Lesson Three-Phase Power · Three-phase topology
Delta connection
In a delta connection the three windings form a closed triangle (like the Greek letter Δ). Line voltage equals phase voltage, but line current is √3 times phase current: ILine = IPhase × 1.732. No neutral is required for a pure three-wire delta, though high-leg and grounded-delta variants appear in the field.
Building a delta
Connect the end of each winding to the start of the next until the loop closes. The three corners are the line terminals. Each winding sees line-to-line voltage directly.
ELine = EPhase
If the winding is rated 480 V, the lines are 480 V apart.
Current relationship
At each corner, two phase currents combine vectorially into the line. Result:
ILine = IPhase × √3 IPhase = ILine / √3
Example pattern: phase current 10 A → line current ≈ 17.32 A on a balanced delta.
Wye vs delta cheat sheet
| Quantity | Wye | Delta |
|---|---|---|
| Voltage | ELine = √3 EPhase | ELine = EPhase |
| Current | ILine = IPhase | ILine = √3 IPhase |
| Neutral | Available at center | Not inherent on 3-wire delta |
Memorize by pairing: wye “boosts voltage”; delta “boosts current” on the line relative to the winding.
Why deltas are used
- Three-wire economy when no neutral loads are needed
- Transformer secondaries and motor windings often delta or dual-voltage reconnectable
- Some services are closed delta; others are open delta (two transformers) with reduced capacity
High-leg / wild-leg awareness
Some delta services center-tap one winding for 120 V loads, creating a high leg (~208 V to neutral) on the opposite corner. Paint and labeling exist so you do not land 120 V gear on the high leg. Always identify the service type before connecting.
Open delta note
Two transformers can supply three-phase in an open delta at reduced capacity (often discussed as 57.7% of a full three-transformer bank). Recognize it when you see only two cans on a pole providing three-phase.
Numbers you should be able to work cold
IPhase = ILine / 1.732 If nameplate FLA (line) = 17.3 A, winding current ≈ 10 A on a delta motor.
Voltages: if ELine=240 V on delta, each winding sees 240 V — not 240/1.732.
High-leg identification
On a 240 V delta with center-tapped leg for 120 V:
- Two legs to center ≈ 120 V
- High leg to center ≈ 208 V
- All L-L ≈ 240 V
Color conventions exist (often orange high leg) — verify locally; do not trust paint alone without metering.
Open delta capacity
Two transformers in open delta deliver three-phase at reduced capacity versus three units. If one can of a bank is out and three-phase remains, you may be on open delta — derate expectations.
Field case
Situation. A tech measures 480 V across each motor winding (delta-connected) and also 480 V line-to-line, then expects line current to equal nameplate phase current stamped on a winding diagram.
What happened. On delta, line current is higher by √3 than winding (phase) current. Overload settings must follow the line conductors and the motor nameplate FLA (which is line current).
Applied lesson. Nameplate FLA is almost always line current. Do not divide or multiply by √3 unless you know you are converting to winding current for a specific task.
### 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
Delta 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.
In the field
Symptom
Current math “off by 1.73”; 120 V load burned on high leg; confusion vs wye
Where to look
Transformer/motor connection diagrams; service labels
Likely causes
- Applied wye formulas to delta
- high-leg miswire
What to measure
- ELine vs EPhase (should match on delta)
- ILine vs IPhase if accessible
What not to do
- Land neutral loads on unmarked legs of a delta high-leg service
Checklist
- I sketch a closed delta
- I use ELine = EPhase
- I use ILine = IPhase × 1.732
- I contrast with wye identities
- I watch for high-leg delta services
- I treat nameplate FLA as line current unless told otherwise