Single-phase current
I = (kVA x 1000) / V
Electrical calculator
Transformer Calculator
Calculate transformer primary current, secondary current, full-load current, load current, and voltage ratio for single-phase or three-phase transformers.
Instant transformer solver
Enter transformer values
Use line-to-line voltage for three-phase transformers. Results update automatically as you type.
Primary current at load
0 A
Full-load primary current: 0 A
Secondary current at load
0 A
Full-load secondary current: 0 A
500 kVATransformer rating
26.51:1Primary:secondary voltage ratio
Three-phaseCalculation type
Formulas
Transformer current formulas used
Three-phase current
I = (kVA x 1000) / (sqrt(3) x V)
Load current
Iload = Ifull-load x (load % / 100)
Voltage ratio
Ratio = Vprimary / Vsecondary
Worked example
Example: 500 kVA 11 kV to 415 V transformer
For a balanced three-phase 500 kVA transformer with an 11,000 V primary and 415 V secondary, the full-load current is calculated on each side using the three-phase current formula.
- Primary current = 500 x 1000 / (sqrt(3) x 11000) = 26.2 A.
- Secondary current = 500 x 1000 / (sqrt(3) x 415) = 695.6 A.
- Voltage ratio = 11000 / 415 = 26.51:1.
Guide
How to use this transformer calculator
- Select single-phase or three-phase based on the transformer and system being checked.
- Enter transformer apparent power in kVA from the nameplate or specification.
- Enter primary and secondary voltages. For three-phase systems, use line-to-line voltage.
- Set load percentage to estimate current at partial load, or leave it at 100% for full-load current.
Reference
What the transformer results mean
Primary current
The current drawn on the supply side of the transformer for the entered kVA and load percentage.
Secondary current
The current available or flowing on the load side at the entered transformer rating and load percentage.
Voltage ratio
The primary-to-secondary voltage relationship. For an ideal transformer, this is approximately the turns ratio.
kVA rating
The apparent power rating used for current sizing. Transformer loading, temperature rise, impedance, and cooling class also matter.
Assumptions and limits
Before using the result for design
- The three-phase calculation assumes a balanced transformer and line-to-line voltage.
- The calculator estimates current from apparent power and voltage. It does not include transformer impedance, inrush current, protection settings, voltage regulation, tap position, harmonics, or fault current.
- Use transformer nameplate data, applicable electrical codes, manufacturer documentation, and qualified engineering judgement for final design and protection selection.
Useful next checks
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Transformer calculator questions
How do I calculate transformer full-load current?
Use I = kVA x 1000 / V for single-phase transformers. For three-phase transformers, use I = kVA x 1000 / (sqrt(3) x V), where V is line-to-line voltage.
Should I use primary or secondary voltage?
Use primary voltage to calculate primary current and secondary voltage to calculate secondary current. For three-phase transformers, enter line-to-line voltage on each side.
Can this calculator size transformer protection?
No. It gives current estimates from kVA and voltage. Protection sizing also depends on electrical codes, transformer impedance, inrush current, conductor ratings, grounding, and manufacturer data.
Is transformer voltage ratio the same as turns ratio?
For an ideal transformer, voltage ratio and turns ratio are approximately the same. Real transformer values can differ due to regulation, tap settings, losses, and manufacturing tolerances.
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