Calculator Inputs
Example Data Table
| Case | Supply V | Load V | Phase | Load A | Correction V | Approx Bank kVA |
|---|---|---|---|---|---|---|
| Motor boost | 208 | 230 | Single | 50 | 22 | 1.10 |
| Panel buck | 240 | 208 | Single | 40 | 32 | 1.28 |
| Three phase boost | 460 | 480 | Three | 60 | 20 | 2.08 |
| Machine correction | 220 | 240 | Single | 35 | 20 | 0.70 |
Formula Used
Voltage correction: Correction V = absolute value of required load voltage minus supply voltage.
Single phase load kVA: kVA = load voltage × load current ÷ 1000.
Three phase load kVA: kVA = 1.732 × load voltage × load current ÷ 1000.
Transformer bank kVA: bank kVA = load kVA × correction voltage ÷ required load voltage.
Margin sizing: sized bank kVA = bank kVA × (1 + safety margin ÷ 100).
Per unit size: per unit kVA = sized bank kVA ÷ transformer unit count.
Real power estimate: kW = load kVA × power factor.
How to Use This Calculator
- Enter the measured supply voltage at the equipment location.
- Enter the voltage required by the connected load.
- Select single phase or three phase service.
- Enter load current. Leave it blank only when load kVA is known.
- Choose the transformer unit count for the intended bank.
- Add power factor, efficiency, frequency, and safety margin.
- Press calculate and review the result above the form.
- Download CSV or PDF records after the result appears.
Planning Voltage Correction
A buck boost transformer is used when voltage is slightly low or high. It does not replace a full design review. It helps estimate corrective transformer size before checking nameplate data. The method supports motors, heaters, lighting, panels, and small machines. The key idea is simple. Only the voltage difference is transformed. The load still receives the final working voltage.
Why Capacity Is Smaller
The transformer is wired as an autotransformer. Its required kVA is often much smaller than load kVA. A ten percent voltage lift may need about ten percent of load kVA. This makes the setup compact and efficient. Still, the selected unit must handle line current. It must also match insulation limits, temperature rise, and installation rules. Always compare the calculated bank size with a listed catalog size.
Important Design Checks
Begin with the measured supply voltage. Use voltage measured under normal load when possible. Then enter the required output voltage. The calculator finds the buck or boost amount. It also estimates load current from kVA. That happens when current is not entered. For three phase systems, it uses the square root of three factor. Power factor gives real power notes. It does not change basic transformer kVA sizing.
Selection Guidance
Use the safety margin field to avoid an undersized unit. A margin covers meter error, future load growth, and heating. Choose the next standard kVA size above the required per unit value. If the value sits close to a catalog size, choose larger. That choice is usually safer.
Practical Notes
Check the connection plan before ordering parts. A boost connection raises voltage. A buck connection lowers voltage. The series winding must carry load current. The primary winding must match the available supply. Three phase banks need matching units and proper phasing. Label conductors clearly during installation.
Limitations
This calculator gives planning numbers. It cannot verify terminal diagrams, grounding, short circuit current, or code compliance. It also cannot confirm the correct transformer count for every circuit. Review the manufacturer wiring diagram before energizing. Confirm primary voltage, secondary voltage, frequency, enclosure type, and temperature rating. For critical equipment, ask a qualified electrician or engineer to review the installation. Keep records with the final panel documentation.
FAQs
What does a buck boost transformer do?
It raises or lowers voltage by adding or subtracting a smaller transformer voltage. It is often used when equipment voltage does not match the available supply.
Why is the transformer kVA smaller than load kVA?
In buck boost wiring, only the voltage difference is transformed. The load kVA may be large, but the corrective voltage portion is smaller.
Can this calculator size three phase banks?
Yes. Select three phase and choose the transformer unit count. Then verify the final bank arrangement with the manufacturer diagram.
Should I enter current or kVA?
Enter load current when it is known. If current is blank, the calculator uses load kVA to estimate current.
What safety margin should I use?
Many planning estimates use 10 to 25 percent. Use a higher margin for hot spaces, uncertain loads, or future expansion.
Does power factor affect transformer kVA?
The kVA sizing is based on voltage and current. Power factor is included to estimate real power in kilowatts.
Can this replace a wiring diagram?
No. This calculator estimates size only. Always follow the transformer nameplate and wiring diagram before energizing the circuit.
When should I ask an electrician?
Ask a qualified electrician for final installation, grounding, overcurrent protection, conductor sizing, and code compliance.