How to calculate transformer voltage ratio and line current
Divide input line-to-line voltage by output line-to-line voltage for the nominal voltage ratio. For each side of a balanced three-phase transformer, line current in amps equals MVA × 1,000 ÷ (√3 × line-to-line kV). The example assumes the same apparent power on each side and excludes losses.
- Nominal input/output voltage ratio = input kV ÷ output kV.
- Input line current = MVA × 1,000 ÷ (√3 × input kV).
- Output line current = MVA × 1,000 ÷ (√3 × output kV).
- Use MVA rather than MW. Convert MW using the project's power factor first.
- The line-voltage ratio is not necessarily the winding turns ratio; winding connections matter.
Step-down and step-up worked examples
A 100 MVA, 230 kV input / 34.5 kV output requirement has a nominal ratio of 6.67:1, approximately 251.02 A input current and 1,673.48 A output current. These are requirement calculations, not matching-stock offers or approved unit ratings.
| Apparent power | Input → output | Input amps | Output amps |
|---|---|---|---|
| 100 MVA | 230 → 34.5 kV | 251.02 A | 1,673.48 A |
| 100 MVA | 34.5 → 230 kV | 1,673.48 A | 251.02 A |
| 0.5 MVA / 500 kVA | 13.8 → 0.48 kV | 20.92 A | 601.41 A |
The calculation does not approve operating direction or capacity
Match each candidate's actual nameplate, drawings, cooling stages, permitted operating direction, grounding, taps, insulation and protection. Reversing the voltage inputs numerically does not establish that a GSU can supply the proposed load in reverse.
The Royal SMIT 525/69 kV listing retains its separate 250 MVA maximum for step-down duty. Its 240/320/400 MVA cooling-stage and step-up information must not be used as a 400 MVA step-down offer. Enter the permitted duty-specific apparent power in the calculator.
This tool does not calculate short-circuit duty, cable ratings, breaker selection, protection settings, losses, overload capability or parallel compatibility. Use the approved project design for those decisions.
Turn the result into a transformer quotation request
Send required MVA or kVA, input and output voltages, actual power-flow duty, quantity, project location and needed-on-site date. Add frequency, winding connections, grounding, cooling, taps, impedance and the one-line or nameplate. J&J compares published equipment and additional supply options against that requirement.
Common questions
Written quotes and supporting records control unit-specific ratings, availability, condition, price, and delivery.
Can this calculator be used for both step-down and GSU transformer inquiries?
Yes. Enter the actual source and output voltages for the proposed duty. The calculator estimates voltage ratio and balanced three-phase line current; it does not approve a particular transformer for that duty.
Should I enter 480 or 0.48 for a 480 V secondary?
Enter 0.48 kV. The calculator uses kV throughout. Likewise, enter 0.5 MVA for 500 kVA.
Does a higher secondary current mean the transformer creates more power?
No. In the simplified calculation, the same apparent power is used on both sides. Lower voltage corresponds to higher line current. Actual equipment also has losses and duty-specific limits.
Does the voltage ratio prove that a transformer can be operated in reverse?
No. Reverse duty requires review of the actual unit records and the project's winding, grounding, tap, insulation, protection and operating requirements. A calculated ratio alone does not establish suitability.