A building gets 600 V delta from the utility and needs 120 V receptacles and 208 V equipment. Which transformer connection fits?

B. Delta-wye (600 V delta to 208Y/120)

Why is delta-wye so common for step-down transformers in buildings?

B. It gives a neutral for line-to-neutral loads, and the delta primary traps triplen harmonic currents instead of passing them upstream

D. It is the only connection that works on 60 Hz

You need to check a 3-phase transformer’s secondary before loading it. Which set of readings confirms a healthy 208Y/120 secondary?

A. Three line-to-line readings near 208 V and three line-to-neutral readings near 120 V

B. One line-to-line reading of 208 V

C. Three line-to-neutral readings near 208 V

D. Line to ground reading of 0 V on every phase

Troisième année Lesson 4 of 13 · about 15 min

Three-phase transformer connections

What you’ll learn

The four ways three transformer windings can be connected: delta-wye, wye-wye, delta-delta and wye-delta.
What each connection gives you on the secondary — a neutral or not, which voltages.
How to work full-load current on each side, and the voltage and current in each winding.
The six readings that prove a secondary is connected right before you load it.

Before this: Transformer basics · Wye and delta

A three-phase transformer is three pairs of windings — in one tank on one core, or three separate single-phase cans in a bank. The primary three are joined in wye or delta, the secondary three in wye or delta, and the name just says which: primary first, secondary second. Delta-wye means a delta primary and a wye secondary.

Delta-wye, the everyday building transformer. Three wires in at 600 V (or 480 V in much of the US), four wires out: three lines at 208 V to each other and a neutral at 120 V to each line.

The four connections

Connection	Secondary gives you	Where you meet it
Delta-wye	A neutral: line-to-line and line-to-neutral loads (208Y/120, 480Y/277, 600Y/347)	Building step-down transformers; utility service transformers
Wye-wye	A neutral on both sides	Utility distribution; the primary neutral is normally tied to the system neutral so it stays put
Delta-delta	Three wires, no neutral	Industrial motor loads; banks that can run as an open delta if one can fails
Wye-delta	Three wires, no neutral	Stepping UP at generators; stepping down to a three-wire delta supply

Delta-wye earns its place for two reasons. The wye secondary makes a neutral you can bring out and ground, so one transformer feeds both motors and lighting. And a closed delta primary gives triplen harmonic currents from electronic loads a loop to circulate in, instead of sending them back up the supply. The price is a 30° shift between primary and secondary voltages — the next lesson explains why that matters when two transformers meet.

Line values and winding values

The full-load current on each side uses the line voltage and the √3, exactly like any three-phase load: I = kVA × 1000 ÷ (√3 × V line). Inside the tank, though, each winding sees the phase value for its connection — which is what the turns ratio is built on.

Wye windingvoltage = line ÷ √3, current = line current
Delta windingvoltage = line voltage, current = line ÷ √3

Worked: 150 kVA, 600 V delta to 208Y/120 V

Primary full-load current: 150,000 ÷ (1.732 × 600) = 144.3 A per line.
Secondary full-load current: 150,000 ÷ (1.732 × 208) = 416.4 A per line.
Each primary (delta) winding: 600 V, carrying 144.3 ÷ 1.732 = 83.3 A.
Each secondary (wye) winding: 208 ÷ 1.732 = 120 V, carrying the full 416.4 A.
Check one winding pair: 600 × 83.3 ≈ 50,000 VA and 120 × 416.4 ≈ 50,000 VA. Each of the three pairs carries a third of 150 kVA.
Turns ratio = 600 ÷ 120 = 5 : 1. Line voltage ratio = 600 ÷ 208 = 2.88 : 1. Different numbers — both right.

The same transformer in the US

A 150 kVA, 480 V delta to 208Y/120 unit: primary 150,000 ÷ (1.732 × 480) = 180.4 A; secondary still 416.4 A. Turns ratio 480 ÷ 120 = 4 : 1.

Prove it before you load it

On a new or reconnected wye secondary, take six readings before anything is connected: A-B, B-C, C-A and A-N, B-N, C-N. All three line-to-line should match (about 208 V), and all three line-to-neutral should match (about 120 V). If one winding is connected backwards, the two line-to-line readings that include it drop to about 120 V. Check the phase rotation too if motors will run on it.

Sur le chantier

Tips

Read the nameplate diagram before you land a lead. Taps, H and X numbering and the connection are all there.
Say the name primary-first: “delta-wye” is delta on the H side.
The secondary of a delta-wye is a new system — its neutral and grounding start at that transformer. How it is grounded and bonded is a code question with its own section in your book.
Canadian 600Y/347 and US 480Y/277 distribution are wye-connected secondaries of the utility’s transformer; the 347/277 V lighting runs line to neutral.

Common mistakes

Using the line ratio as the turns ratio on a delta-wye (or the reverse).
Forgetting the √3 in full-load current and getting a number 1.73 times too big.
Expecting a neutral from a delta secondary.
Only reading line to line on a wye secondary — a reversed winding can hide from one reading.
Swapping primary and secondary: feeding the X side turns a step-down into a step-up.

Check yourself

0 right of 0 answered 7 questions · pick one answer each

1.A building gets 600 V delta from the utility and needs 120 V receptacles and 208 V equipment. Which transformer connection fits?
2.A 150 kVA, 600 V to 208Y/120 V three-phase transformer. Full-load secondary current?
3.In that 600 V delta – 208Y/120 transformer, what voltage does each SECONDARY winding see?
4.Which connection gives you NO neutral on either side?
5.A wye-connected secondary winding carries 416 A line current. How much current is in each secondary winding?
6.Why is delta-wye so common for step-down transformers in buildings?
7.You need to check a 3-phase transformer’s secondary before loading it. Which set of readings confirms a healthy 208Y/120 secondary?

Answers

B. Delta-wye (600 V delta to 208Y/120) — Only a wye secondary gives a neutral point for 120 V loads while also giving 208 V line to line. The delta primary suits the three-wire 600 V supply. The same pattern in the US is 480 V delta to 208Y/120.
C. 416 A — I = kVA × 1000 ÷ (√3 × V) = 150,000 ÷ (1.732 × 208) = 416 A. 144 A is the primary (600 V) current. 721 A forgot the √3.
B. 120 V — Each wye winding sits between a line and the neutral point, so it sees 208 ÷ 1.732 = 120 V. Each primary delta winding sees the full 600 V, which makes the winding turns ratio 600 : 120 = 5 : 1, not the 2.88 : 1 line ratio.
C. Delta-delta — A neutral comes from the centre point of a wye. Delta-delta has no wye on either side, so it is a three-wire system in and out — common for industrial motor loads.
B. 416 A — In a wye, line current and winding (phase) current are the same. It is a DELTA winding whose current is the line current ÷ √3.
B. It gives a neutral for line-to-neutral loads, and the delta primary traps triplen harmonic currents instead of passing them upstream — The wye secondary provides the neutral (and a point to ground the system), and the closed delta primary lets triplen harmonic currents circulate in the windings rather than flowing back into the supply. It does introduce a 30° phase shift — covered next lesson.
A. Three line-to-line readings near 208 V and three line-to-neutral readings near 120 V — Read all six: A-B, B-C, C-A should each be about 208 V and A-N, B-N, C-N about 120 V. One wye winding connected backwards shows up as TWO line-to-line readings of about 120 V instead of 208 V (the two that include the reversed winding).

Questions fréquentes

What is the most common three-phase transformer connection in buildings?

Delta-wye step-down: a three-wire delta primary at the distribution voltage (600 V in much of Canada, 480 V in much of the US) and a four-wire 208Y/120 V secondary for receptacles, lighting and small equipment. The same idea at higher voltages feeds whole buildings from the utility.

Can I use three single-phase transformers instead of one three-phase unit?

Yes — that is a transformer bank, and it is connected in exactly the same patterns. A bank is larger and has more connections to make, but if one can fails you replace one can, and a delta-delta bank can keep running on two as an open delta. The next lesson covers polarity, which matters a great deal when you build a bank yourself.

Why is the turns ratio of a delta-wye not the same as the voltage ratio?

Because each side’s winding sees a different fraction of its line voltage. A delta winding sees the full line voltage; a wye winding sees line ÷ √3. A 600 V delta to 208Y/120 unit has windings of 600 V and 120 V — a 5 : 1 turns ratio — while its line voltages are 600 : 208, or 2.88 : 1.

Voir aussi

Transformer full-load current & ratio — winding volts and amps for any connection
Polarity, phase shift and open delta — the next lesson
Transformer basics — one pair of coils
Wye and delta: line vs phase
Wye and delta (glossary)
Rusty on transformer connections?

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