In a three-phase system, a minimum of how many supply lines are required?

In a three-phase system, a minimum of how many supply lines are required?

• A. One
• B. Two
• C. Four
• D. Three supply lines

Answer: (D)

Three-phase power requires three separate conductors, each carrying one phase of the electrical cycle. The voltages are offset by about 120 degrees, which lets motors and other equipment experience nearly constant power. If you only have one line, you’ve got a single-phase supply; with two lines you still lack the third phase, so it isn’t a complete three-phase system. A neutral may be present for returning current in unbalanced loads, but the minimum to achieve true three-phase operation is three supply lines.

Three-phase power requires three separate conductors, each carrying one phase of the electrical cycle. The voltages are offset by about 120 degrees, which lets motors and other equipment experience nearly constant power. If you only have one line, you’ve got a single-phase supply; with two lines you still lack the third phase, so it isn’t a complete three-phase system. A neutral may be present for returning current in unbalanced loads, but the minimum to achieve true three-phase operation is three supply lines.