Three-Phase Systems and Transformers

Commercial startup and troubleshooting means understanding three-phase. The one relationship that unlocks it all: the √3 (1.732) factor.

1. Wye (Y) — the √3 rule

• Line-to-line voltage = √3 × line-to-neutral voltage (√3 ≈ 1.732).
• 208Y/120: 120 V phase-to-neutral, 208 V phase-to-phase (120 × 1.732 ≈ 208).
• 480Y/277: 277 V phase-to-neutral (common for commercial lighting), 480 V phase-to-phase.
• In wye, line current = phase current.

2. Delta

• Line voltage = phase voltage (they're equal).
• Line current = √3 × phase current.