Transformer Secondary Overcurrent Protection Calculator

Calculate proper overcurrent protection for transformer secondary circuits

Secondary Overcurrent Protection Calculator

Compliant with NEC Article 450 & IEC 60076 Standards

Protection Results:

Secondary Full Load Current: 0.00 A

Required Protection Rating: 0.00 A

Standard Breaker Size: 0 A

Minimum Wire Size (AWG): -

Transformer Primary Current Calculator
Transformer Secondary Current Calculator

How to Use Transformer Secondary Overcurrent Protection Calculator

Single Phase Transformers

  1. Select "Single Phase" configuration
  2. Enter transformer kVA rating
  3. Input secondary voltage (line-to-line)
  4. Choose protection type (125% standard)
  5. Select conductor material
  6. Click calculate for results

Three Phase Transformers

  1. Select "Three Phase" configuration
  2. Enter transformer kVA rating
  3. Input secondary voltage (line-to-line)
  4. Choose appropriate protection percentage
  5. Select conductor type
  6. Review calculated protection values

Protection Standards

  1. Standard: 125% for normal transformers
  2. High Impedance: 167% for special cases
  3. Custom: User-defined percentage
  4. NEC Article 450 compliance
  5. Consider ambient temperature

Transformer Secondary Protection Theory

Protection Purpose

Secondary overcurrent protection serves to:

  • Protect secondary conductors from overload
  • Prevent transformer damage from secondary faults
  • Ensure safe operation under normal conditions
  • Comply with electrical codes and standards

NEC Requirements

  • Article 450.3 - Overcurrent Protection
  • 125% of full load current (standard)
  • 167% for high impedance transformers
  • Next standard size up rule applies

Key Protection Principles:

• Protection must be sized to carry normal load

• Must trip on overload and fault conditions

• Coordination with primary protection required

• Consider inrush current characteristics

Calculation Formulas

Single Phase Secondary Current:
I = (kVA × 1000) / V

Three Phase Secondary Current:
I = (kVA × 1000) / (√3 × V)

Protection Rating:
Protection = I × (Percentage / 100)

Real-Life Calculation Example

Given: 75 kVA, 3-phase, 480V secondary transformer

Step 1: Calculate secondary full load current

I = (75 × 1000) / (√3 × 480)

I = 75000 / (1.732 × 480) = 75000 / 831.36

I = 90.21 A

Step 2: Apply protection factor (125%)

Protection = 90.21 × 1.25 = 112.76 A

Step 3: Select standard breaker size

Next standard size up = 125 A breaker

Step 4: Determine wire size

Based on 90.21 A and copper conductor = #3 AWG

Final Result: 125A breaker, #3 AWG copper wire

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