Three-phase transformer current formula
For a balanced three-phase transformer, full-load line current can be calculated from the total transformer kVA rating and the line-to-line voltage.
Apply the formula separately to the primary and secondary rated voltages.
Single-phase transformer current formula
For a single-phase transformer, full-load current is:
Transformer calculation example
Consider a three-phase 1,000 kVA transformer rated 11,000 V on the primary and 400 V on the secondary.
The primary full-load current is approximately 52.49 A.
The secondary full-load current is approximately 1,443.38 A.
Transformer voltage ratio
The rated voltage ratio is the primary winding voltage divided by the secondary winding voltage.
An 11,000 V to 400 V transformer therefore has a line-voltage ratio of 27.5:1.
Step-down and step-up transformers
When the primary rated voltage is higher than the secondary rated voltage, the calculator labels the transformer as step-down.
When the secondary rated voltage is higher, it is labeled step-up. Equal rated voltages produce an isolation or equal-voltage result.
Why current changes with voltage
In the ideal transformer model, apparent power is treated as the same on both sides. Lower voltage therefore corresponds to higher current, while higher voltage corresponds to lower current.
For the 11 kV / 400 V example, the secondary current is about 27.5 times the primary current, matching the inverse of the voltage relationship.
Transformer kVA vs. kW
Transformers are commonly rated in apparent power, such as kVA, rather than only real power in kW. The kW associated with a load also depends on its power factor.
If you know kW and power factor, use the Power Factor Calculator to determine the corresponding kVA.
Voltage ratio vs. winding turns ratio
For an ideal single-phase transformer, the voltage ratio follows the winding turns ratio.
Three-phase transformers can use different delta and wye winding connections, so a line-to-line voltage ratio should not always be interpreted directly as the physical winding turns ratio without considering the winding connection.
Full-load current vs. actual operating current
The calculated currents are nameplate full-load values at rated kVA and rated voltage. Actual current depends on the connected load and operating conditions.
Related electrical calculations
For conductor sizing, use the Wire Size Calculator.
For feeder voltage loss, use the Voltage Drop Calculator.
For AC kW, kVA, kVAR and power factor relationships, use the Power Factor Calculator.
This calculator provides ideal nameplate full-load current and voltage-ratio calculations. It does not size protection, conductors, grounding, transformer impedance, fault current, inrush current, efficiency or voltage regulation.
Frequently asked questions
How do I calculate transformer full-load current?
For single-phase, divide VA by voltage. For balanced three-phase, divide VA by √3 times the line-to-line voltage.
What voltage do I enter for three-phase?
Enter the rated line-to-line voltage for the winding when using the three-phase current calculation.
Why is secondary current higher on a step-down transformer?
In the ideal model, apparent power is approximately conserved, so reducing voltage increases the corresponding current.
Is voltage ratio always the same as turns ratio?
It is directly related for an ideal single-phase transformer. For three-phase transformers, delta and wye winding connections must also be considered.
Does this calculate transformer fault current?
No. Fault-current calculations require additional information such as transformer impedance and upstream source characteristics.
Using this calculator effectively
The Transformer Calculator estimates primary and secondary full-load current from transformer kVA and rated winding voltages. It supports single-phase and balanced three-phase systems and also reports the rated voltage and current ratios.
Common uses
- Checking full-load current on the primary and secondary sides of a transformer.
- Comparing current levels before and after a transformer voltage change.
- Finding the rated voltage ratio of a step-down, step-up or equal-voltage transformer.
Tips for a better estimate
- For three-phase calculations, enter total transformer kVA and the rated line-to-line voltage of each winding.
- Use nameplate rated voltage and kVA when calculating nameplate full-load current.
- Do not use the ideal full-load result as a substitute for protection, fault-current, conductor or transformer-engineering calculations.
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