Buck Boost Transformer Sizing Calculator
Screen buck-boost unit kVA from supply volts, desired load volts, and load amps. Default: 208 V → 240 V @ 50 A → ~2 kVA. For full distribution kVA use the transformer kVA calculator.
Quick Calculator
Results update instantly. Advanced options: phase and 3-phase connection below.
About this calculator
Vendor-neutral buck-boost sizing screen for small voltage corrections—not a Federal Pacific / HPS / Acme SKU picker. After kVA, verify taps with the OEM chart. More tools on the power calculator hub · transformer section.
Calculation Results
208 V → 240 V @ 50.0 A · Single-phase (1 unit)
Calculated unit = 1.600 kVA (1600 VA) → catalog 2.0 kVA
Unit VA ≈ Iload × ΔV. Round up to catalog; confirm winding configuration with the manufacturer.
Engineering disclaimer
This calculator provides a preliminary buck-boost unit kVA screen only. Wiring diagrams, temperature rise, inrush, and code compliance require manufacturer data and a qualified electrician or engineer. Not a substitute for OEM selection software.
Buck-boost sizing chart (common corrections)
Unit kVA ≈ (I × ΔV) / 1000 for 1φ / open-delta. Open a row in the calculator. Suggest: buck boost transformer sizing chart, 208 to 240.
Buck-boost sizing formula & explanation
Buck-boost transformers are usually wired as autotransformers. The unit only handles the correction power, not the full load kVA:
ΔV = |V_load − V_supply|
% of supply = (ΔV / V_supply) × 100
Unit VA ≈ I_load × ΔV (1φ or each open-delta unit)
3φ wye: Unit VA ≈ I_load × ΔV / √3
Catalog kVA = round up Unit VA / 1000
Worked example — 208 to 240: ΔV = 32 V, I = 50 A → VA = 1600 → 1.6 kVA → catalog 2 kVA. Full-load kVA at 240 V would be 240×50/1000 = 12 kVA—do not buy a 12 kVA isolation transformer for a 15% boost.
If you need a pad-mount or dry-type sized from plant kW/PF, switch to the transformer sizing calculator. Voltage ratio alone: turns ratio calculator.
Last reviewed: 2026-08-09. Estimate / assumption: infinite catalog ladder; ignore temperature rise and harmonic derating.
When to use buck-boost vs distribution kVA
| Situation | Tool | Why |
|---|---|---|
| 208 V utility, equipment needs 230–240 V | This page | Small % boost, low unit kVA |
| New plant feeder / pad-mount from diversified kW | Transformer kVA | Full load apparent power |
| Nameplate turns / voltage ratio only | Turns ratio | Ideal a = V1/V2 |
| Secondary fault from %Z | %Z guide | Isc screen after kVA known |
Frequently Asked Questions
How do you size a buck boost transformer?
Compute ΔV = |V_load − V_supply|, then unit VA ≈ I × ΔV (1φ / open-delta). Divide by 1000 and round up to catalog kVA. Example: 208→240 @ 50 A → 1.6 → 2 kVA.
What is the difference between a buck-boost and a distribution transformer?
Buck-boost corrects a few percent of voltage at low unit kVA. Distribution transformers carry full load kVA—use transformer kVA calculator.
How do I size a buck boost for 208 to 240 V?
ΔV = 32 V. Unit kVA ≈ amps × 0.032. At 50 A → 1.6 → 2 kVA. Open the preset.
Do I need one or two buck-boost transformers for three-phase?
Open-delta banks usually use two identical units; wye uses three. This tool sizes each unit. Confirm the OEM diagram.
Is this the same as an Acme or HPS buck-boost selector?
No—OEM selectors pick SKUs and wiring. This page is a vendor-neutral kVA screen from volts and amps.
Can I use a large isolation transformer instead?
You can, but it is usually oversized and costlier for a 5–20% correction. Buck-boost autotransformer sizing keeps unit kVA near I×ΔV.