How does voltage drop calculation differ between single-phase and three-phase circuits?

• A. Three-phase uses the same formula as single-phase
• B. Three-phase uses a different formula that accounts for line-to-line voltage and phase relationships, typically yielding different drop values.
• C. Voltage drop is only defined for DC
• D. Voltage drop is irrelevant in three-phase

Answer: (B)

In a three-phase system, voltage drop isn’t just a single I times Z path like in a lone conductor circuit. There are three conductors carrying currents that are 120 degrees apart, and the voltages of interest between lines are line-to-line voltages, which are the vector differences of those phase voltages. Because of this, you must use phasor relationships to relate line voltages, phase voltages, and the line impedances. The result is a different calculation approach—often a per-phase drop that is then converted to a line-to-line drop using the geometry of three-phase relationships (involving factors like sqrt(3) and the 120-degree phase separation). So the voltage drop you get in a three-phase system is typically different from the single-phase case because the line-to-line voltages and the phase angles change how the current and impedance interact.