About the Transformer Ratio Calculator
What is the Transformer Ratio Calculator?
A transformer's voltage ratio equals its transformer turns ratio, and currents change in the opposite ratio. This calculator gives the ratio, the secondary turns if you know the primary turns, and the full-load current on each side.
It supports single-phase and three-phase transformers, such as the common 11 kV / 433 V distribution transformer.
How to use the Transformer Ratio Calculator
- Type (sample: Three-phase)
- Primary Voltage (V) (sample: 11,000)
- Secondary Voltage (V) (sample: 433)
- Rating (kVA) (sample: 100)
- Primary Turns (0 if unknown) (sample: 0)
- Press Calculate Transformer Ratio (results also refresh as you type). The result card shows the Turns Ratio, along with Ratio as a number, Primary full-load current, Secondary full-load current and Type.
Formula used
Np ÷ Ns = Vp ÷ Vs = Is ÷ Ip
- Full-load current (3-phase) = kVA × 1,000 ÷ (√3 × V)
- Full-load current (1-phase) = kVA × 1,000 ÷ V
Distribution transformers use 433 V (not 415 V) secondaries to allow for voltage drop under load.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Type | Three-phase |
| Primary Voltage (V) | 11,000 |
| Secondary Voltage (V) | 433 |
| Rating (kVA) | 100 |
| Primary Turns (0 if unknown) | 0 |
| Result | Value |
|---|---|
| Turns Ratio | 25.404 : 1 |
| Ratio as a number | 25.4042 |
| Primary full-load current | 5.25 A |
| Secondary full-load current | 133.34 A |
| Type | Step-down |
Tips for using the Transformer Ratio Calculator
- Use the full-load current to size cables and protection on each side.
- Real transformers have small losses, so the actual secondary voltage drops slightly under load.