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3.4: Three-wire Power Supply System

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    • Contributed by Camosun College
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    Electrical energy to most individual and small commercial buildings in North America is distributed through a 120 V/240 V AC, single-phase, three-wire system.

    Several advantages are gained by using this method of distribution:

    • Copper conductor current requirements can be reduced.
    • Two different voltages (120 V and 240 V) are available.
    • Improved safety is established through grounding the neutral.

    Source connections

    A three-wire circuit is accomplished by connecting two 120 V sources in a series-aiding configuration. The conductor taken from the common point between the two sources is called the neutral conductor. Conductors taken from the two outer points are called the line or hot conductors.

    As shown in Figure \(\PageIndex{1}\):

    • Voltage measured line-to-line is 240 V.
    • Voltage measured from either line to neutral is 120 V.

    This allows connection to either 120 V loads (such as lighting) or 240 V loads (such as ranges or clothes dryers). See Figure \(\PageIndex{2}\).

    Figure \(\PageIndex{1}\): Three-wire circuit (CC BY-NC-SA; BC Industry Training Authority)
    1. 3_wire_distributiont.png
      Figure \(\PageIndex{2}\): Three-wire distribution (CC BY-NC-SA; BC Industry Training Authority)
    2. fault_ungrounded_wiring.png
      Figure \(\PageIndex{3}\): Accidental fault in an ungrounded wiring system (CC BY-NC-SA; BC Industry Training Authority)
    3. fault_grounded_neutral.png
      Figure \(\PageIndex{4}\): Accidental fault in a grounded neutral writing system (CC BY-NC-SA; BC Industry Training Authority)
    4. grounded_wiring_system.png
      Figure \(\PageIndex{1}\): A properly grounded wiring system (CC BY-NC-SA; BC Industry Training Authority)

    Although properly grounded wiring systems do not eliminate shock hazard, they certainly lower the odds of being shocked!

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