What is 277 volt

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What is 277 volt

Covered on this page: 277 volt is a commonly used, single-phase voltage in the US that is derived from 480 volt commercial 3-phase service. Typical residential home would not have 277 volt.

Covered on separate page: 277 volt single-phase can also be achieved by converting another service voltage into 277 volt using on-site 3-phase transformer, Buck boost transformers

In all cases, 480 volt 3-phase WYE is required to achieve 277 volt single phase because of standardization of voltages and the mathematics of electricity.

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277 volts is a standard single-phase voltage that comes from 4-wire 3 phase 480 volt commercial electric service.

Fig-1 shows 2 different commercial electric services:
- 3-phase 480-480-480-277 volt WYE in the background ....
- 3-phase 240-208-120 High Leg Delta in the foreground.

Each service looks the same, each has 3 transformers, and each transformer bank provides 4-wire 3-phase to the end user, but only one service has 277 volt ... circled in pink in the background. The difference is how the 3 transformers are wired. Larger image What is high leg delta

Commercial electric services typically have three transformers as seen in Fig-1.
Transformers are required to convert 3-phase 7200 volt distribution power into usable standard voltages like 277 volt.
Note: A 'service' is the voltage and amount of power (KVa) that is supplied from the grid to the building.

277 volts a standard voltage available in the US, found in commercial applications such as business or buildings.
480-480-480-277 volt is one of several 3-phase electric services that can be installed by the power company. Other commonly used standard commercial voltages can include 208-208-208-120, 240-208-120 high leg, 480-415-240 high leg, 600-600-600-347 etc. Other 3-phase and single-phase voltages found in commercial applications include 220-220-220-127, 230-230-230-133, 460-460-460-266, 380-380-380 Delta, and 575-575-575 volt Delta. See transformer voltages

Voltages are a function of power generation, mathematics of electricity, transformer configuration, availability of conductive materials, cost, safety, and need for standardization.
Standardization makes it possible to mass produce electric wire, switches, appliances etc. It also means standard electric practices, and allows power companies to generate electric power that is shared across interconnected grids.

Transformers are used throughout the grid to raise and lower voltages as needed

Power plant generators produce four wire 3-phase that is transmitted over 3 Hot wires accompanied by a 4th wire called the Neutral. The Neutral is bonded to a ground (earth) connection at each pole, substation, end user service etc, forming a giant array of grounding that stabilizes the grid.

On the customer side of the 480-277 volt service, there are still 3 Hot wires and the Neutral is used with any one of the three 480 volt Hot wires to produce 2-wire single-phase 277 volt.

Fig-2 shows the three 7200 volt Hot wires and a Neutral connecting to three transformers at local drug store. The high voltage distribution wires are bare, while the secondary wires that drop into the building are insulated.

Four wires are standard for electric transmission and distribution to commercial buildings ... with each hot wire requiring a separate transformer that is needed to reduce high voltage into usable standard voltages.

Inside each transformer are 2 sets of coils, called the primary coil and secondary coil. When voltage is applied to the primary coil, it energizes the secondary coil. Different voltages can be achieved by varying the number of wraps or turns of wire in each coil. This called the turns ratio, with more turns on the primary and fewer on the secondary coil. The power company must install 3 transformers, each with the correct turns ratio to convert 7200 volt into 480 volt.

The high voltage distribution wires connect to the 3 primary coils located inside the 3 transformers, while the customer side wires connect to the 3 secondary coils inside each transformer.

Full image

3-phase arrives at building
Commercial voltages and 277 volt:
-Typical buildings, businesses and factories have 3 transformers that provide three-phase service.
The 3-phase 480-277 volt service is the source for 277 volt, but not every commercial installation has 277 volt, unless specified for the service.
Why select 480-277 volt? Higher voltages like 480-277 are more efficient for motors and large area uses than lower voltages like 208 and 240 volt etc.

How it happens: If a 480-270 volt service is specified, then the power company connects three-phase 7200 volt distribution lines to the three transformers on the primary side. Correct wiring is selected on the secondary side to produce 480-480-480-277 Volt for the customer.
Transformers are also selected by the amount of power (KVA) they deliver to ensure the service can meet expected loading (power use).

-The resulting service gives 3 wiring options:
3-wire 3-phase 480 volt for motors etc
2 wire 480 volt single-phase for lights etc
2-wire 277 volt single-phase for lights and other circuits

With this type of 3-phase service:
480 is the hot-to-hot line voltage,
277 is the hot-to-neutral phase voltage.

120 volt for ordinary plug-in outlets is available by stepping down 277 volt using small on-site transformer. (#7, #23, #63 acme transformer wiring)
Resources:
Difference between single-phase and 3-phase
What is 208 volt
Identify transformer configurations
What is 3 phase
On-site transformers

Inside the 3 transformers
Fig-4 from JEA.com shows 3 transformers and the coils inside each transformer for a 480-277 volt WYE-WYE service, same as illustration above.
Resource1 Resource2

To achieve 480-277V on the customer side, or secondary side, the correct transformers with the correct turns ratio to

convert the primary voltage into 480-277 must be installed and then transformers connected in WYE configuration on the secondary side.

The high-voltage primary wires A B C connect to H1 tap on each of the 3 transformers. The H2 taps on each of the 3 transformers are connected together and connected to the N Neutral. The Neutral wire is always bonded to the ground wire that is connected to the ground rod.
This describes a WYE primary configuration.

If the Primary side was Delta, the transformers would look the same, but the Primary wires would connect differently. See delta configuration

On the secondary side, each transformer has X1 tap that is connected to a-b-c Hot wires. The X2 taps on all 3 transformers are connected together, and then connected to the N Neutral wire. The Neutral wire is always bonded to the ground wire that is connected to the ground rod. This describes a WYE secondary configuration. 480-277 volt requires a WYE connection on the secondary as shown. See delta secondary configuration

Note the Line voltage, or Hot to Hot is 480 volts. The phase voltage, or Hot to Neutral is 277 volts.
This shows that 1 Hot wire and 1 Neutral wire are required for 277 volt connection.
This means that 277 volt is a 2-wire connection and not a 3-wire connection. Since 277 volts is 2 wire, then it is alway single-phase, that is derived from 3-phase 480 volt service.
Likewise, an electrician can connect 2 Hot wires and achieve 480 volt single-phase.
Source for illustration

Phasor diagrams are simple way to show transformer connections.
Phasor diagrams are used for Secondary or Primary, depending on what is being explained.
Start with the secondary connections from Fig 4 above, and then simplify into the Phasor diagram that summarizes the wires coming off a transformer bank.

There are 3 transformers. each with X1 X2
Since the three X2 taps are all connected to the Neutral wire, the phasor diagram represents the X2 taps as a single connected point in the center.
The resulting 3-phase service is wired in WYE ... meaning the 3 secondary coils are wired in parallel resulting in 3 Hot wires and a Neutral.

Fig-W2 shows the 'phasor diagram' for 480-480-480-277 volt WYE A phasor diagram is a shorthand method for showing the three secondary coils inside each transformer. Fig-W2 only shows the 3 secondary coils ... it does not show the 7200 volt primary coils. The three secondary coils are represented by curled lines that share a common neutral point located in the center. The neutral point represents actual field wiring where one end of each transformer coil is connected to the same Neutral wire. Relationship between 480 volt and 277 volt The mathematical relationship of voltages with 3 phase is calculated using √3 or 1.732, meaning that 480 volt divided by √3 or 1.732 = 277 volt .... or 277 volt x √3 or 1.732 = 480 volt Fig-W2 shows that 277 volt is achieved using one Neutral wire and any one of the three 480 volt Hot wires. This means 277 volt uses 2 wires to complete a circuit, and therefore 277 volt is single-phase, not 3-phase. It also means 480 volt 3-phase WYE service is required to achieve 277 volt single phase ... with 480 as the hot-to-hot line voltage, while 277 volt is the hot-to-neutral phase voltage calculated using the ratio of √3 or 1.732. Sources for 480 volt WYE and Delta If the three secondary coils are wired in parallel, it is called a WYE (or star) configuration as seen Fig-W2. If the three secondary coils are wired in series, or end to end, then it is called a Delta configuration as seen Fig-D2 and Fig-D3. The choice between Delta and WYE depend on what voltages and power objectives are needed for the building, and other factors having to do with the mathematics of electricity. Resource: Difference between Delta and WYE There are 3 choices of transformer wiring that provide 480 volt to end user: 1) 4-wire 480-415-240 high leg delta with Neutral at midpoint on one transformer coil (Fig-D2) Note 277 volt can be achieved from 240V delta by using small on-site transformer (#70-71 acme transformer wiring) 2) 3-wire 480-480-480 volt Delta without Neutral (Fig-D3). Note 277 volt can be achieved from 480V delta by using small on-site transformer (#31 acme transformer wiring) 3) 4-wire 480-480-480-277 volt WYE with a Neutral (Fig-W2). Only the 480-480-480-277 volt WYE can produce 277 volts. .
Variety of standard voltages: -Different transformers and different wiring configurations allow selection of voltage choices for business and industry with 3 wire and 4 wire varieties of WYE and Delta including 208-120, 480-277V and 600-347V etc, with 480V being the Hot-to-Hot line voltage, and 277 being the Hot-to-Neutral phase voltage calculated using a ratio of √3 or 1.732. Residential homes have 120-240 volt single phase -In the US, there are usually no choices for residential homes, and 120-240 is standard. This means you would encounter 277 volts in a building or business, but not at a residential house or apartment. -Voltages are not exact: For example a 240 volt service can provide 250 volt, 236 volts, etc and still be within acceptable range. Typically, anything less or more than 80% rated value can cause failure. In a commercial setting, a non-standard voltage can be changed to standard using buck boost transformer Resources: Buck boost transformers Difference between commercial 3-phase and household single phase Basic household wiring What is 3-phase How to wire 3-phase Neutral currents in 3-phase WYE .pdf
Larger image 480-277 volt 4-wire 3-phase Fig 6. is a phasor diagram that shows the wiring connections for 3-phase WYE on the secondary side. It does not show coils and connections on the primary side. It also shows the ground wire connection at the Neutral. Throughout the grid, at every pole, substation, transformer, and installation, the Neutral wire is bonded (connected) to the ground or earth to stabilize the grid against overvoltages etc, helping to ensure grid reliability. . At the other end of each of the 3 coils are the 3 Hot wires that carry power from the transformer to the end user's main breaker box. The resulting 3-phase service has 3 Hot wires, 1 Neutral wire and a ground wire that enter the main breaker box, and thus it is a 4-wire 3-phase service (the ground is always required to be present but not counted among the wires).

480-277 volt 3-phase service panel/ with Ground
3-phase Power leaves the transformer and enters electric meter that registers billable wattage.

From the meter, the electric power passes through a 3-pole safety switch, and then enters main panel box where 3 Hot wires connect to 3-pole main breaker, while Neutral wire and ground wire connect to separate busbars
3-pole means there are 3 places for wires to connect. And each pole is electrically separated from other poles.

Main breaker box is same for WYE or Delta service, with 3 hot wires. In some cases of Delta or WYE, there is no Neutral wire present.
WYE services like 208-120, 480-277, 600-347 etc volt have a Neutral wire.

Main breaker feeds each hot wire to separate hot busbar.
There are 3 Hot busbars, each electrically separated from the other, and each carries power from one Hot wire.

There are 3 ways to wire from the WYE main panel.
a) Install three-pole circuit breaker and draw 480 volt 3-phase power where 3 wires leave breaker and go to motor switch, or other use etc.
b) Install 2-pole breaker and and draw 2 Hot wires for 480 volt single phase.
c) Install single-pole breaker and draw 1 Hot and a Neutral wire for single-phase 277 volt.

Resources
How to wire 3-phase electric
See wiring for 3-phase non-balanced water heater

Larger image

Example single-pole breaker in 3-phase panel
Color added for illustration

Single and double breaker can be added to 3-phase panel
Or single-phase can be drawn from one or 2 poles of a 3-pole breaker.

In the case of 277 volt, 1 Hot wire is drawn from the circuit breaker and 1 Neutral wire is drawn from the Neutral busbar.

Larger image	Many timers and other devices are rated for 277 volt Look for rating label Illustration shows 277 volt timer controlling 3-pole contactor that switches power ON-off to a contactor that switches a 3-phase load. 1 Hot and 1 Neutral are drawn off the 480V 3-phase line to yield 277 volt. Inline fuse required on Hot wire pulled from 3-phase. Buy: GM40AV 120-208-240-277V GM40AVQ w/ back-up battery Tork TU series DPDT/ 120-277V universal timer Tork P1104FM 240V SPDT with fireman switch Buy Tork DTU digital timer 3 pole contactor 277volt coil 3-pole contactors with 277V coil 30 amp 277V coil 50 amp 3-pole 277V coil Mars 60 amp 277V coil Mars 75 amp 277V coil Mars 90 amp 277 V coil Mars Resources: How to wire Intermatic GM40 timer How to wire 3-phase timer 3-phase astronomic timer Fuseholders
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Amps and watts with 277 volts Calculations are the same for all electricity around the world: Volts x Amps = Watts. Or watts divided by volts = amps. Compare 120, 240 and 277 volt. For example if the Load (motor, lights etc) consumes 8000 watts. 8000 divided by 277 volts = 28.88 amps requiring 30 amp breaker and #10 gauge wire. 8000 divided by 240 volts = 33.3 amps requiring 40 amp breaker and #8 gauge wire. 8000 divided by 120 volts = 66.6 amps requiring 70 amp breaker and #4 gauge wire. Amperage is heat. If wire gets hot, then energy is being wasted. The more heat on the wire, the larger the wire and circuit breaker must be to maintain efficiency. Larger wire and the conduit needed to hold the wire costs more, especially if the load is spread over a large area like commercial lighting. The mathematics shows that using 277 volts is a less expensive way to handle large loads. "Industrial and commercial 3-phase electric systems differ from residential single-phase: 3-phase power is more complex, but more efficient in motor applications, and large area uses. Higher voltages of 277/480v and 347/600v distribution systems are more efficient, but considerably more dangerous, and should only be maintained and modified by trained and qualified electricians." Resource: Difference between single-phase and 3-phase
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277 volt locking receptacle
277 volt is derived from 3-phase 480-277 service.
One Hot leg and a Neutral wire are pulled from the breaker box to create 277 volt potential
Extreme danger from high voltage 480-277, wiring should be done by licensed electrician
Buy:

Connectors
L7-15R 277 volt 15 amp
L7-20R 277 volt 20 amp
L7-30R 277 volt 30 amp

Outlets
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Resources:
What is 277 volt
How to wire 3-phase
Difference single-phase and 3-phase

277 volt twist lock
The center wire is the Neutral. The top wire is Hot, and bottom wire is ground.
Note also that Neutral is connected to ground.
The ground is not a Neutral and must serve as unobstructed pathway directly to earth.

Resource:
How to wire twist lock plugs
Why you need ground wire

Elements

Contactors/ relays

Z-wave/ wifi

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