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208 volts
208 volt is usually derived from commercial 3-phase transformer configurations.
-208 volts is a standard voltage available in the US, usually found in commercial applications such as business or apartment building.

-There are a variety of standard voltages across the world.
Voltages are a function of power generation, mathematics of electricity, transformer configuration, availability of conductive materials, cost, safety, and need for standardization.
Standardized voltage means mass production of electric wire, switches, and appliances is possible.

-End-user voltage is specified by the builder or designer according to normal practices, followed by installation and wiring of transformer(s) by the power company. Power company transformers are manufactured by standard specifications established in the industry and can be wired different ways to achieve desired voltage.

-Transformers work on the principle of electromagnetics where applying electricity to one coil of wire will energize the other coil of wire with electricity. Inside the power company transformer are two coils of wire called the primary and secondary coils. Each coil is wrapped around a metal core. The metal core is shared by both coils, but both coils of wire 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 lower or raise voltage. Raising and lowering voltage has the inverse effect on amperage: when volts are raised by the transformer, amps are lowered. This inverse relationship is represented by the formula E volts x I amps = P watts.
-Lower amps means wires run cooler, and thus high voltage is used to transmit and distribute electricity over long distance. Transmission lines might carry 500,000 volt to the transformer substation, subtransmission lines carry 69,000 volt to local substation transformer, and 7200 volt is used to distribute electricity to each home and business. High voltage requires large expensive switchgear making it impractical to use inside a home or business. Each home or business has a transformer(s) to covert 7200 volt distribution electricity into usable voltages.

-Different voltages are achieved throughout the grid by varying the number of turns on the primary and secondary coils.
-For example a typical residential home has 1 transformer with a 30:1 turns ratio (30 turns on primary side for each turn on secondary side) that converts single-phase 7200 volt distribution line into 120-240 volt for household. The power company connects 7200 volt to the primary coil, and that produces 120-240 volt on the secondary coil. Normally a residential home in US does not have 208 volt, but 208 volt is frequently available in residential apartment buildings. ResourcesSee inside main breaker box

-Typical buildings, businesses and factories have 3 transformers instead of 1. This is called three-phase service. The 3-phase service is the usual source for 208 volt. The power company connects three-phase 7200 volt distribution line to the three transformers on the primary side. Correct wiring is selected to achieve a 34.6:1 turns ratio (primary volts divided by secondary volts) to produce 208-208-208-120Volt on the secondary side.
-The resulting 208 volt service gives several wiring options: 3-wire 3-phase 208 volt for motors etc, 2 wire 208 volt single-phase for appliances and commercial lighting, and 120 volt single-phase for outlets and light bulbs etc.
Resources: Difference between single-phase and 3-phase     Transformer configurations

-Another way to achieve 208 volt is wiring the 3 transformers for High-leg Delta. This configuration gives several wiring options: 3-wire 3-phase 240 volt, 2-wire single phase 208 volt, and single-phase 120 volt.

-Different transformers and different wiring configurations allow a selection of voltage choices for business and industry with 3 wire and 4 wire varieties including 208-120, 240-120,  480-277V and 600-347V etc.
Usually, there are no choices for residential homes, and 120-240 is standard. This means you would more likely encounter 208 volts in a building or business, but not at a residential house. ResourceBasic household wiring
-It all starts with standardization, and is determined by the transformer(s) installed at each location.
3-phase distribution panel
Larger image
Example 3-phase distribution panel with 208-120 volt service

4-wire installation with 3 Hot wires, 1 Neutral + ground.
The ground wire is always connected to the Neutral (not shown)

3 Hot wires and 1 Neutral enter meter box
3 Hot wires and 1 Neutral enter distribution panel and connect to main breaker.
Main breaker supplies power to 3 separate busbars.
3-pole breakers snap onto the 3 busbars.
3-pole breaker supplies 208-208-208 volt power to 3-phase circuit with 3 Hot wires, plus Neutral wire and ground wire

The Neutral wire means this panel has both 208V and 120V.
208 divided by √3 (1.732) = 120 volt
120 volt multiplied by √3 (1.732) = 208 volt

Voltage opportunities from 208-120 panel include:
-3-wire 3-phase 208 volt for motors etc: 3-pole breaker snaps over the 3 Hot  busbars and then Blue, Red and Black wires travel to load.

-2 wire 208 volt single-phase for appliances (water heater) and commercial lighting: use double pole breaker or pull 2 legs off the 3-pole breaker.

-2-wire 120 volt single-phase for outlets and light bulbs etc: use single breaker or pull 1 leg off the 3-pole breaker.
wiring options: 3-phase water heater label
Single-phase 240-208 water heater 
208 volt as it applies to water heater
All appliances have a label.
The purpose of a label is to specify what voltages can be used with the appliance.
Typical residential water heater label shows 240-208 rating. This means the water heater will work with 240 volt or 208 volt.
Note if the water heater is connected to 208 volt, the wattage of element drops to 4125 watts. This is because the water heater comes with elements rated for 240 volt, and because elements are like light bubs. When household voltage drops, the light bulb becomes dim. Same with water heater element: when 208 volt is applied to 240 volt element, the element burns a bit cooler and heats slower. Each element has a rating that appears on the end.
In the very unique case of water heaters, you can connect 120 volts to the 208-240V heater. So voltages lower than the highest rating are ok, but this applies to water heaters and not other household appliances.
The reason residential water heater will work with 120-208-240 volt is the elements are resistive (like a light bulb) and the thermostats are mechanical. 
If the water heater is connected to higher voltages, such as 480 volts instead of 208-240, the elements will burn out because the elements are rated for 240 volt maximum. If the elements are changed to 480Volt rating, the water heater will function correctly using same thermostats etc, but the label must be changed on water heater to meet code.

Electrical equipment and timers that are rated for 240 volt are often rated for 208 volt. But do not assume. Always check the label. Connecting to wrong voltage can be dangerous or damage equipment.
Resources:
Figure volts amps watts for water heater
Connect water heater to 120 volt
Connect water heater to generator
Single-phase 208 elements
208 volt as it applies to elements
Elements are called resistance elements.
Any ordinary resistance element can be connected to 208 volts.
For example, connect 208 volt to element rated for 240 volt, and the wattage output is lowered because the element burn cooler than the coil resistance was engineered for.
The lower wattage still heats acceptable amount of water using 208 volts on 240 elements.
If 208 volts is connected to element rated for 120 volt  will burn out element
If 208 volts is connected to element rated for 480 volts, the wattage drops 

Resource:
More about elements
Figure volts amps watts for water heater
Single-phase elements
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Figure volts amps watts
What is 3-phase?
3-phase is generated at the power plant. It is more efficient than single-phase household voltages, but requires more wiring.
In a single-phase application, there are two wires and each device requires both wires to complete the circuit.
With 3-phase, each circuit requires 3 wires. When power is switched on-off, the switch turns off all three wires.

Consider another perspective that partially explains the difference:
120 volt single phase is like a bicycle without pedal straps. You can only push the pedal downward, and the upward stroke provides no power.
240 volt single phase is like a bicycle with pedal straps, so while one pedal is pushing down, the upward stroke is being lifted. Result: 240 has more power and efficiency than 120.
The limitation with single-phase 120-240 is the lag when pedals are at the top and bottom of the stroke.
3-phase is different.
3-phase is like a 3-legged bicycle with pedal straps (an interesting invention indeed) where the pedals are each 60 apart so one pedal is always in the downward stroke and never are both pedals at the top and bottom at same time. Result the average power is greater.
Resource:
What is 3-phase: power plant to end user
3U element Three-phase
There are two types of three phase water heaters for commercial application: 3-phase balanced and three phase unbalanced. 3-phase water heaters can be converted to single-phase household voltage.
3U bundle elements are for 3-phase balanced applications and have 3 elements in 1
Wiring uses 3 or 6 wires depending on specification of heater, illustrations shown below.

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3U bundle element
3U elements

Resource:
More about 3-phase water heaters
3 phase elements Three-phase water heater elements
Water heater will specify WYE or Delta
3-phase tankless requires delta (call manufacturer)

3-phase tank-type heater can be connected to wye or delta (call manufacture- look at spec sheet- look at label on side of tank)

6 wire leads:
can be used single phase, or 3-phase delta or 3-phase wye

3 wire leads:
can be used for 3-phase wye only

Resource:
3-phase water heater manual p17
How to wire 3-phase

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Single-phase
Residential homes in US do not have 208 volt
Residential services have 1 transformer. Inside the transformer are two coils represented by illustrations on left.
The transformer receives 4500-7900 volts from power company on primary side, and the output coil or secondary puts out 120-240 volt power.
Contrast with commercial 3-phase services have 2 or three transformers, each transformer has 2 coils inside.
No matter if the service is 1 transformer or 3 transformers, each transformer has 2 coils inside.
Most residential household electric services are 120-240 volt with 2 hot legs and 1 center-tap neutral. (ground wires not shown connect to center-tap neutral and to the soil.)

Hot leg to neutral gives 120 volt
Hot leg to Hot leg gives 240 volts
208 volts is not available.
Exception: some apartments buildings have 120 and 208 volt that is supplied by the building's 3-phase service.

Illustrations show the primary and secondary coils inside the transformer.
Each transformer consists of primary and secondary coils.
Primary wires are the power lines coming from substation.
Secondary wires connects to lines going into customer's breaker box

Resources:
See inside main breaker box
Electricity generation: power plant to home
Why you need ground wire


Larger image
Three-phase and single-phase
Some buildings & apartments have 208 volt

208-120 WYE or 208 Delta service
1 hot leg and 1 neutral provide 120V single phase (wye only)
2 hot legs provide 208V single phase, and wiring for 208 volt looks identical to household 240 volt wiring.
3 hot legs provide 208V three phase

Note: the delta configuration does not have neutral connection so 120V is not available

Images show 3 transformers, normally found on the power pole near each business, or located underground in waterproof electrical vault..

Resource:
How to identify transformer wiring
Transformer resources
How to wire 3-phase

high leg delta
Larger image
Three-phase and single-phase
Commercial buildings with high leg delta service have 208 volt
High leg delta or split-phase Delta service
120-208-240 volts configurations

208 volt is derived from the center-tap neutral and the Hot leg furthest from the center tap.

Illustration shows 3 transformer coils on secondary side, primary coils are not shows.
Each transformer consists of primary and secondary coils.
Primary connects to power lines coming from substation. (not shown)
Secondary connects to wires going into customer's breaker box

Not shown are Open delta and open Wye

Resource:
How to identify transformer wiring
Transformer resources
How to wire 3-phase
Distribution wiring diagrams
Difference between Delta and Wye
Delta transformer coils are wired in parallel.
WYE transformer coils are wired in series.
Delta has lower amps/ meaning less heat.
For example many transformer banks are wired Delta on the Power company or Primary coils, and Wye on the customer or secondary coils. The reason for this: the primary side voltage can be 4500-7900 volts, and using delta configuration means lower amps which means the manufacturer can use smaller wires on the primary side coils and save money.
Materials used for the power grid are a balance of cost, safety, and function.
Star-Delta (WYE-delta) is also a configuration used for motor starter timers, such as a star-delta timer. With motors, they consume a lot a of amperage to start so the motor is started with the star or wye configuration, and after the motor gets going, the circuit is switched to delta for higher efficiency.
Resource:
How to identify transformer wiring
How to calculate the voltage for WYE
Test Hot to ground = voltage
Test Hot to Neutral voltage multiplied by √3 (1.7333) = Hot to Hot voltage
Test Hot to Hot voltage divided by √3 = Hot to Neutral voltage
Calculations assume balanced voltage across all three lines

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Copper ground wire
Use same size as other wire in circuit

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