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How to identify transformer wiring

Three phase Wye or Delta?
WYE transformer

Quick way to identify WYE or DELTA

Applies to 3-transformer installation.
Three wires drop down from overhead power lines.

If one of the drop wires connects to only one transformer then that connection is WYE
If the drop wire connects to 2 different transformers, then that connection is DELTA
Difference between Delta and Wye
WYE transformer coils are wired in parallel, like connecting 4 light bulbs to the hot wire.
On the 3-can transformer bank for WYE, you would observe 3 hot wires drop down from the 3 overhead dristribution lines. Each hot wire connects to 1 transformer.
Delta transformer coils are wired in series, like stacking batteries end to end.
On the 3-can transformer bank for Delta, you would observe 3 hot wires drop down from the 3 overhead dristribution lines. Each hot wire connects to 2 different transformers.

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 4400-7900 volts, and using delta configuration means lower amps which means the manufacturer can use smaller wires on the primary side coils and save money. It also reduces heat and need for transformer cooling. Using WYE on the secondary, or customer side, supplies a Neutral connection, and allows for more voltage combinations and higher-amp service, with equal votage on all 3 legs.
Materials used for the power grid are a balance of cost, function and safety, so reducing amps using Delta on primary side saves money overall. However not every customer application can be met using Delta primary. There are a wide variety of volt-amp requirements at end user locations, and many ways to wire transformers to meet electrical needs.
WYE is frequently called Star, for example Star-Delta motor starter or timer. Motors consume a lot a of amperage when starting, so the star or wye configuration (more amps) is used for starting, and after the motor gets going, the circuit is switched to delta (low amps) for higher efficiency. This basic strategy meets electrical regulations intended to reduce voltage surges on the grid.
What is electricity
Name part on electric poles
Pulling overhear power wire
Electricity from power plant to end user
How to wire 3-phase
3-phase timer
3-phase surge protectors/

Delta primary/ WYE secondary

1 supply wire connects to TWO transformers
Each hot wire from the power pole connects to two transformers
Ground wire to each transformer NOT shown


1 supply wire connects to ONE transformer
Each hot wire from the power pole connects to one transformer
Ground wire to each transformer shown

Delta Delta

Ground wire to each transformer NOT shown
Transformer wiring
Larger image
Original image

Delta WYE

Transformer wiring/ how to identify transformer wiring

Image shows Delta Primary and 4-wire WYE Secondary
Primary is Delta: How do we know? Each hot wire connects to two transformers.... so they are wired in series
Secondary is WYE. One wire connects to all three transformers, and to Neutral. One wire from each transformer is Hot... so they are wired in parallel
System neutral connects to Neutral and ground
There are many different transformer configurations

This is 'most common type of wiring' because Delta has lower amps/ meaning less heat, so insulation on primary coil is less expensive.

Transformers can be wired different ways depending on the incoming electric supply and end user need.
Two key factors for the grid are economic performance and reliability.
Reliability is necessary because electricity must run clean without big-voltage spikes and low-voltage brownouts. Otherwise motors and HVAC equipment will have shorter lifespan.

Transformer configurations
Transformer manuals
What is 3-phase electricity/ power plant to end user

Normally, the Neutral service wire is located on top of other three Hot wires
Primary wires are not insulated/ Secondary wires to service are insulated.
Neutral on pole connectes to Neutral going to service, and to ground wire that bonds each transformer to the earth, illustration does not clearly show this. Installations might use bare solid copper, or use the stranded aluminum per more recent practice.
3-phase service
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Larger image of grid from power plant
Delta-WYE Delta primary/ WYE secondary ... because Delta carries less amperage, so primary coils can use smaller wire ... saves cost of transformer
Normally, the Neutral service wire is located on top of other three How wires
Primary wires are not insulated/ Secondary wires to service are insulated.
Neutral on pole connectes to Neutral going to service, and to ground wire that bonds each transformer to the earth.
electric grid
Larger image
Larger image of grid from power plant
Residential 120-240 volt service
Normally, the 3 service wires to house are triplex, with 1 bare stranded Neutral and 2 insulated Hots.
The Neutral wire runs continuously across the grid to bond all grounds into one giant array of earth connections that stablize the grid from overvoltage, short circuits, lightning etc ... and to protect and assist circuit breakers and wires. Ground wires are present at each pole, transmission tower, and electrical connection across the grid.

Residential 120-240 volt service
Larger image of grid from power plant
Neutral on pole connectes to Neutral going to service, and to ground wire that bonds each transformer to the earth.

Larger image
Larger image of whole grid
Ground Loop or Earth Loop
Note the Primary Neutral wire (static line, lightning protector, when the Neutral runs along top of pole) does not visibly connect to the secondary Neutral wire that drops down to the service.
Instead the Primary Neutral wire on pole is connected to the ground wire on pole, and ground wire runs to bottom of pole into the earth, and also connects to each metal transformer.
So each transformer is bonded to earth and bonded to the primary Neutral that comes from local substation.
The secondary Neutral or service drop, is pulled from the transformer's secondary coil inside the transformer. The secondary Neutral connects to the customer service but also to metal transformer and to the ground on the pole. The entire Neutral system is bonded to earth by the ground connections.
Neutral wire is part of the grounding system
The Neutral wire is not continuous from the power plant to end user, except for the ground connections at each substation and transformer, The system of grounding bonds the entire Neutral system together, protecting both customer and grid from overload and surge caused by lightning, and short circuits etc.
The ground wire must have sufficiently low resistance, or a short circuit might cause overload on breaker panel, circuit breaker and wire. This will damage the service panel and create hazardous wiring conditions.
Each breaker has a rating for amps, but also a rating for ground resistance. For example, a breaker cannot absorb a direct lightning strike, or it will burn out. That's an extreme example of too much voltage, but the earthing or ground system plays a role for circuit breakers.
When a short circuit occurs, the overload should move rapidly through an unobstructed ground path and be absorbed in the earth. At the same moment, the breaker absorbs the overload and trips. If the resistance is too high on the ground, that affects heat on the breaker. So the ground wire assists the breaker.
The overall ground at each location can be tested for earth loop impedance (resistance). Impedance measures how much the ground is impeded or slowed by poor earthing.
There is no simple explanation for measuring earth loop impedance, but the general theory is as follows: the local ground rod at each business is electrically bonded to all other grounds back to the main power plant via the Neutral wire. The total ground loop or earth loop offers impedance. The impedance can be calculated, but it is usually advisable to buy proper measuring device.

Delta Primary / WYE secondary/ same as above

wye delta
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WYE Primary / DELTA secondary/ 

distribution transformers at substation handle high voltage

Shows 3 transformers, with 3 primary coils and 3 secondary coils
Primary WYE 69,000 volts ... because 1 69,000 volt wire connects to 1 transformer
Secondary Delta 4,400 volts ... because 1 4400 volt wire connects to 2 transformers

Generally 4,400 volts is lowest voltage for distribution without heat loss from high amperage
The lower the voltage, the higher the amperage.

High amperage causes heat loss.
The transmission of electric power requires high voltage and low amperage.
When amperage is high, the distribution distance is reduced.

However high voltage switchgear is expensive
It is impractical for end user to install high voltage appliances and motors because of the expense

As a result, the end user must receive low volts with higher amperage
To meet demands on the grid, transformers are used to raise and lower volts and amps in inverse proportion
When volts are reduced, amps are increased.

What is 3-phase electricity/ power plant to end user
Transformer wiring

Why does transformer wiring differ.

1) Electricity is mathematical
For example: Cosine of phase angle is known as power factor
P is power factor
Power factor is cosine of phase difference between v and I

Research star-delta for more information
And read below

Power generation by Singh
3-phase power generation book at Amazon
Electric Power generation books at Amazon
3-phase wiring books at Amazon

2) Cost
Delta configuration reduces amperage for the same amount of voltage

3) How does WYE DELTA affect cost
3-phase is more efficient because it runs at higher average power vrs single phase.
It's like having a 3 legged bicycle rider on a 3 pedal bike, with each pedal spaced evenly apart vrs a two legged bike rider on an ordinary bicycle.

4) Cost for 3-phase motors and commercial equipment:
"During start-up, motors develop high currents of up to eight times the rated current, and they have high starting torque.
The high starting currents often lead to voltage drops in the supply network which can affect reliability of other equipment, and cause surges that shorten life of expensive equipment.
And the high starting torque puts strain on mechanical parts, which wears out motors.
Electric companies determine limiting values for motor starting so the power surges and heavy-amp draws do not affect other users. Such as requiring 2-stage motors or star-delta or auto-transformer motor starters etc.
A 2-stage motor means the motor starts out in 'low gear,' and when it gets to higher speed, it shifts to 'higher gear.' This saves energy and wear and tear and helps prevent surge.
Motors have start winding for getting the motor started. And then the run winding kicks in for motor at full speed.
WYE configuration is used for the start winding. And then Delta is used for full speed.
The reason is that Wye supplies more amps per each volt.
For example: voltage is applied to the WYE (also called STAR) start windings. The voltage is reduced by a factor of 1√3 = 0.58 this connection amounts to approximately 30% of the delta values. By reducing the voltage, the starting current is reduced to one third of the direct starting current. When motor reaches speed, the run winding (wired in Delta) takes over and motor receives full voltage.
This staged approach to motor starting, when applied to all motors in a home, and in a business, add up to considerable savings.

Transformers and motor starting
3-phase formula
Connect transformer to wire

Connecting High voltage wire to transformer

Hot clamp

Hot clamp



Read about ohm's law/ electricity/ and what transformer does
Transmission and distribution lines
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Sub-Transmission and distribution lines

Transmission lines run between power plant substation and local substations
Subtransmission run from local substations to local substation
Distribution lines run from local substation to the end-user location

Generally transmission lines are higher off the ground because the air space acts as the insulator for bare conductor wire. Air is a non-conductive insulator.

Transmission lines might be 500,000 V and sub-transmission 69,000 V
Distribution lines are from local substation to each building and house, and are lower to ground and carry lower voltages such as 4400 to 6900.

Image shows transmission lines that run from local  Rosenberg substation out to another substation in rural area of county.

Distribution lines leave from the same Rosenberg substation and distribute power in one section of town.
Distribution lines can be identified because transformers connect to the distribution lines.

Transmission and distribution wires are bare.
They have no insulation covering the wire.
This helps keep the wire cool and increases transmission distance.
Wires are kept away from the poles using purified glass insulators and other non-conductive materials such as silicone.
Birds sitting on the bare wire are not electrocuted as long as they do not touch two wires or any part of the pole.
Electricity on the wire is a potential.
The voltage potential is only reached if two wires come together or one wire comes together with a ground source.

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