Buck Boost Transformer Wiring Diagram
Color-coded wiring diagrams for all 6 common single-phase and 3-phase configurations — boost and buck.
X2 → Neutral/H2 (junction)
X1 → Output Line
Output = Input + Sec. Voltage
Secondary ADDS voltage
X1 → Neutral/H2 (junction)
X2 → Output Line
Output = Input − Sec. Voltage
Secondary SUBTRACTS voltage
Single-Phase Boost Wiring Diagrams
⬆️ BOOST
| Terminal | Connect To | Note |
|---|---|---|
| H1 | L1 Input (Line) | Primary energized from source |
| H2 | N Input (Neutral) + joined to X2 | ✓ Key junction point |
| X1 | Output L1 = 132V | Boosted output line |
| X2 | Short to H2/Neutral | ✓ Series-aiding connection |
| H3, H4, X3, X4 | Not connected | Leave open / insulate |
⬆️ BOOST
| Terminal | Connect To | Note |
|---|---|---|
| H1 + H3 (tied) | L1 Input | Parallel primary — 120V each winding |
| H2 + H4 (tied) | N Input + joined to X2+X4 | ✓ Common junction |
| X1 + X3 (tied) | Output L1 = 240V | Parallel secondary output |
| X2 + X4 (tied) | Short to H2+H4 / Neutral | ✓ Series-aiding junction |
Use this diagram for the most common North American application: powering 240V equipment from a 208V 3-phase panel.
⬆️ BOOST
| Terminal | Connect To | Note |
|---|---|---|
| H1 | L1 Input | 120V primary winding |
| H2 | N Input + joined to X4 | ✓ Common junction (24V series secondary) |
| X2 → X3 | Bridged (short X2 to X3) | ✓ Two 12V windings in series = 24V |
| X1 | Output L1 = 144V | Top of 24V series secondary |
| X4 | Short to H2 / Neutral | Bottom of 24V series secondary |
Single-Phase Buck Wiring Diagrams
⬇️ BUCK
| Terminal | Connect To | Note |
|---|---|---|
| H1 + H3 (tied) | L1 Input (240V) | Parallel primary — 120V each |
| H2 + H4 (tied) | N Input + joined to X1+X3 | ✓ Reversed junction (vs boost) |
| X1 + X3 (tied) | Short to H2+H4 / Neutral | ⚠️ X1 to Neutral in BUCK (opposite of Boost!) |
| X2 + X4 (tied) | Output L1 = 208V | Opposing output — subtracts 32V |
⬇️ BUCK
| Terminal | Connect To | Note |
|---|---|---|
| H1 | L1 Input (240V — series primary) | Use series H1-H2-H3-H4 for 240V |
| H2 | N Input + joined to X1 | ✓ X1 to neutral = BUCK mode |
| X1 | Short to H2 / Neutral | Opposing secondary connection |
| X2 | Output L1 = 228V | 240V − 12V = 228V out |
Three-Phase 208V → 240V Boost
⬆️ 3-PHASE BOOST
| Phase | H Terminals | X Terminals | Input | Output |
|---|---|---|---|---|
| Phase A | H1=L1, H2+H4=N | X1+X3→Out-L1; X2+X4→N | L1-N (120V) | L1′ boosted |
| Phase B | H1=L2, H2+H4=N | X1+X3→Out-L2; X2+X4→N | L2-N (120V) | L2′ boosted |
| Phase C | H1=L3, H2+H4=N | X1+X3→Out-L3; X2+X4→N | L3-N (120V) | L3′ boosted |
Quick Reference — All 6 Configurations
| # | Input → Output | H1 | H2 (Common) | X1 | X2 | Type |
|---|---|---|---|---|---|---|
| 1 | 120V → 132V | L1 | N + join X2 | Out (132V) | → H2/N | Boost +12V |
| 2 | 208V → 240V | L1 (w/H3) | N + join X2,X4 | Out (w/X3) | → N (w/X4) | Boost +32V |
| 3 | 120V → 144V | L1 | N + join X4 | Out (144V) | → X3 bridge | Boost +24V |
| 4 | 240V → 208V | L1 (w/H3) | N + join X1,X3 | → N (w/X3) | Out (w/X4) | Buck −32V |
| 5 | 240V → 228V | L1 | N + join X1 | → H2/N | Out (228V) | Buck −12V |
| 6 | 3Ø 208 → 240V | L1/L2/L3 | N + join X2,X4 | Out (w/X3) | → N (w/X4) | 3Ø Boost |
1. Verify input voltage matches H terminal rating (H1-H2 = 120V, or H1-H2 + H3-H4 series = 240V).
2. Double-check BOOST vs BUCK: X2→Neutral = Boost; X1→Neutral = Buck. Reversing them creates wrong voltage.
3. Overcurrent protection per NEC 450.4 must be sized for the auto-connected kVA (not nameplate kVA).
4. Always verify polarity with a voltmeter at no load before connecting equipment.
🔗 Related Transformer Guides
Frequently Asked Questions
For boost (step-up) wiring, the key rule is: X2 connects to the Neutral/H2 side.
Step-by-step: Connect H1 to L1 (input line). Connect H2 to Neutral. Then short X2 to H2 (this is the junction point). X1 becomes your boosted output line. Load connects between X1 and Neutral.
Output voltage = Input voltage + Secondary winding voltage (12V or 24V added). Example: 120V + 12V = 132V.
BOOST: X2 connects to Neutral (common side), X1 is the output line. Secondary voltage adds to source.
BUCK: X1 connects to Neutral (common side), X2 is the output line. Secondary voltage subtracts from source.
The H terminal wiring is identical in both cases. Only X1 and X2 roles are swapped — this reverses the secondary polarity relative to the load circuit.
This uses a parallel-primary, parallel-secondary configuration (Config 2):
• H1 + H3 tied → L1 input (parallel primary, each winding sees 120V)
• H2 + H4 tied → Neutral, then also connected to X2+X4
• X1 + X3 tied → Output L1 (240V boosted line)
• X2 + X4 tied → Short to H2+H4/Neutral
Requires a transformer with a separate 24V secondary (or two 12V windings connected in series). Output: 208V + 32V = 240V.
Reversing X1/X2 switches between boost and buck mode. If you wire for boost (X2→Neutral) but physically reverse to X1→Neutral, you get buck instead — the output voltage will be lower than input by the secondary voltage, not higher.
Always measure output voltage with a voltmeter at no load before energizing. A correct boost connection will show Input + Secondary V. A correct buck will show Input − Secondary V.