Three-phase power is the standard for commercial and industrial distribution because three current-carrying conductors, spaced 120° apart, deliver smoother and more efficient power than a single phase can. As one phase peaks, the other two sit at intermediate values, which produces constant, non-pulsating power in a balanced load - a real advantage for motors. How that power gets wired, though, comes down to two configurations that behave differently enough to matter on the exam.
Wye: the one with a neutral
In a wye (Y) connection, one end of each of the three phase windings ties to a common neutral point; the other ends are the three phase terminals. This gives you two different voltages to work with: phase voltage, measured from a phase terminal to neutral, and line voltage, measured between two phase terminals. The relationship between them is fixed: Vline = √3 × Vphase (about 1.732×). A 208Y/120V system has 120V phase-to-neutral and 208V phase-to-phase. A 480Y/277V system has 277V phase-to-neutral and 480V phase-to-phase. In a perfectly balanced wye system, the neutral carries zero current - it only carries the unbalanced portion of the load.
Delta: no neutral, different math
A delta (Δ) connection wires the three windings end-to-end into a triangle, with no neutral point at all. Here line voltage and phase voltage are the same number - Vline = Vphase - but current is where the √3 shows up instead: Iline = √3 × Iphase. Delta is common for motor loads and on the primary side of distribution transformers.
One wrinkle worth knowing: a 240V delta system with a center-tapped neutral produces a "high leg" (sometimes called the wild leg or stinger) that reads 208V to neutral from one particular phase, not 120V. It has to be identified and kept away from standard 120V loads, or it will fry whatever's connected to it.
Why this shows up so often
The exam likes to test whether you know which relationship applies where - line-to-phase for wye is a voltage relationship, line-to-phase for delta is a current relationship. Mixing those up is the single most common way to get a three-phase calculation wrong even when the arithmetic itself is easy.
Full coverage of wye/delta systems, common system voltages, and the three-phase power formula (P = √3 × Vline × Iline × PF) is in the Study Guide, with practice problems in the Exam Companion.