Transformer Current Calculator
Formula Used
Single phase current: I = (kVA × 1000) / V
Three phase current: I = (kVA × 1000) / (√3 × V)
Primary current with efficiency: Ip = (Load kVA × 1000) / (Phase factor × Vp × Efficiency)
Secondary current: Is = (Load kVA × 1000) / (Phase factor × Vs)
Voltage ratio: Vp / Vs
Ideal current ratio: Ip / Is = Vs / Vp
Real output power: kW = Load kVA × Power factor
Estimated loss: Loss kW = Input kW − Output kW
How to Use This Calculator
- Select single phase or three phase.
- Enter the transformer rating in kVA.
- Enter the expected load percentage.
- Enter primary and secondary voltage values.
- Add efficiency, power factor, and safety factor.
- Optional current fields can estimate current on the opposite side.
- Press the calculate button to view results above the form.
- Use the CSV or PDF button to save the result.
Example Data Table
| Rating |
Phase |
Primary Voltage |
Secondary Voltage |
Load |
Efficiency |
Use Case |
| 25 kVA |
Single phase |
2400 V |
240 V |
80% |
97% |
Small distribution service |
| 100 kVA |
Three phase |
11000 V |
415 V |
100% |
98% |
Commercial panel supply |
| 500 kVA |
Three phase |
33000 V |
400 V |
75% |
98.5% |
Industrial load planning |
Why Transformer Current Matters
Transformer current is a key value in every electrical design. It tells you how much current each winding must carry. This affects cable size, breaker selection, fuse rating, heat rise, and voltage drop. Primary current flows on the supply side. Secondary current flows on the load side. A small voltage change can create a large current change. That is why both sides should be checked together.
Practical Design Notes
This calculator uses apparent power because transformers are usually rated in kVA. The formula works for single phase and three phase systems. For three phase work, use line to line voltage. The load percentage lets you test normal loading, light loading, or overload conditions. Efficiency adjusts the primary side estimate. It shows the extra input needed to cover transformer losses. The safety factor gives a design current for protective devices and wiring checks. It should not replace local code rules.
Interpreting Results
Rated current is based on the full transformer rating. Operating current is based on the selected load. Design current includes the safety factor. The voltage ratio also shows the ideal turns ratio. When voltage steps down, current usually steps up. When voltage steps up, current usually steps down. Known current fields help you estimate the opposite side current from a measured value. These estimates assume balanced loading and normal transformer action.
Good Use Cases
Use the tool during panel planning, motor control design, power supply sizing, and maintenance checks. It is helpful before ordering cable, switchgear, fuses, and meters. It also supports quick comparisons between transformer sizes. Always confirm nameplate data before final work. Real installations may include harmonic heating, inrush current, ambient temperature, altitude, grounding method, and service rules. Large transformers need detailed engineering review. Use this calculator as a planning guide. Then verify the final design with qualified standards and site conditions.
Limits and Safety
The result is not a fault current study. It does not calculate short circuit duty, inrush peaks, or protection coordination. Nameplate impedance and utility data are needed for those tasks. Keep conductor temperature and enclosure ventilation in mind. For critical equipment, use manufacturer data and a licensed professional. Document input assumptions so future checks remain clear and easy.
FAQs
What is primary current?
Primary current is the current drawn from the supply side of the transformer. It depends on transformer power, primary voltage, phase type, load, and efficiency.
What is secondary current?
Secondary current is the current delivered to the load side. It rises when secondary voltage is lower for the same kVA rating.
Why is kVA used instead of kW?
Transformers are rated by apparent power. kVA includes voltage and current capacity without depending only on power factor.
How is three phase current calculated?
Three phase current uses √3 in the denominator. The formula is kVA times 1000 divided by √3 times line voltage.
Does efficiency affect secondary current?
Secondary current is mainly based on delivered apparent power and secondary voltage. Efficiency mostly affects the primary input current estimate.
What does the safety factor do?
The safety factor increases the design current. It helps compare wiring and protection margins during early planning.
Can I use this for step-up transformers?
Yes. Enter the lower voltage on the primary side only when it is truly the supply side. Current direction follows the entered values.
Is this suitable for final installation design?
Use it for estimates and planning. Final design should follow local codes, nameplate data, protection rules, and qualified engineering review.