Transformer Inrush Current Calculator
Calculate peak inrush current and protection requirements for power transformers
Transformer Inrush Current Calculator
Compliant with IEEE C57.12.00 & IEC 60076 Standards
Results:
Full Load Current: 0.00 A
Peak Inrush Current: 0.00 A
Inrush Duration: 0.1 - 1.0 sec
Protection Setting: 0.00 A
How to Use the Transformer Inrush Current Calculator
Step 1: Select Transformer Type
- Choose your transformer type from the dropdown
- Power transformers: 8-12x inrush factor
- Distribution transformers: 12-20x inrush factor
- Dry type transformers: 6-10x inrush factor
- Select "Custom" for specific inrush factors
Step 2: Enter Transformer Data
- Select phase configuration (Single or Three Phase)
- Enter transformer kVA rating
- Input primary voltage (line-to-line for 3-phase)
- Verify the inrush factor is appropriate
- Click "Calculate" to get results
Step 3: Interpret Results
- Full Load Current: Normal operating current
- Peak Inrush Current: Maximum transient current
- Inrush Duration: Time for current to stabilize
- Protection Setting: Recommended overcurrent setting
- Use results for protection coordination
How to Calculate Transformer Inrush Current
Understanding Inrush Current
Transformer inrush current occurs when:
- Transformer is first energized
- Magnetic flux asymmetry develops
- Core saturation causes high current
- Current gradually decreases to normal
Calculation Steps
- Calculate Full Load Current (FLC)
- Determine appropriate inrush factor
- Multiply FLC by inrush factor
- Consider protection coordination
Key Formulas:
Single Phase FLC: I = kVA × 1000 / V
Three Phase FLC: I = kVA × 1000 / (√3 × V)
Inrush Current: I_inrush = K × I_FLC
Real-Life Calculation Example
Given: 500 kVA, 11kV/415V, Three-phase Distribution Transformer
Step 1: Calculate Primary Full Load Current
I_primary = 500 × 1000 / (√3 × 11000) = 26.24 A
Step 2: Calculate Secondary Full Load Current
I_secondary = 500 × 1000 / (√3 × 415) = 695.7 A
Step 3: Apply Inrush Factor (K = 15 for distribution transformer)
Primary Inrush = 15 × 26.24 = 393.6 A
Secondary Inrush = 15 × 695.7 = 10,435.5 A
Step 4: Protection Setting (typically 1.25 × FLC)
Primary Protection = 1.25 × 26.24 = 32.8 A
Secondary Protection = 1.25 × 695.7 = 869.6 A
Result: Peak inrush current is 393.6 A on primary side, lasting 0.1-1.0 seconds
Calculation Formulas
Single Phase FLC:
I = kVA × 1000 / V
Three Phase FLC:
I = kVA × 1000 / (√3 × V)
Inrush Current:
I_inrush = K × I_FLC
Protection Setting:
I_protection = 1.25 × I_FLC
Typical Inrush Factors
- Power Transformers: 8-12 times FLC
- Distribution Transformers: 12-20 times FLC
- Dry Type Transformers: 6-10 times FLC
- Current Transformers: 20-40 times FLC
- Small Transformers (<10kVA): 25-40 times FLC
Factors Affecting Inrush Current
- Transformer core material and design
- Switching angle (worst at voltage zero)
- Residual flux in the core
- System impedance
- Transformer size and rating