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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.

Blue color added for illustration <> in real breaker box, the 2 hot wires are black

The 2 hot wires connect to the Main Breaker <> a lug is tightened very tight to hold wire in place. Neutral is connect to neutral busbar.

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.

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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 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 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 or connection points located ontop 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 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 forthon 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

See basic 120V and 240V water heater circuits
What is difference between 120Volt and 240Volt?
Inside main breaker box are Three busbars. 
One busbar is Neutral busbar. 
Two busbars are Hot busbars.
Each Hot busbar is 'out of phase' with the other.

Bare ground wires connects to Neutral busbar as redundant safety.
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
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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