Delta vs. Wye
Explained.
Delta and wye are the two ways to connect the windings of a three-phase transformer. The choice decides whether you get a neutral, which voltages you can serve and how the system is grounded. This guide explains both and shows how to read the voltage strings on a nameplate or a BPS listing.
The short
version.
- Wye (Y) ties one end of each winding to a common neutral point. You get two voltages, line-to-line and line-to-neutral, as in 480Y/277 or 208Y/120.
- Delta (Δ) connects the windings end to end in a closed loop. There is no neutral point and only one voltage, line-to-line, as in 480 Delta. The exception is 240/120 Delta, which has a center tap on one winding.
- Neither one is “the high-voltage connection.” Both are used on primaries and on secondaries. BPS three-phase listings offer wye and delta primaries (12470Y/7200, 13800 Delta) and wye and delta secondaries (480Y/277, 480 Delta).
- Match the secondary to your loads and the primary to the utility. For 277 V or 120 V loads you need a wye secondary (or 240/120 Delta for 120 V). The primary has to match the system you connect to, so get the exact voltage string from the utility.
Two Ways
to Connect.
A three-phase transformer has three windings on each side. “Delta” and “wye” describe how those three windings are joined, and each side of the transformer can use either one.
The three windings meet at the neutral (X0). Three lines (X1, X2, X3) plus the neutral.
The windings connect end to end. Three lines and no neutral point.
Wye (Y), also called star
One end of each winding connects to a common point, the neutral. The other ends connect to the three lines. The secondary neutral terminal is marked X0 and the primary neutral is H0.
- Two voltages. Line-to-line voltage is √3 (about 1.732) times line-to-neutral voltage: 277 × 1.732 is about 480, and 120 × 1.732 is about 208.
- Four-wire service: three phase conductors plus a neutral. Single-phase loads such as 277 V lighting or 120 V receptacles connect line-to-neutral.
- The neutral can be grounded. On a solidly grounded wye, each line’s voltage to ground stays at the line-to-neutral value.
- The neutral carries the unbalance between phases. It also carries triplen harmonic currents (3rd, 9th, 15th and so on) from nonlinear loads. Triplens add up in the neutral instead of cancelling, so heavily nonlinear sites may need a larger neutral.
Delta (Δ)
The three windings connect end to end in a triangle. Each winding sits directly between two lines, so the winding voltage equals the line-to-line voltage.
- One voltage: line-to-line. A 480 Delta secondary gives 480 V between any two phases and nothing lower.
- Three-wire service. There is no neutral point, so there is no neutral conductor and no neutral current.
- Current: on a balanced load, line current is √3 times the current in each winding.
- Triplen harmonics circulate inside a delta winding instead of passing through to the lines on that side. That is one reason transformers serving nonlinear loads often have a delta winding on one side.
- Special case: 240/120 Delta (four-wire delta). One winding has a center tap brought out as a neutral. That gives 120/240 V single-phase from two of the phases. The third phase, the “high leg,” measures about 208 V to neutral and must not feed 120 V loads.
Common myth“Delta has lower neutral current.” A delta winding has no neutral at all, so the comparison doesn’t apply. If you need a neutral, you need a wye winding (or the 240/120 Delta center tap).
Read the
Voltage String.
Nameplates and BPS listings write each winding as a short voltage string, primary first and then secondary. Learn a few patterns and you can read almost any of them.
The patterns
12470Y/7200: number, Y, slash, number means a wye winding. The first number is line-to-line and the second is line-to-neutral (12,470 ÷ 1.732 ≈ 7,200).13800 Deltaor13800Δ: a delta winding at 13,800 V line-to-line. BPS listings spell out the word “Delta.”12470GrdY/7200: GrdY means grounded wye. The winding is meant for a system whose neutral is solidly grounded. BPS uses this form on single-phase listings: the one primary winding connects from a line to the grounded neutral, so it sees 7,200 V.120/240: two numbers with no Y on a single-phase winding means a center-tapped winding: 240 V end to end and 120 V from either end to the center tap (the neutral).240/480follows the same pattern.- A single number such as
7200or12000on a single-phase listing is the winding voltage by itself. Whether it connects line-to-neutral or line-to-line depends on the system, so tell us the system’s full voltage string.
When a spec just says “480 V,” ask which one: 480Y/277 and 480 Delta are both 480 V line-to-line, but only one has a neutral. For kVA math, three-phase formulas use the line-to-line voltage; see the transformer sizing guide.
| String | Where it appears | Connection | What it means |
|---|---|---|---|
| 12470Y/7200 · 13200Y/7620 · 13800Y/7970 · 24940Y/14400 · 34500Y/19920 | Three-phase primaries (pad-mount, substation, dry-type) | Wye | Line-to-line / line-to-neutral. For four-wire distribution systems. |
| 12000 Delta · 13200 Delta · 13800 Delta · 34500 Delta (also 2400 to 7200 Delta) | Three-phase primaries | Delta | Line-to-line only. For three-wire systems. |
| 208Y/120 · 415Y/240 · 480Y/277 · 600Y/347 | Three-phase secondaries | Wye, 4-wire | Motors line-to-line; lighting and receptacles line-to-neutral. |
| 240 Delta · 480 Delta | Three-phase secondaries | Delta, 3-wire | Three-phase loads only. No neutral. |
| 240/120 Delta | Three-phase secondaries | Delta, 4-wire (center tap) | 240 V three-phase plus 120/240 V single-phase from two phases. High leg about 208 V to neutral. |
| 12470GrdY/7200 · 13200GrdY/7620 · 24940GrdY/14400 | Single-phase primaries (pad-mount, pole-mount) | Line-to-neutral on a grounded-wye system | The winding sees the second number (7,200 V on a 12,470 V system). |
| 120/240 · 240/480 | Single-phase secondaries | Center-tapped | Three-wire single-phase. |
| 115000 GrdY · 138000 GrdY · 161000 GrdY | Large substation primaries | Grounded wye | High-voltage primary on a solidly grounded system. |
Strings are copied from current BPS listings. Options vary by kVA, and each listing shows its own primary and secondary choices.
Primary +
Secondary.
Transformers are described primary first, then secondary. These four combinations cover most distribution transformers. The example strings are real BPS listing options; whether a pairing suits your site is a utility and engineering decision.
Grounded Wye to Grounded Wye
A common choice for three-phase pad-mounts on four-wire, multi-grounded utility distribution systems.
- Neutral on both sides. On many pad-mounts the primary and secondary neutrals share one bushing, marked H0X0.
- No phase shift between primary and secondary.
- Ground current passes through to the primary side, which the utility accounts for in its protection. Some utilities specify this connection and others limit it for certain loads.
Wye Primary to Delta Secondary
Serves three-wire loads, such as motor-heavy plants, from a four-wire utility system.
- No neutral on the secondary.
- 30° phase shift between primary and secondary.
- Ask about the primary neutral (H0). With H0 grounded, a wye-delta unit can act as a grounding source for the utility system and carry current during primary faults and unbalance. Many utilities therefore want H0 left ungrounded on this connection. Check before you order.
Delta to Delta
Found on three-wire systems, often at older industrial sites. The 240/120 Delta version suits shops that need three-phase motors plus some 120 V.
- No neutral on either side (except the 240/120 center tap).
- No phase shift.
- On an ungrounded delta system a single line-to-ground fault draws little current, so these systems commonly use ground-fault detection.
Delta Primary to Grounded Wye
The usual way to serve commercial and industrial buildings from a three-wire primary. The secondary creates a new neutral that can be grounded at the service.
- Four-wire secondary from a three-wire primary.
- 30° phase shift.
- Secondary ground faults don’t send ground current into the primary, and triplen harmonics from the loads circulate in the delta winding.
Common myth“Delta is for high voltage, wye is for low voltage.” Either connection can be used on either side. BPS three-phase listings include wye primaries (12470Y/7200, 13200Y/7620, 24940Y/14400) and delta primaries (12000 Delta, 13800 Delta), with both wye and delta secondaries.
Neutral &
Grounding.
This is where delta and wye differ most in practice, and where mistakes are expensive. These are the concepts. Your engineer of record, the NEC and the utility set the actual rules for a site.
Wye: a neutral you can ground
- A grounded neutral gives fault current a low-impedance path back to the source, so breakers and fuses clear ground faults quickly.
- It holds each line’s voltage to ground at the line-to-neutral value (277 V on 480Y/277, 120 V on 208Y/120).
- Line-to-neutral loads need the neutral, and the neutral has to be sized for unbalance and harmonics.
- The secondary neutral is bonded to ground at one point, as the NEC requires. Where that happens depends on whether the transformer is a utility service or a separately derived system on your side of the meter.
Delta: no neutral
- No neutral conductor, so no line-to-neutral loads.
- Delta systems are run ungrounded, corner-grounded, or given a ground reference another way, such as a grounding transformer. Each behaves differently during a fault, so this is an engineering call.
- If you need 120 V or 277 V from a delta secondary, you need either a different secondary (wye or 240/120 Delta) or a separate step-down transformer for those loads.
The 30-degree phase shift
Delta-wye and wye-delta transformers shift the secondary voltages by 30 degrees relative to the primary. Wye-wye and delta-delta units have no shift. Under the ANSI convention used in North America, the low-voltage side lags the high-voltage side by 30 degrees.
- One transformer feeding its own load: the shift doesn’t matter. Your equipment never sees it.
- Where two sources meet, it does matter: paralleling transformers, closed-transition transfer schemes, or tying into an existing bus. Transformers in parallel need the same phase shift, the same voltage ratio and closely matched impedance.
- Replacing a unit? Match the existing connection unless the engineer says otherwise, and send a nameplate photo with the quote request.
Single-phase from a three-phase system
- On a four-wire grounded-wye primary, single-phase transformers connect from one phase to neutral. On a 12470Y/7200 system the winding sees 7,200 V. That is why BPS single-phase pad-mount and pole-mount listings show primaries such as 12470GrdY/7200.
- On a three-wire delta primary, single-phase units connect phase to phase, so the winding is rated at the full line voltage, such as 12000.
- From a three-phase secondary: 208Y/120 gives 120 V line-to-neutral and 208 V (not 240 V) line-to-line. 480Y/277 gives 277 V for lighting. 240/120 Delta gives 120/240 V from the center-tapped winding.
- Three single-phase transformers can be banked to make three-phase service. The bank follows the same delta and wye rules. Talk to us before you plan a bank.
Delta vs. Wye
at a Glance.
The differences that matter when you spec or replace a transformer.
| Feature | Wye (Y) | Delta (Δ) |
|---|---|---|
| Neutral point | Yes: H0 (primary), X0 (secondary) | No |
| Wires | 4: three phases plus neutral | 3 (4 with the 240/120 center tap) |
| Voltages available | Two: line-to-line and line-to-neutral | One: line-to-line (plus 120/240 V on 240/120 Delta) |
| Voltage across each winding | Line-to-neutral (line voltage ÷ 1.732) | Line-to-line |
| Current in each winding | Equal to line current | Line current ÷ 1.732 |
| Grounding | Neutral usually solidly grounded | Ungrounded, corner-grounded or another method |
| Triplen harmonics | Add up in the neutral | Circulate inside the winding |
| BPS secondary strings | 208Y/120, 415Y/240, 480Y/277, 600Y/347 | 240 Delta, 240/120 Delta, 480 Delta |
| BPS three-phase primary strings | 4160Y/2400 up to 34500Y/19920 | 2400 Delta up to 34500 Delta |
| Phase shift | 0° to another wye, 30° to a delta | 0° to another delta, 30° to a wye |
Copy the voltage strings exactly as the utility letter or the old nameplate writes them.
- kVA and phase: single- or three-phase.
- Primary voltage string from the utility, such as 12470Y/7200 or 13800 Delta, and whether the system is three-wire or four-wire.
- Secondary voltage string, such as 480Y/277, 208Y/120, 480 Delta or 240/120 Delta.
- Unit type: pad-mount, pole-mount, substation or dry-type.
- Grounding requirements from the utility or engineer, for example whether the primary neutral (H0) is grounded.
- Replacing or paralleling? Send a nameplate photo so we can match the connection, phase shift and impedance.
- Quantity, site location and need date, plus any listing requirements (UL Listed options available).
Delta / Wye
Answers.
Send a photo of the existing nameplate or the utility’s service letter with your quote request to sales@bearpowersolutions.com, or call the team.
Call 601-912-3512Does a delta transformer have a neutral?
No. A delta winding has no neutral point, so a standard delta secondary such as 480 Delta is three-wire and carries no neutral current. The exception is 240/120 Delta, where a center tap on one winding provides a neutral for 120/240 V single-phase loads.
Is delta for high voltage and wye for low voltage?
No. Either connection can be used on either side. BPS three-phase listings include wye primaries such as 12470Y/7200 and 24940Y/14400, delta primaries such as 12000 Delta and 13800 Delta, and both wye (480Y/277, 208Y/120) and delta (480 Delta, 240 Delta) secondaries. The right choice depends on the utility system and your loads.
What does the Y in 12470Y/7200 mean?
It means the winding is wye-connected. The first number is the line-to-line voltage (12,470 V) and the second is the line-to-neutral voltage (7,200 V). GrdY, as in 12470GrdY/7200, means the winding is intended for a system with a solidly grounded neutral.
Can I get 120 V or 277 V from a 480 Delta secondary?
Not directly, because there is no neutral. Choose a wye secondary instead (480Y/277 for 277 V lighting, 208Y/120 for 120 V), or add a separate step-down transformer for the 120 V loads.
Can I replace a delta-wye transformer with a wye-wye unit?
Only after an engineering check. The two have different phase shifts (30° vs. 0°) and different grounding behavior. That matters if the unit runs in parallel with another source or the utility has a standard. For a like-for-like replacement, match the existing nameplate unless your engineer or utility says otherwise.
Know your
voltage strings?
Send kVA, phase, the primary and secondary strings exactly as written (for example 12470Y/7200 to 480Y/277), quantity and where it’s going. We keep hundreds of units in stock and will tell you what’s ready; typical lead time is 12–16 weeks. UL Listed options available.