Transformer Secondary Current Calculator

Calculator Inputs

Large screens show three columns, medium screens show two, and mobile screens show one.

Phase Type

Transformer Rating (kVA)

Primary Voltage (V)

Secondary Voltage (V)

Efficiency (%)

Load (%)

Power Factor

Measured Primary Current (A)

Primary Turns (optional)

Secondary Turns (optional)

Required Load Current (A)

Reset

Use turns values for ratio-based estimation when available. Otherwise, the tool falls back to voltage ratio for approximation.

Formula Used

1) Rated secondary current

Single phase: I₂ = (kVA × 1000) / V₂

Three phase: I₂ = (kVA × 1000) / (√3 × V₂)

2) Rated primary current

Single phase: I₁ = (kVA × 1000) / V₁

Three phase: I₁ = (kVA × 1000) / (√3 × V₁)

3) Operating secondary current

I_2,load = I_2,rated × (Load % / 100)

4) Ratio-based secondary current estimate

I_2,ratio ≈ I_1,measured × Current Ratio × Efficiency

Current ratio uses N₁ / N₂ when turns are supplied, otherwise V₁ / V₂.

5) Current margin

Margin = Operating Secondary Current − Required Load Current

How to Use This Calculator

Step 1: Select single-phase or three-phase operation.

Step 2: Enter transformer rating, primary voltage, and secondary voltage using consistent nameplate values.

Step 3: Add load percentage, efficiency, and power factor to estimate operating current and power performance.

Step 4: Enter measured primary current for an approximate ratio-based secondary current.

Step 5: Add turns values when available for a stronger ratio estimate than voltage-only conversion.

Step 6: Enter required load current to compare available operating current against demand and identify remaining margin.

Example Data Table

Item	Value
Phase type	Three Phase
Transformer rating	150 kVA
Primary voltage	11,000 V
Secondary voltage	415 V
Efficiency	98.2%
Load	80%
Power factor	0.92
Measured primary current	6.40 A
Required load current	160.00 A
Rated secondary current	208.68 A
Operating secondary current	166.94 A
Rated primary current	7.87 A
Ratio-based secondary current	166.95 A
Current margin	6.94 A

FAQs

1. What does this calculator estimate?

It estimates rated secondary current, operating current at a selected load, ratio-based secondary current, and remaining current margin against required demand.

2. When should I choose three-phase mode?

Choose three-phase mode when the transformer and connected system use balanced three-phase line voltages and currents on the selected side.

3. Why is power factor included?

Power factor helps estimate real power output and input power. It does not replace nameplate current formulas based on apparent power.

4. Why are efficiency losses shown?

Efficiency converts estimated output power into estimated input power. The difference is shown as losses for quick operating review.

5. Are turns values required?

No. Turns are optional. When absent, the tool uses the voltage ratio as an approximation for ratio-based secondary current.

6. Does this replace detailed transformer design checks?

No. It is a planning and review tool. Final design should also consider temperature rise, insulation class, connection type, harmonics, and code requirements.

7. What does a negative margin mean?

A negative margin means the selected operating current is below the required load current. That indicates insufficient headroom at the chosen load point.

8. Can I export the results?

Yes. After calculation, use the CSV button for spreadsheet-friendly output or the PDF button for a quick printable report.