Complete Guide to Circuit Breaker Sizing
The Breaker Size Selector picks standard amp rating per NEC 240.6. Our free tool prevents nuisance trips and ensures safety.
What is Breaker Sizing?
Breaker ≥ 125% continuous, FLA × factor for motors. Must be standard size. Our Breaker Size Selector automates NEC rules.
Why Size Correctly?
Avoids:
- Nuisance Trips: Undersized.
- Overheating: Oversized.
- Code Violation: Inspector rejection.
- Fire Risk: Wrong protection.
Use with Wire Size and Transformer Size.
NEC Rules
| Load | Rule | Max % FLA |
|---|---|---|
| Non-Continuous | 100% | — |
| Continuous | 125% | — |
| Squirrel Cage | Table 430.52 | 250% |
| Wound Rotor | Table 430.52 | 150% |
| Synchronous | Table 430.52 | 200% |
Standard Breaker Sizes (A)
NEC 240.6: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000.
Calculation Steps
- Determine Load: Continuous or motor.
- Apply Factor: 125% or Table 430.52.
- Round Up: Next standard size.
- Check Wire: Breaker ≤ wire ampacity.
Example: 50 HP Motor
480V 3-phase, FLA = 65A, Squirrel Cage.
- 250% × 65 = 162.5A
- Next standard = 175A
- Wire: ≥ 175A ampacity
Motor Starting
Inrush: 6–8× FLA. Inverse-time breaker allows startup.
| Motor HP | FLA @ 480V | Max Breaker |
|---|---|---|
| 10 | 14 | 35A |
| 25 | 34 | 80A |
| 50 | 65 | 175A |
| 100 | 124 | 300A |
Continuous vs Non-Continuous
- Continuous: Lighting, HVAC ≥3h → 125%.
- Non-Continuous: Receptacles, tools → 100%.
Practical Applications
For:
- Factories: Motor circuits.
- Offices: Lighting panels.
- Retail: Receptacle circuits.
- EV Charging: Dedicated breakers.
Tips
- Round Up: Never down.
- Motor Nameplate: Use FLA.
- Coordination: Upstream > downstream.
- Label: Per NEC 408.4.
Common Mistakes
- No 125%: Continuous loads.
- Wrong Table: Motor type.
- Non-Standard: 42A breaker.
- Ignore Inrush: Nuisance trips.
Advanced
- Selective Coordination: Time-current curves.
- Arc Flash: Lower let-through.
- Ground Fault: GFP breakers.
- Smart Breakers: Remote monitoring.
Conclusion
Proper breaker sizing ensures safety and reliability. Our Breaker Size Selector follows NEC 240.6 and 430.52. Pair with Wire Size and Construction tools. Protect circuits with 1000 Calculators.