Wye connection

Connect one end of each phase winding together. The three free ends are the line terminals A, B, C. The junction is neutral N (may be grounded).

Phase voltage: across one winding (line-to-neutral). Line voltage: between two lines (line-to-line).

Because phases are 120° apart, two phase voltages do not add to 2×EPhase. Vector sum gives:

ELine = EPhase × √3 EPhase = ELine / √3

With √3 ≈ 1.732:

In a wye, each line feeds exactly one winding:

ILine = IPhase

If ammeters in the line and in the winding both read 10 A, that is expected — not a coincidence to “fix.”

Residential 120/240 V is typically a single-phase center-tapped transformer: the two hot legs are 180° apart, so they add to 240 V. Three-phase wye legs are 120° apart, so 120 V windings yield 208 V line-to-line. Different geometry, different sum.

On a balanced wye load, neutral current is ideally zero. Unbalanced phase loads push current onto the neutral. Undersized or open neutrals cause odd L-N voltages — lights bright on one phase, dim on another.

• Label panels as 208Y/120 or 480Y/277 clearly
• Use L-N for single-phase branch loads; L-L for three-phase equipment
• Verify neutral continuity before energizing sensitive loads
• Never assume “240” when the nameplate says 208

208 / 1.732 ≈ 120 480 / 1.732 ≈ 277 240 × 1.732 ≈ 416 (less common wye)

If L-N voltages are 125, 118, 110 with wild swings under load, suspect neutral problems before blaming the utility for “bad phases.”

• L-N loads (lights, receptacles on wye): balance them across A-N, B-N, C-N
• Three-phase motors: use L-L, no neutral needed
• Never open the neutral under load on a wye supplying L-N loads

A 208 V motor on 208Y is using L-L. Its coil connection might be wye or delta internally — the supply being wye does not force the motor winding topology. Read the motor plate and connection diagram separately.

Situation. A tech replaces a 208 V three-phase motor starter and sets overloads using “120 × 2” thinking for expected current math, then mis-compares nameplate FLA to clamp readings.

What happened. Mixing L-N voltage with L-L formulas skewed VA estimates. Using ELine and ILine with the three-phase power formula (later lesson) fixes it.

Applied lesson. On wye, keep EPhase/ELine and IPhase/ILine identities straight before any power math.

### Teaching pause — say this out loud

Before you leave this lesson, explain the main idea to an imaginary first-month helper in under one minute. If you need the book open to do it, reread How it works once more. Field diagnosis only helps after the concept is yours.

Also sketch the key diagram from memory (triangle, wye/delta, filter shape, or charge curve — whichever this lesson used). Labels beat artistic skill.

### Why this lesson matters on Monday morning

Wye connection is not trivia. You will meet it when a meter reading looks “impossible,” when a replacement part is almost right, or when a helper asks why the book uses √3 or lead/lag. Master the model here so the next call is pattern recognition, not panic.

Common Monday uses: verify a nameplate against clamps, explain a PF or capacitor change to a customer, or catch a miswired series/parallel or wye/delta assumption before energizing.