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What is a 3-Phase Power Supply?

Updated February 21, 2017

Did you know that the electricity you enjoy in your everyday life is vastly different than the electricity that your electric company sends out? Well, theoretically different, that is. When electric is generated and issued by the electric company, it is done so in what's called 3-phase power. The electricity most people have in their home is single phase, but it's drawn off of the three phase grid. So, what exactly is a 3-phase power supply and how does it affect our lives?

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Across the world, and most prevalently in the United States, 3-phase power supply systems are the most commonly used forms of electricity delivery. Three-phase power supply is also used to power large motors or other types of equipment that draw very high loads.


Three phase power supply is delivered on three lines of power, each line is considered a "phase." Each phase carries the same voltage at the same frequency. One line is chosen as the reference line. The voltage flow in the second line is then delayed by a third of the electrical cycle. The third line is also delayed, but at two-thirds the cycle. The reason for this delay in the phases is so power transfer is happening at all times over the alternating current's three cycles. This is also the reason why three-phase power supply is effective at creating the magnetic power field required by electric motors to run.


Three-phase power supply is preferred by electric companies because it allows for certain benefits not attainable through other delivery methods. - There is no need for a neutral line in a 3-phase delivery system. This is because each of the three phases are carrying the exact same current, which results in a linear balanced load. - In a linear balanced load, power transfer is constant. This allows generators to work more efficiently with less vibration. - Electric motors that use 3-phase power supply are simple to design. - Three-phase power supply requires less conductor material than other single or 2-phase delivery systems. This allows for the same level of electric to be delivered but in a much more economical fashion.


Three-phase power rarely enters the home in the United States. The vast majority of homes are powered by a single-phase system. On the event that three-phase does enter a home, it must first enter a distribution panel where it is split out into various single-phase loads. Electric motors that are fed by three-phase power can have their direction reversed by switching the L1 and L3 lines.


While the United States National Electric Code does not require colour-coded phase identification, there are some areas of the country where the local regulation may require it. It is also common practice among electricians to colour code the phases for easy identification. The most commonly used colour code is: Line 1 - Black Line 2 - Red Line 3 - Blue Neutral (if used) - White or Gray Ground - Green, Green with a Yellow Stripe or Bare Copper If the power delivery is a grounded Delta system, where one leg has a higher potential to ground than the other two, the Delta line is always coloured Orange and it is always the L2, or centre line.

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About the Author

Based in Atco, NJ, Dave Donovan has been a full-time writer for over five years. His articles are featured on hundreds of websites, and have landed him in two nationally published books "If I Had a Hammer: More Than 100 Easy Fixes and Weekend Projects" by Andrea Ridout and "How to Cheat at Home Repair" by Jeff Brendenberg.

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