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How to identify transformer wiring
This page covers Wye or Delta connections on commercial 3-phase electric service transformer banks

Single-phase and 3-phase Dry type industrial and buck boost transformers covered at: Buck-boost transformers
Residential single-phase transformers covered at: Single-phase vs 3-phase
Overview of grid on following page: Name parts on electric pole
Low voltage doorbell transformers at following page: Identify doorbell wiring
Low voltage swimming pool transformers: Intermatic PX series
Substation and power plant transformers are not included on this website 2022
WYE transformer Quick way to identify WYE or DELTA
Applies to primary side of 3-transformer installation.
Three Hot wires will drop down from overhead power lines.

If each drop wire connects to only one transformer then that connection is WYE
If each drop wire connects to 2 different transformers, then that connection is DELTA

Resource:
3-phase transformers .pdf
Transformer .pdf files
Difference between delta and wye
Power plant dispatches 3-phase WYE
Generally, at a conventional (coal, gas) power plant, the generator produces 30,000 volt power that travels through a step up transformer that dispatches 200,000-500,000 volt power onto groups of 3 Hot wires that travel to local substations, where voltage is lowered, and eventually distributed to each home and business etc.
Resources: Read more     Image showing grid
Power plant step-up transformers are WYE-WYE, and by design, other types of generators (solar, wind, etc) will connect to the same transmission lines and dispatch same power to match existing grid. Therefore the grid delivers 3-phase to end users in WYE.     Delta is not used for transmission or distribution.
Resources: Difference between Delta and WYE      Generating 3-phase AC power

Each residential and commercial service uses transformers
A transformer is required at each end user to convert high voltage distribution power into lower volt power that can be safely controlled using correctly-sized circuit breaker and wire that supplies power to moderately priced switches, outlets, bulbs, and appliances. Higher voltages would require heavier, larger, and more expensive wire, breaker, switchgear, appliances etc.
Residential homes receive power from a single transformer, which is connected in only one manner, and is not part of this page since it is neither WYE nor Delta.
Commercial transformer banks with two or three transformers are covered on this page.
Transformer basics
All end user transformers have two sides, the primary and secondary -or- the primary coil and secondary coil that are located inside the transformer can. While the 3-phase distribution circuit arriving from power plant is WYE, the end user transformer bank consisting of two or three transformers can be wired in Delta or WYE on either the primary side or secondary side. Some transformer designs contain the 3 primary and 3 secondary coils inside a single can, instead of 3 cans.

Generally, the difference between Delta and WYE is not the transformers, but how the transformers are wired.
While transformers look similar during casual observation, they vary based on the KW or power rating required by end user ... plus internal number of taps, size of wire, number of turns of wire in primary and secondary coils, cooling fins, diameter etc. For example, residential transformers have a 30:1 turns ratio between primary and secondary, and they are smaller than commercial transformers since they deliver fewer amps. Read More Commercial transformers are larger and deliver more power. They can have taps for 30:1 ratio, but offer other taps so higher voltages can be achieved.

Difference between Delta and Wye transformer wiring
WYE transformer coils are wired in parallel.
Applies to transformer banks with 3 transformers:

Primary side WYE: On the 3-transformer bank for WYE, you would observe 3 Hot wires drop down from the 3 overhead distribution lines. Located on top of each transformer are two bushings that connect to the primary coil inside each transformer. Each Hot wire connects to 1 bushing on a different transformer. The Neutral wire connects to the second bushing on all 3 transformers.
Secondary WYE: Each transformer has two or three taps located on side of transformer can. These taps connect to the secondary coil inside each transformer. The commercial building with WYE service receives 3 Hot wires and a Neutral from the secondary side of the transformer. 1 Hot wire is connected to 1 tap on each transformer. The Neutral is connected to the other tap on all 3 transformers.

Delta transformer coils are wired in series, like stacking batteries end to end in a flashlight.
 Applies to transformer banks with 3 transformers:
Primary Delta: On the 3-can transformer bank for Delta, you would observe 3 hot wires drop down from the 3 overhead distribution lines. Each hot wire connects to one bushing on 2 different transformers. The Neutral wire is not connected to any of the bushings.
Secondary side Delta: The commercial building receives 3 Hot wires. Each hot wire is connected to a tap on two different transformers. The Neutral might or might not be present. If present, the Neutral will most commonly connect to the center tap on middle transformer.

Reasons for choosing WYE or Delta
Delta can deliver same power with lower amps/ meaning less heat.
For example, transformer banks are often wired Delta on the Power company or Primary coils, and Wye on the customer or secondary coils. According to one manufacturer's manual, the reason for Delta primary is to save money since the delta configuration means lower amps for the same power output, which means the transformer manufacturer can use smaller wires on the primary side coils. It also reduces heat and need for transformer cooling.
Using WYE on the secondary, or customer side, supplies a Neutral connection, and gives more voltage combinations and higher-amp service, with equal voltage on all 3 legs. WYE offers Line voltage, single-phase Hot-to-Hot or 3-phase Hot-to-Hot-to-Hot, and offers Phase voltage measured Hot to Neutral which is single-phase line voltage divided by square root 3. What is 3-phase 208 volt WYE
Contrast Delta without a Neutral and the service supplies fewer voltage options: single-phase Hot-to-Hot or 3-phase Hot-to-Hot-to-Hot.
Delta with a center tap Neutral offers the widest range of voltages, but also can create additional load balance problems. What is High Leg Delta
Using small customer-installed Buck Boost or Industrial transformers can supply additional voltages at any commercial location. Buck-boost transformers
Materials used for the power grid are a balance of cost, function and safety, so reducing amps using Delta on primary side can save 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 large amounts 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.
Resources:
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
WYE WYE
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.

Resources:
Transformer configurations
Transformer manuals
What is 3-phase electricity/ power plant to end user
electric grid
Larger image
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 connects to Neutral going to service, and to ground wire that bonds each transformer to the earth


3-phase service
Larger image
Larger image of grid from power plant
WYE-WYE
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 connects 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.

WYE-High Leg Delta
Identify High Leg transformer bank: 3 wires come off one transformer and 1 wire (the high leg) comes off another transformer

-Typical buildings, businesses and factories have 3 transformers that provide three-phase service.
Some services that require less power have 3-phase from 2 transformers.

The 3-transformer 3-phase service is the source for High Leg Delta, but not every commercial installation has High Leg, unless specified for the service.

How it happens: If High Leg Delta service is specified by the architect, builder, end user or electrical engineer, then the power company brings the correct transformers, and connects three-phase 4500-7200 volt distribution line to the three transformers on the primary side in a WYE configuration as shown. Primary side can also be wired in Delta.
To achieve high leg, the transformers on the secondary, or the service side, are wired in Delta configuration.
3-phase open wye and open deltaOpen Delta
2 transformer 3-phase open delta for smaller loads

Identify High Leg open delta. There are 2 transformer, one is larger than other, 3 wires come off large transformer and 1 wire (the high leg) comes off another transformer
Open Delta is a lower-power 3-phase Delta option that uses 2 transformers instead of 3. One of the transformers is larger than other.
On the primary side, you would observe 2 hot wires drop down from the overhead distribution lines. Each hot wire connects to one bushing on 2 different transformers. The Neutral wire is not connected to any of the bushings.
On the secondary side, the commercial building receives 3 Hot wires and a Neutral. The smaller transformer has 2 taps. The larger transformer has 3 taps: The Neutral comes from center tap. One Hot wire comes from another taps. Another hot wire comes from the third tap on the large transformer plus one tap from smaller transformer. The third hot wire comes from the second tap on small transformer. See image

High Leg Delta (or open delta) can be achieved using 2 transformers that pull 2 Hots and a Neutral off the distribution line, instead of 3 Hots and  Neutral.
Open Delta can also be identified by 3 drop wires (two Hots and a Neutral) connecting to 1 transformer, and 1 drop (Hot wire) connected to the smaller transformer. The secondary side still supplies 4-wire high leg to the business, with 3 wires coming off 1 transformer and 1 wire (the high leg) coming off the smaller transformer.

This configuration is common for businesses that consume less power, but still want 3-phase, and/or want the selection of voltages found with high leg ... that is 240 volt, 120 volt, and 208 volt.
For example, this service provides power to a windshield repair company, with an office and garage for dispatching trucks.

This configuration is also used in case of emergency to restore power if one transformer goes bad in a 3-transformer bank.

If the business needs more power, then a third transformer can be added at any time ... and this would change the high leg open delta into ordinary high leg delta.
A 3-transformer service is more efficient at delivering high power than 2-transformer service.

Two transformers can also be configured to deliver Open WYE, and can supply 4 wires, with 2 sets of single-phase, but not 3 phase. This would generally be found in a repair condition when 1 transformer in a 3-transformer bank fails.

Larger image
Larger image of whole grid
Delta-Delta
Neutral on pole connects to Neutral going to service, and to ground wire that bonds each transformer to the earth.
Delta Primary / WYE secondary/ same as above
wye delta
Larger image
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.

Resources:
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

Resources:
Research star-delta for more information
And read below

Buy
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.

Resources:
Transformers and motor starting
3-phase formula Formula for calculating power for 3-phase
1.732 is square root 3 and is standard for calculating 3-phase ... for example measuring Hot wire to Hot wire yields the Line voltage while Hot to Neutral is called the Phase voltage which is calculated by multiplying line voltage by square root 3.

KVA is Kilo Volt Amps or 1000 VA or 1000 watts or 1 kilowatt.
Watts and power are different words for same thing.
Each transformer has a power rating to ensure that the building is able to run operations without power loss due to undersized transformers -or- undersized service.

KVA is used in commercial applications instead of kilowatts because of power factor What is power factor
Connect transformer to wire
Connecting High voltage wire to transformer
Hot clamp Hot clamp
Fuse
Transformer
Resources:
Read about ohm's law/ electricity/ and what transformer does
Transmission and distribution lines
Larger image
Rosenberg Texas

Note that the commercial three-phase transformer bank in the foreground is connected to the distribution lines, drawing a hot wire from each of the 3 lines. The transformers then supply power to local business.

This is called 3-phase electric service.

Distribution Transformers are designed to convert distribution voltage into usable, safer, low voltage for homes and business.

Because volts and amps are inversely proportional, then the safer lower voltage at home and business comes with higher amps. To solve problem of higher amperage, correctly-sized wire and circuit breaker are required, and distance the electricity can travel before voltage falls below nominal can be reduced to a few hundred feet.

Electric services are connected to distribution lines, but are not connected to high voltage transmission or subtransmission lines.
3 categories of power lines:
1) Transmission lines (not shown in image) might be 270,000 to 500,000 (775-500-270 kV) volts. Generally transmission is anything over 115 kV.
Voltages are measured Hot-to-Hot, also called the line voltage ... contrast with Hot-to-Ground phase voltage which is line voltage divided by square root 3. Read

2) Subtransmission lines might be 69,000 Volts (110-69-33 kV), and travel from substation to local substation and are located higher off the ground than local distribution lines.
Transmission and subtransmission make up the bulk power system.

3) Distribution lines travel from local substation to each home and business, and are much lower to the ground and carry less dangerous 12000+ volt, measured between hot wire to hot wire ... and 7200 volts (7.2 Kv) measured between hot wire to ground.

"The distribution voltage classes for most utilities can be 5 kV, 15 kV, 25 kV, and 35 kV." Voltages are determined at local substation transformer and vary depending on local demand, distances etc.

Below 4000 volts (4kV) on in-town distribution line can become inefficient because large number of loads will pull large amount of power (watts). If volts are too low, then the loads will draw too many amps from the distribution line, causing higher resistance on lines, heat loss and ultimately damage to power lines, motors etc.
Because volts and amps are inversely proportional, then raising voltage at the substation transformer will deliver the needed wattage (power) for each load on the distribution line, but at lower amp draw, resulting in less heat loss, higher efficiency and more reliable power.
Raising voltage on distribution line might require larger wire, higher poles, different insulators etc depending on expected loading.
Lower voltages can be used on distribution lines that serve very few homes.

Local distribution lines can be less than a mile to more than 20 miles long. There are generally 3 categories for distribution: Radial, Loop, & Network (not discussed further).

Resource:
Design-overhead-distribution.pdf
Mechanical design overhead lines pdf
Electric-power-distribution-system-operation.pdf

Residential 120-240 volt serviceResidential 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 stabilize 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.

Larger image of grid from power plant

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