Calculator Inputs
Example Data Table
| Scenario | Phase | Load Basis | Entered Load | Primary Voltage | Power Factor | Efficiency | Load Factor | Estimated Primary Current |
|---|---|---|---|---|---|---|---|---|
| Distribution unit | Single | Apparent | 50 kVA | 240 V | 0.95 | 98% | 80% | 170.068 A |
| Industrial feeder | Three | Real | 150 kW | 11,000 V | 0.90 | 97% | 75% | 6.764 A |
| Plant auxiliary load | Three | Apparent | 500 kVA | 6,600 V | 0.92 | 98.5% | 60% | 26.633 A |
Formula Used
Ip = Sin / Vp
Ip = Sin / (√3 × Vp)
Sin = Pout / (η × PF)
Actual load = Rated load × (Load factor / 100)
Single-phase: Ip ≈ (Vs × Is) / (Vp × η)
Three-phase: Ip ≈ Vs × Is / (Vp × η)
This calculator treats three-phase voltage as line-to-line voltage and three-phase current as line current. Efficiency raises required input current because losses make the primary draw more power than the secondary delivers.
How to Use This Calculator
- Choose single-phase or three-phase operation.
- Select whether your entered load is apparent power or real power.
- Enter the rating or connected load and choose the correct unit.
- Provide the expected load factor, power factor, efficiency, and primary voltage.
- Optionally add secondary voltage and current to compare a ratio-based estimate.
- Press the calculate button to show results above the form.
- Review actual current, full-load current, design current, and the plotted load curve.
- Use CSV or PDF export for documentation, checking, or design records.
FAQs
1. What is transformer primary current?
Transformer primary current is the current drawn from the supply side. It depends on input voltage, load power, phase arrangement, power factor, and efficiency losses inside the transformer.
2. Why does efficiency affect primary current?
A transformer is not perfectly lossless. Core and copper losses force the primary to draw more real power than the secondary delivers, so the primary current rises when efficiency decreases.
3. When should I use apparent power instead of real power?
Use apparent power when the transformer is rated directly in VA, kVA, or MVA. Use real power when the connected equipment load is given in watts or kilowatts.
4. Which voltage should I enter for three-phase systems?
Enter the primary line-to-line voltage for balanced three-phase systems. The calculator already applies the √3 relationship needed to convert apparent power into line current.
5. What does design current mean here?
Design current is the calculated operating current multiplied by your chosen margin. It gives a conservative value for checking conductors, protective devices, and thermal loading allowances.
6. Why compare results using secondary voltage and current?
Secondary data offers a fast cross-check. If the secondary-derived primary current differs sharply from the main result, one of the assumed load, efficiency, or power factor values may need review.
7. Does this calculator include inrush current?
No. The calculator estimates steady operating current. Transformer energization inrush is a short-duration phenomenon and needs separate analysis using core flux, switching angle, and system impedance.
8. Can this help with breaker or cable sizing?
Yes, as a preliminary guide. Use the design current and suggested standard device as screening values, then confirm sizing with applicable electrical codes, temperature correction, and installation conditions.