Calculating the rated current from a transformer's power and voltage is a daily skill for engineers and technicians: you need it to select breakers, cables, and current transformers, and to understand instrument readings in the field. The formula is simple, and the important thing is to pay attention to the units.
The Basic Formulas
For a single-phase transformer:
For a three-phase transformer:
Where S is the apparent power in volt-amperes (VA), and V is the voltage in volts. Be careful to convert kVA to VA (×1000) and kV to V (×1000).
A 1000 kVA transformer with a secondary voltage of 400 volts:
The same transformer from the 13.8 kV side:
Notice how the current decreases as the voltage increases — this is the very essence of the idea of transmitting power at high voltage.
Why Does This Number Matter to You?
- Selecting protection breakers and cable sizes on both sides of the transformer.
- Selecting the ratio of current transformers (CTs) for measurement and protection.
- Comparing the actual operating current with the rated current to assess the loading percentage.
Try the transformer current calculator on the site for an instant calculation that shows the formula.
Sample answer: I = 500,000 ÷ (1.732 × 400) ≈ 722 amps. This is the line current on the secondary side at full load, and it is used as the basis for selecting the breaker and cables on that side.
Forgetting the √3 in three-phase calculations, or mixing up units (using kVA directly with V without conversion). Always check your units before substituting values.
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