Transformer Calculator quickly solves voltage, current, winding ratio and apparent-power questions for common single-phase and three-phase systems. It combines four practical modes in one page so technicians, students and homeowners can compare the ideal electrical relationship before checking a real unit’s nameplate.
Calculate transformer voltage, current, turns ratio or power
Choose a calculation mode, enter known primary and secondary values, then calculate ideal transformer results.
Transformer Calculator formulas
An ideal transformer relates primary and secondary voltage to the number of turns in each winding. The same ratio works in reverse for current.
Single-phase VA = V × I
Three-phase VA = √3 × V × I
Estimated output power = input VA × efficiency
Here, V is volts, I is amperes, and N is winding turns. Subscripts p and s mean primary and secondary.
Transformer Calculator worked example
A transformer has 1,000 primary turns connected to 240 V and 100 secondary turns. The turns ratio is 10:1, so the ideal secondary voltage is:
If the primary current is 1 A, the ideal secondary current is 10 A and the apparent power is 240 VA. At 95% efficiency, estimated delivered power is about 228 W when the load power factor is assumed to be 1.
Transformer sizing and safety
| Check | Practical guidance |
|---|---|
| Load VA | Add the connected load volt-amperes and allow suitable design margin. |
| Voltage | Match rated primary frequency and voltage; confirm the required secondary voltage. |
| Current | Do not exceed the winding current rating or nameplate VA rating. |
| Protection | Use appropriately rated overcurrent protection, wiring and enclosure. |
For selection work, begin with the load voltage and total load current, calculate the required VA, and choose a transformer whose continuous rating meets the application and applicable code requirements. Loads with high starting current, harmonics or poor power factor may require additional capacity. Ambient temperature, ventilation, duty cycle and enclosure conditions can also change the correct selection.
The Transformer Calculator is a planning aid, not a substitute for a transformer nameplate, manufacturer data or a qualified electrician. Review transformer efficiency guidance from the U.S. Department of Energy. Three-phase line values depend on the winding connection and system configuration.
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Frequently asked questions
What is a transformer turns ratio?
It is the primary winding turns divided by the secondary winding turns. In an ideal transformer, this equals the primary-to-secondary voltage ratio.
Does a step-down transformer increase current?
Ideally, yes. When voltage decreases, available current increases by the inverse ratio while apparent power remains approximately constant.
What is the difference between VA and watts?
VA is apparent power. Watts are real power. For AC loads, watts equal VA multiplied by power factor; efficiency also affects delivered output.
Can I use this for three-phase transformers?
Use the three-phase option for apparent power based on line voltage and line current. Always verify delta or wye connections and nameplate ratings.
Why is actual secondary voltage lower?
Winding resistance, magnetic losses, regulation and load current cause voltage drop. The ideal turns-ratio result does not model every loss.