Energy Academy
Level 1/Electrical Science Refresher
Three-Phase Systems5 / 10

Three-Phase Basics

Why three-phase is used, phase angles, and line vs phase voltages.

10 min read

Most large buildings and all industrial sites use three-phase AC. It's more efficient and simpler than single-phase for high-power equipment.

Why three-phase?

In single-phase AC, power oscillates: P = V Γ— I Γ— sinΒ²(Ο‰t). It dips to zero 50 times per second (at 50 Hz), causing flickering in lights and pulsing in motors.

In three-phase, three sine waves are 120Β° apart:

  • Phase A: 0Β°
  • Phase B: 120Β°
  • Phase C: 240Β°

They're offset so that power is delivered smoothly and constantly β€” no dips. P = 1.5 Γ— V_phase Γ— I Γ— cos(Ο†) β€” constant!

Voltages and connections

Phase voltage (V_phase): voltage between one phase and neutral (UK: ~230 V) Line voltage (V_line): voltage between two phases = V_phase Γ— √3 β‰ˆ 400 V (UK standard)

In a Y (star) connection: neutral is the common point; phases are at 230 V from it. In a Delta connection: phases are 400 V apart with no neutral.

Most commercial buildings use Y: 400 V between phases, 230 V phase-to-neutral.

Power in three-phase

Total power: P = √3 Γ— V_line Γ— I_line Γ— cos(Ο†)

or equivalently: P = 3 Γ— V_phase Γ— I_phase Γ— cos(Ο†)

Example: A three-phase 400 V system with 10 A per phase and PF 0.95 delivers: P = √3 Γ— 400 Γ— 10 Γ— 0.95 β‰ˆ 6,600 W (6.6 kW) from each phase, for a total of 19.8 kW.

Advantages for motors and large loads

  • Smooth power delivery β€” constant, no pulsing
  • High torque β€” motors start and run reliably
  • Efficiency β€” three-phase motors are 85–95% efficient; single-phase are 50–70%
  • No neutral current in a balanced system β€” saves wiring

This is why boiler pumps, HVAC fans, compressors, and industrial equipment are three-phase.