Transformer current and kVA calculator

Use this page when a transformer schedule, replacement note, switchboard record or estimate needs the current on both winding sides to be visible. The arithmetic is simple, but the record can still go wrong when the current is copied without its side, voltage or rating basis.

The useful output is a transformer relationship record: kVA, primary voltage, secondary voltage, primary current and secondary current shown together. That record can support a quote, a board schedule, a cable conversation or a protection discussion. It does not choose the transformer, prove spare capacity or decide switchgear suitability.

Field use cases

The last point is often the practical risk. A transformer record that says "current 721.69 A" is incomplete. A record that says "TX-1, 500 kVA, 11 kV/400 V three phase, primary 26.24 A, secondary 721.69 A" can be checked by another person.

Solve-mode matrix

The modes are record modes, not decision modes. They answer which arithmetic relationship was used. A calculated kVA from secondary current may be useful for an estimate, but a selected transformer rating still needs load profile, duty, manufacturer data and project requirements.

Primary and secondary side interpretation

Transformer current changes with voltage. For a 500 kVA three-phase transformer at 11 kV and 400 V, the primary current is about 26.24 A and the secondary current is about 721.69 A. Both values describe the same apparent power, but they belong to different parts of the installation.

Primary current is usually the current carried into upstream supply, HV feeder or primary protection discussion. Secondary current is usually the value carried into low-voltage switchboard, cable, busbar or load-side review. Mixing those two values can create a serious record error even when the formula itself is correct.

The calculator therefore keeps both side values in the result panel. When a review current is entered, the selected review side should match the equipment or record being checked. A secondary-side switchboard value should not be compared with the primary-side current.

Review workflow

1. Identify the transformer reference from the drawing, schedule, asset register, nameplate or quote note.
2. Confirm whether the task starts from transformer kVA, primary current or secondary current.
3. Select single phase or three phase. Use line-to-line voltages for balanced three-phase transformer records.
4. Enter primary and secondary voltages exactly as they are being used in the project record.
5. Enter the known value for the selected solve mode.
6. If a review current is used, select the side it belongs to before reading the margin.
7. Carry the result forward only with the voltage side, phase arrangement and data source attached.

This workflow keeps the calculator as a transformer relationship tool. It creates a clear current record, but it does not replace the transformer schedule, product data sheet, protection study or installation design.

Worked records

The examples show why transformer records need both side labels. A high-voltage primary current can look small compared with the low-voltage secondary current. That is normal for the same kVA relationship, but it must be recorded clearly.

Boundary with neighbouring calculations

Keeping this boundary clear prevents one calculator from pretending to solve the whole transformer job. Current and kVA are important inputs, but transformer work also depends on installation context and product data.

Australian context

The default project context is Australian 230/400 V a.c., 50 Hz, with editable voltage fields because transformer records often involve higher primary voltages or special secondary voltages. The calculator does not reproduce controlled standard tables and does not make a standards determination.

Transformer records may be affected by current Australian standards, local authority requirements, DNSP conditions, site earthing arrangements, manufacturer instructions, temperature and enclosure conditions, protection coordination and short-circuit levels. Use the calculated currents as traceable arithmetic values, then keep the stronger project and product checks visible.

Stop points

• The transformer reference cannot be matched to a drawing, schedule, asset register, nameplate or quote.
• The entered current may belong to the wrong winding side.
• The voltage basis is uncertain or copied from a different transformer.
• A calculated current is being used to select protection, fusegear, switchgear or cable without product and project review.
• The actual question is fault current from impedance rather than load current.
• The transformer is being selected for purchase or final design without manufacturer data and duty information.
• The result is being used as spare capacity without load profile and maximum-demand review.