Calculate transformer rating, primary and secondary voltage, full-load current, or turns ratio for single-phase and three-phase systems.
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Transformer Formula
The transformer calculator uses kVA, volts, amps, and turns ratio. It converts MVA to kVA, kV to V, and mA to A before calculating.
\begin{aligned}
S &= \frac{F \times V_p \times I_p}{1000} \\
V_p &= \frac{S \times 1000}{F \times I_p} \\
I_p &= \frac{S \times 1000}{F \times V_p} \\
TR &= \frac{V_p}{V_s} \\
V_s &= \frac{V_p}{TR} \\
I_s &= I_p \times TR
\end{aligned}- S = transformer rating in kVA
- F = phase factor, 1 for single phase or 1.73 for three phase
- Vp = primary transformer voltage in volts
- Vs = secondary transformer voltage in volts
- Ip = primary full load current in amps
- Is = secondary full load current in amps
- TR = turns ratio, calculated as primary voltage divided by secondary voltage
If you leave the transformer rating blank, the calculator uses primary voltage and primary current to find kVA. If you leave primary voltage or primary current blank, it rearranges the kVA formula. If you leave secondary voltage, secondary current, or turns ratio blank, it uses the turns ratio formulas.
Phase Factors and Unit Conversions
| Phase selection | Factor used | Rating formula |
|---|---|---|
| Single phase | 1 | kVA = V × A / 1000 |
| Three phase | 1.73 | kVA = 1.73 × V × A / 1000 |
| Input unit | Base unit used | Conversion |
|---|---|---|
| MVA | kVA | 1 MVA = 1000 kVA |
| kV | V | 1 kV = 1000 V |
| mA | A | 1 mA = 0.001 A |
Example Calculations
Example 1: Find three-phase transformer rating
You have a three-phase transformer with a 480 V primary voltage and 120 A primary full load current.
Use the rating formula:
S = 1.73 × 480 × 120 / 1000 = 99.648 kVA
The transformer rating is about 99.65 kVA.
Example 2: Find secondary voltage from turns ratio
You have a primary voltage of 2400 V and a turns ratio of 10.
Use the secondary voltage formula:
Vs = 2400 / 10 = 240 V
The secondary transformer voltage is 240 V.
FAQ
What does transformer full load current mean?
Full load current is the current a transformer carries when it is delivering its rated kVA at the specified voltage. Primary full load current is on the input side. Secondary full load current is on the output side.
Why is 1.73 used for three-phase transformers?
The value 1.73 is an approximation of the square root of 3. Three-phase apparent power uses this factor when voltage and current are line values. That is why the three-phase kVA formula is kVA = 1.73 × V × A / 1000.
How many fields should be entered?
Enter exactly 6 fields and leave the value you want to calculate blank. Phase must be selected. The calculator cannot calculate phase because it is a system type, not a value derived from the other entries.