Buck-Boost Transformer Calculator

Related tools: Transformer kVA Calculator, Power Factor Calculator, and Electrical Load Calculator.

A buck-boost transformer is used when an available supply voltage is close to, but does not match, the voltage required by a load. The central sizing value is the Required buck-boost kVA—the winding kVA needed to change the voltage for the stated load current after applying the selected planning allowance.

Unlike a conventional isolation transformer calculation, buck-boost sizing is based on the voltage being added or subtracted, not the full load voltage. A 208 V supply raised to 240 V, for example, requires only a 32 V boost. The connected load may be 7.2 kVA, while the buck-boost transformer winding requirement is substantially smaller.

This calculation is commonly used during early equipment-voltage review for branch circuits, feeders, machinery, motor loads, HVAC equipment, controls, and other loads that need a modest voltage adjustment. It helps establish the preliminary transformer kVA requirement before a wiring arrangement, transformer catalog selection, overcurrent protection design, conductor ampacity review, voltage-drop review, or field installation is finalized.

Voltage Change and Operating Mode

Supply voltage (V) is the voltage available at the source. Target voltage (V) is the voltage required by the load.

The calculator first determines the voltage change:

\(\displaystyle \text{Voltage change} = \left| \text{Target voltage} - \text{Supply voltage} \right|\)

It then identifies the connection purpose:

• boost when Target voltage is higher than Supply voltage
• buck when Target voltage is lower than Supply voltage

For a 208 V supply and a 240 V target voltage:

\(\displaystyle \left|240 - 208\right| = 32\text{ V}\)

The result is a 32 V voltage change in boost mode. The transformer arrangement must add 32 V to the available 208 V supply.

The arithmetic does not establish the transformer connection. Actual buck-boost installations require a verified manufacturer wiring diagram and a transformer configuration suitable for the supply, target voltage, phase, and load.

Load kVA and Winding kVA

Load current (A) is the load current used for the calculation. Phase selects either Single-phase or balanced three-phase kVA math.

The calculator reports Load kVA using the target load voltage:

\(\displaystyle \text{Load kVA} = \frac{\text{Target voltage} \times \text{Load current} \times \text{Phase multiplier}}{1000}\)

For Single-phase, the phase multiplier is 1. For balanced three-phase calculations, the phase multiplier is \sqrt{3}.

The buck-boost transformer does not process the full Load kVA as a standard full-voltage transformer would. Its windings process the voltage difference:

\(\displaystyle \text{Buck-boost winding kVA} = \frac{\left|\text{Target voltage} - \text{Supply voltage}\right| \times \text{Load current} \times \text{Phase multiplier}}{1000}\)

This is the electrical basis for using a comparatively small buck-boost transformer to make a modest voltage correction on a larger kVA load.

Sizing Multiplier

Sizing multiplier (%) applies a planning multiplier to the calculated Buck-boost winding kVA:

\(\displaystyle \text{Required buck-boost kVA} = \text{Buck-boost winding kVA} \times \frac{\text{Sizing multiplier}}{100}\)

The output Sizing multiplier used confirms the percentage applied to the calculation. The multiplier is not a transformer nameplate rating, conductor adjustment factor, insulation temperature correction factor, or code-required ampacity adjustment. It is a user-entered planning allowance applied only to the winding kVA result.

Calculation Example

Using the stated values:

Field	Value
Supply voltage (V)	208 V
Target voltage (V)	240 V
Load current (A)	30 A
Phase	Single-phase
Sizing multiplier (%)	115%

The calculation proceeds as follows.

\(\displaystyle \text{Voltage change} = |240 - 208| = 32\text{ V}\)

Because the target voltage exceeds the supply voltage, the result is:

\(\displaystyle \text{Buck/boost mode} = \text{boost}\)

The full connected load is:

\(\displaystyle \text{Load kVA} = \frac{240 \times 30 \times 1}{1000} = 7.2\text{ kVA}\)

The buck-boost winding requirement is:

\(\displaystyle \text{Buck-boost winding kVA} = \frac{32 \times 30 \times 1}{1000} = 0.96\text{ kVA}\)

Applying the 115% sizing multiplier:

\(\displaystyle \text{Required buck-boost kVA} = 0.96 \times 1.15 = 1.104\text{ kVA}\)

The calculator results are therefore:

Result	Value
Voltage change	32 V
Buck/boost mode	boost
Load kVA	7.2 kVA
Buck-boost winding kVA	0.96 kVA
Required buck-boost kVA	1.104 kVA
Sizing multiplier used	115%

Field Verification

The Required buck-boost kVA result supports preliminary transformer capacity screening. It does not select a specific transformer or establish a compliant installation.

Verify the following separately before installation:

• Transformer catalog ratings, available voltage taps, and manufacturer-approved buck-boost connection diagrams
• Actual single-phase or balanced three-phase load conditions
• Load type, including motor starting characteristics, inrush, non-linear loading, and continuous-load considerations
• Branch-circuit or feeder conductor ampacity, AWG or kcmil size, terminal rating, insulation temperature rating, and applicable correction or adjustment factors
• Overcurrent protection, disconnecting means, grounding and bonding, and equipment listing requirements
• Raceway fill, conductor routing, available working space, and voltage drop at the corrected load voltage
• Local amendments, utility conditions, and AHJ requirements

The calculation provides a voltage-change winding kVA estimate only. It does not provide a wiring diagram, catalog selection, grounding method, overcurrent protection result, listing determination, or code-compliance decision.