UNIT 3.1 — THREE-PHASE

Three-phase power: wye, delta, and √3

CERT 3 · Commercial & Industrial Systems · ~12 min

Community content — not verified by a licensed electrician. Lessons and questions here are written for study practice and pinned to NEC 2023. Code editions and local amendments vary by jurisdiction. Always confirm against your adopted code book and your licensing board's published exam outline before test day.

Commercial power arrives as three sine waves offset 120° from each other. In a wye (star) connection the three phases share a neutral point: phase-to-neutral voltage × √3 = phase-to-phase voltage. That is where the two workhorse systems come from — 120/208 V (120 × 1.732 ≈ 208) and 277/480 V (277 × 1.732 ≈ 480).

Delta connections have no neutral by default; the three windings form a triangle and phase-to-phase is the only voltage. Motors love delta; lighting panels love wye.

Three-phase power is P = √3 × V_L × I_L × PF. Rearranged for current — I = P ÷ (√3 × V × PF) — it is the single most-used commercial exam formula: sizing feeders, breakers, and transformer secondaries all start here.

Exam focus: recognize the system from the voltages given, apply √3 in the correct direction (÷ to get phase-to-neutral, × to get line), and drill the power formula both ways.

√3

1.732

Wye systems

120/208 · 277/480

3φ power

P = √3 × V × I × PF

Knowledge check

Quiz — 3 questions

1. The phase-to-neutral voltage of a 480 V wye system is:

2. Line current for a 50 kW, 208 V three-phase load at PF 1.0 (nearest):

3. The three phases of a 3φ system are separated by:

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