:
See 30 amp switch turning off water heater
Difference between single-phase and 3-phase
More detail
Amps is a measure of the number of electrons that are being pushed by voltage through the matrix of a conductor (wire). Voltage remains steady throughout the grid, with small variations.
The voltage and amp designs of the grid are standardized to allow mass production of electrical devices.
Each device has a specific Volt rating and requires a range or amount of Amps to operate correctly.

By NEC code, all wire and cable used for permanent installation inside homes and businesses are rated 600 volt.
This does NOT include extension cords that cannot be used for permanent wiring and have strictly limited applications.
The reason: Standardization. 600 volts is the maximum voltage that distribution transformers are designed to supply to the secondary side of a business transformer. But 600 volts is only supplied to commercial installations when that specific service is required. Homes never receive 600 volt service, and there are no household devices or appliances made for such voltage.
Most businesses receive 120-208-240-277-480 volts depending on requirements. 600 volt service is not common.
Homes receive standard 120-240 volt service, which can range from 115 to 250 volts since voltages are never precise, but expected to be within range to ensure appliances and devices operate reliability. The voltage might vary slightly over time, but is generally stable and unchanging.

If there is a short circuit, where electrons (amps) start running wildly into the ground wire (assuming installation is correctly grounded), then the wires can melt.
Why you need ground wire
a) The voltage does not increase during a short circuit. The voltage didn't spike over 600 volts. The grid transformer doesn't suddenly change it's construction parameters and deliver more voltage.
b) The amp rating of the copper wire does not change. Each wire has a maximum amp rating. For example 10 gauge wire has maximum 30 amps. NEC Code says: Maximum is 80% of rating ... 80% x 30 amps = safe maximum 24 amps for 10 ga wire. Wires are like lanes on a freeway, and amps are like the cars. Wires only allow so many cars before cars slow down and get backed up. Larger wires have more lanes.

c) So what changes when a short circuit happens? The flow of amps on wire increases because the short has allowed the electrons to flow into earth, and the electrons start to back up because the roadway doesn't have enough lanes.
d) But why does the wire get hot when more electrons are on the wire? Why does a wire melt?
e) The answer is, voltage is pushing more electrons against the matrix of a conductor (wire), causing resistance to go up. The copper wire is a fixed size, and the copper matrix will only allow so many electrons to flow on the wire before the resistance limits the flow of electrons. Electrons begin to pile up and push harder, like an impatient traffic jam. The more electrons on the wire, the more heat on wires.

f) Mathematics
First formula: Volts x Amps = P (Power or watts) illustrates the concept. Volts do not change, but more electrons (amps) are flowing down the wire, so amps are rising. This means P (power or watts) is rising.
Second formula: IČ (amps) x R (resistance) = P (Power or watts). The first formula tells us that Power is going up. If P is going up, then [amps x resistance] must be going up. But the wire limits how many amps (electrons) can flow on the wire. Once the number of electrons approaches the maximum, then the number of electrons is limited. This means the resistance must go up../. which slows the electrons and backs them up like too many cars on the road. Too many electrons on the wire and the wire gets hot causing the breaker to trip. Breakers respond to heat, but breakers are slow-acting, so the amperage can melt the wire almost instantly before the breaker trips.

This tells us that amps (the flow of electrons on the wire), held back by resistance, is the cause of heat. If there was no amperage flowing, there would be no heat.
Resources:
How to install electric meter on water heater
Install circuit breaker
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How to replace circuit breaker
Breakers are held in place at both ends <> one end locks into a cleat -or- snaps onto a bar <> other end of breaker rotates to busbar and then 'stabs' onto the busbar 

Push firmly to make sure breaker is stabbed all the way down

When replacing a breaker, check busbar for damage or burning from previous breaker <> busbars are frequently damaged by aging breakers that arc and burn <> arcing will damage busbar so new breaker will no longer work in the same slot.Read steps 32-37:

When lights flicker, check breaker immediately before sparking causes damage in breaker box. Check breaker and listen for fizzing or crackling sound. Check if the breaker screw is tight against wire. Flickering lights are usually a loose wire at a plug, but it pays to check breaker first to avoid busbar damage.

Always put cover back on breaker box to prevent sparks from leaving breaker panel.
Resources:
Read how to replace circuit breaker

For safety, always put cover over Breaker box circuit breakers can overheat from aged breakers or loose wires and cause sparks that ignite a fire
Electricity leaves power plant Resource: Electricity:
The grid is defined by three parts: transmission, distribution and end user. The following links show how it works
Power plant to end user power pole
Power pole to residential breaker box
Power pole to commercial breaker box

How to wire residential electric
How to wire commercial 3-phase

Why you need ground wire
How to wire 3-phase timer
3-phase water heater
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