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3 Phase Power Defined

December 01, 2020

Last Updated: December 01, 2020

by Chad Soucy

3 phase power is a common method of alternating current electric power generation, transmission and distribution. It is a type of polyphase system and is the most common method used by electrical grids worldwide to transfer power. It is also used to power large motors and other heavy loads. A 3 wire 3 phase circuit is usually more economical than an equivalent 2 wire.

How 3 Phase Power Works

Utility generating stations generate 3 phase power using high-pressure steam and low-pressure steam to turn large turbines called generators. Delta-connected transformers step up the voltage, and then is distributed as high-voltage 3 phase power, which then leaves the power plant onto the power grid.

High-voltage transmission lines distribute 3 phase power to consumers, stopping off at switching stations. These are centralized switchyards at the point of utilization for the purpose of transforming the 3 phase high-voltage transmission to a 3 phase medium-voltage transmission. Medium voltage is a suitable voltage for industrial installations, though transforming to a lower voltage is required for residential and commercial buildings.

It’s important for electrical personnel to understand the relationship between a transformer connected in a Delta and Wye configuration. Transformers are designed to isolate and step up voltage (or step down voltage depending on the application). They are typically configured as Delta or Wye.

3 Phase Power Distribution- Delta vs Wye Configuration

An alternative transformer arrangement found in the USA is called Delta. It gets its name because it is similar to the greek symbol Delta, which is like a triangle.

Delta is commonly used for 3 phase transmission and industrial plants. Wye is commonly used for 3 phase power supply commercial consumer services and a single-phase (1 phase of 3) supply residential consumer service entrances.

Unlike the Wye configuration, which branches out from a common neutral, the three phases all connect head to toe. Delta is typically ungrounded and without any neutral. Described as phase to phase, nominal voltage typically would be 240 volts.

An added center-tap configuration means one of the phases and the neutral results in two legs with voltages of 120. A third available voltage of 208 volts is called the “high leg,” identified (usually) orange in color.

3 Phase Power, Delta Configured: Four-Wire

Phase to phase voltages measures 240 volts, 208 volts and 120 volts. This can be desirable for some commercial and industrial buildings. While a wide variety of office equipment and tools operate at 120 volts, more powerful appliances, like industrial lighting fixtures, require 240 volts and 208 volts.

An electrical technician has to be well versed in practical theory and the national electrical code to perform installation and maintenance to transformers. Often he/she is required to configure a transformer connection to Delta/Wye or Delta/Delta, Wye/Wye or Wye/Delta.

3 Phase Power, Wye Configured: Four-Wire

Wye connection resembles the letter Y, also known as star. A common supply voltage is 120V/208V 3 phase Wye configuration. 120 volts is measured from each phase to neutral (neutral is center-tapped) and 208 volts is measured phase to phase.

Wye configuration is commonly used to supply 3 phase 4 wire service entrances, such as commercial buildings. Single-phase residential service entrances are derived from one of the three Wye-configured 3 phases.

3 phase Wye nominal low voltages 120 volts/208 volts, and 277 volts/480 volts, are commonly available for commercial and lite industrial buildings. 277 volts is used primarily for commercial office lighting, and 480 volts is used to power large HVAC systems.

3 Phase Wye Consumer Supply

Wye 3 phase power can be transformed to lower voltages. Typically, the main service supply for commercial buildings favors the cost savings of Wye-configured 120/208 volts distributed from the panelboard to feeders and branch circuits.

Wye is versatile in terms of the voltage that is available in relation to electrical equipment and appliance power requirements. All 3 phases can supply a 208 volt motor load, a 208 volt industrial water heater, and 120 volts 1 phase to neutral for standard outlets, appliances and office equipment.

3 Phase Power vs Single-Phase Power

3 phase power is the primary form of electrical power at our businesses and factories. Here are the notable differences between 3 phase power and single-phase power:

  • Compared to single-phase power, 3 phase power has a higher power factor, greater efficiency and requires lower current for the same amount of power. It also requires smaller wires, so it is less expensive.
  • 3 phase is more efficient for transmission given the same peak voltage between conductors and the same current in each conductor.
  • 3 phase delivers three times the power, with one and a half times the copper. Three wires, instead of two, doubles the usefulness of each pound of copper or aluminum over long distances, which is a significant cost savings.
  • 3 phase power transforms into single-phase power at the pole top, transformers and pad-mount transformers for underground consumers service entrances nearest our homes.

Electricians working on new construction, remodel work, maintenance and repair work must be a qualified person (QP) as stated by OSHA and NFPA 70E. In addition, they must meet local and state licensing requirements. It is typical for the work environment to classify the work being performed, and a risk analysis based on NFPA 70E, is required.

Whether seeking a career as a residential electrician or a commercial electrician, 3 phase power is part of electrical theory, and involved with practical experience vital to the foundation of your overall knowledge base. With developed instructor videos from experts, knowledge checks and simulations around core topics, Interplay Learning’s SkillMill™ courses are a great place to start.

Chad Soucy, Electrical Expert

Chad Soucy

Interplay Learning Electrical Expert

Chad is Interplay’s electrical expert and is a Master Electrician. Chad has progressed as an electrical professional throughout his career, with early beginnings in rewiring/wiring homes to QA/Commissioning plants, honing his skills in all aspects of the electrical trade along the way. He transitioned his career through Residential, Commercial and Industrial sites, and in 2012, further expanded on his mission to lifelong learning in becoming an electrical instructor. He continued on this path as an online course developer and is steadfastly committed to electrical safety and sound adult learning theories.


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