Generator Breaker Size Calculator Guide
A generator circuit breaker size calculator helps you select the correct breaker for safe operation. It ensures proper protection against overloads and short circuits. Use this guide to calculate accurate breaker size using practical methods and standards.
Breaker Size Calculator
How to Use Generator Breaker Size Calculator
- Enter Generator Rating: Input generator capacity in kW or kVA.
- Select Voltage and Phase: Choose single-phase or three-phase system (Example: 230V or 400V).
- Enter Power Factor: Typical value: 0.8 for generators.
- Apply Derating Factors: Consider ambient temperature, cable grouping, and altitude if applicable.
- Calculate Current: Tool calculates full-load current automatically.
- Apply Safety Margin: Multiply by 125% for continuous load.
- Select Breaker Size: Choose nearest higher standard breaker rating.
How to Calculate Generator Breaker Size
To calculate the correct breaker size, follow these steps:
Generator = 50 kW
Voltage = 400V (three-phase)
Power Factor = 0.8
Step 1: Calculate Full Load Current
I = (50 × 1000) / (√3 × 400 × 0.8)
I ≈ 90 A
Step 2: Apply Continuous Load Factor
Breaker Current = 90 × 1.25 = 112.5 A
Step 3: Apply Derating (Example: 40°C ambient = 0.9 factor)
Adjusted Current = 112.5 / 0.9 = 125 A
Step 4: Select Standard Breaker
Final Breaker Size = 125 A
IEC / IEE / NEC Standards Check
IEC (International Electrotechnical Commission)
- Follow IEC 60947 for low-voltage circuit breakers
- Ensure breaker rated current ≥ design current
- Verify breaking capacity (Icu) ≥ fault level
- Apply correction factors for temperature and installation
IEE (BS 7671 Wiring Regulations)
- Ensure breaker rating (In) ≥ load current (Ib)
- Ensure cable capacity (Iz) ≥ breaker rating
- Apply correction factors (Ca, Cg, Ci)
- Check disconnection times for safety
NEC (NFPA 70)
- Use Article 445 for generators
- Apply 125% rule for continuous loads
- Size breaker not less than generator full-load current
- Verify interrupting rating (AIC) meets fault current
Derating Factors for Accurate Sizing
Always apply derating to avoid overheating and failure:
- Ambient Temperature: Above 30°C reduces breaker capacity (Example: 40°C → multiply by 0.9)
- Cable Grouping: Multiple cables reduce heat dissipation (Apply grouping factor 0.7–0.9)
- Altitude: Above 1000m reduces cooling (Apply correction factor 0.9–0.95)
- Continuous Load: Always apply 125% multiplier
Generator Breaker Size Conversion Chart
| Generator Size (kW) | Voltage | Phase | Base Current (A) | Final Breaker (A) |
|---|---|---|---|---|
| 10 kW | 230V | 1Ø | 54 A | 63 A |
| 20 kW | 400V | 3Ø | 36 A | 50 A |
| 30 kW | 400V | 3Ø | 54 A | 63 A |
| 50 kW | 400V | 3Ø | 90 A | 125 A |
| 75 kW | 400V | 3Ø | 135 A | 160 A |
| 100 kW | 400V | 3Ø | 180 A | 200 A |
Frequently Asked Questions (FAQs)
It calculates the correct breaker size using generator rating, voltage, and safety factors.
It ensures safe operation under continuous load conditions.
It adjusts current capacity based on temperature, grouping, and altitude.
Use IEC, IEE (BS 7671), or NEC depending on your region.
It is the maximum fault current the breaker can safely interrupt.
No. Skipping derating may cause overheating and equipment failure.
Yes. Higher temperature reduces breaker performance.
It may not trip during faults, risking equipment damage.
It will trip frequently and disrupt power supply.
kVA simplifies calculation since it avoids power factor adjustment.