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How to calculate line to line voltage

Updated February 21, 2017

Balanced three-phase transformers increase or decrease alternating current (AC) voltage using three wires or "lines" denoted as phase "a," "b" and "c." "Balanced" indicates that each line carries the same voltage magnitude, but their "phases," i.e. their locations along the characteristic sine wave of AC power, are equally spaced. This spacing provides for smooth power. There are certain calculations in transformer science when you want to know the difference in voltage from one line to another: the line-to-line voltage. We write the line-to-line voltages as Vab, Vbc and Vac, respectively. Depending on your given information, there are two ways to calculate them.

Subtract Ib from Ia. For example, 8 amps minus 6 amps equals 2 amps.

Multiply the result from Step 1 by the internal impedance Z. Using the previous example, 2 amps times 5 ohms (example) equals 10 volts.

Repeat steps 1 and 2 with (Ia - Ic) and (Ib - Ic).

The products from repeating step 2 are the magnitudes of the line-to-line voltages Vab, Vac, and Vbc, respectively.

Multiply the line-to-neutral voltages by the square root of 3 (approximately 1.73). Using an example Van value of 10 volts, Vab = 10 volts times 1.73 which equals 17.3 volts. This is Vab's magnitude.

Subtract 120 degrees from the phase of the line-to-neutral voltage. For example, if Van's phase angle is 150 degrees, then Vab's phase angle will be 30 degrees.

Repeat steps 1 and 2 for Vab, Vbc, and Vca, respectively. Combine the magnitudes from steps 1 with the phase angles from steps 2 to get the line-to-line voltages' magnitudes and phases.

Warning

Note that section 1's results only give magnitudes, omitting phases. This problem requires college-level understanding of power engineering.

Things You'll Need

  • Magnitude of the current of each phase: Ia, Ib and Ic
  • Internal impedance of each line: Z
  • Magnitude of line-to-neutral voltage of each phase: Van, Vbn, Vcn
  • Phase angle of line-to-neutral voltage of each phase (0 to 359 degrees)
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About the Author

Joe Friedman began writing in 2008 while in the U.S. Air Force as a KC-10 tanker pilot. He is now an equipment engineer in the semiconductor manufacturing industry. Friedman holds a Bachelor of Science in engineering physics from Embry-Riddle Aeronautical University and a Master of Science in electrical engineering from Drexel University.