Buck Boost Transformer Sizing Calculator

Screen minimum buck-boost correction-winding kVA from supply volts, desired load volts, load amps and connection. Default: 208 V → 240 V @ 50 A → 1.6 kVA minimum/unit. Jump the 208↔240 amp ladder. For pad-mount kVA use transformer size; for nameplate winding current use transformer FLA.

Quick Calculator

Results update instantly. Advanced options: phase and 3-phase connection below.

Supply voltage (V) Available line voltage before correction.

Desired load voltage (V) Target utilization voltage (boost if higher, buck if lower).

Load current (A) Continuous load amps at the load voltage. Example: 50 A on a 208→240 boost.

Phase

About this calculator

Vendor-neutral buck-boost sizing for small corrections (Suggest: 208 to 240)—not an OEM SKU picker. Confirm continuous load amps first; if you only have distribution kVA, get secondary current from transformer FLA. Hub: power calculator · transformer.

Calculation Results

1.600 kVA minimum / unit

Boost · ΔV = 32.0 V · % of supply ≈ 15.4%
208 V → 240 V @ 50.0 A · Single-phase (1 unit)
Calculated minimum per unit = 1.600 kVA (1600 VA). This does not select an OEM catalog unit.

Default preview — updates when you change inputs.

Unit VA ≈ I_load × ΔV for single-phase/open-delta, or ÷√3 per wye unit. Treat it as a minimum correction-winding requirement; select only from the exact manufacturer's application table.

Continue Your Calculation

Need a full distribution pad-mount instead of buck-boost? Continue in Transformer Size. For winding currents from kVA, use Transformer FLA.

Distribution kVA →

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 (208↔240 ladder + common pairs)

Unit kVA ≈ (I × ΔV) / 1000 for 1φ / open-delta. 208↔240 workflow: pick load amps → open preset → OEM application table. Suggest: buck boost transformer sizing chart, 208 to 240.

Supply → Load	ΔV	Load A	Min kVA/unit (1φ)	Note	Open
208 → 240 V (boost)	32 V	25 A	0.80	Light continuous load	Open
208 → 240 V (boost)	32 V	50 A	1.60	Default screen	Open
208 → 240 V (boost)	32 V	100 A	3.20	OEM table required	Open
240 → 208 V (buck)	32 V	50 A	1.60	Same ΔV, buck direction	Open
208 → 230 V (boost)	22 V	50 A	1.10	Common equipment band	Open
480 → 502 V (boost)	22 V	50 A	1.10 (open-delta each)	3φ bank — OEM diagram	Open
120 → 132 V (boost)	12 V	50 A	0.60	Control / small loads	Open

Need secondary amps from a distribution kVA nameplate instead of a measured load? Use transformer full-load amps, then return here with that current.

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
Minimum unit VA = I_load × ΔV  (1φ or each open-delta unit)
3φ wye: minimum unit VA = I_load × ΔV / √3
Minimum unit kVA = Unit VA / 1000

Worked example — 208 to 240: ΔV = 32 V, I = 50 A → minimum correction winding VA = 1600 → 1.6 kVA per unit. This arithmetic does not identify an OEM catalog unit. Full-load apparent power at 240 V is 12 kVA, but autotransformer suitability and allowable connected load must come from the exact application table.

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.

Before an OEM selector: confirm supply and target voltage are from the same phase convention, the correction is modest, the load amperage is continuous-duty appropriate, the selected connection is permitted by the manufacturer, and the proposed tap/wiring diagram matches the nameplate. A large voltage conversion or a need for isolation belongs on the distribution transformer sizing path instead.

Last reviewed: 2026-09-01. Arithmetic screen only: it does not model an OEM catalog ladder, allowable connected-load tables, temperature rise, harmonics or installation 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
Primary & secondary winding FLA from kVA	Transformer FLA	Nameplate currents (not buck-boost unit kVA)
Secondary fault from %Z	%Z fault-current screen	Isc screen after kVA known

Frequently Asked Questions

How do you size a buck boost transformer?

Compute ΔV = |V_load − V_supply|, then the minimum correction-winding VA from current, ΔV and connection. For 208→240 V at 50 A, the single-phase minimum is 1.6 kVA per unit. Select the actual unit and wiring only from the manufacturer's application table.

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. Minimum unit kVA ≈ amps × 0.032. At 50 A the arithmetic minimum is 1.6 kVA per unit; at 100 A ≈ 3.2 kVA. Verify the OEM application table. Open the 208↔240 ladder.

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.