Circuit Breaker Size Calculator
Safety Gauge
Standard Breaker Ratings
Derating Impact Analysis
Enter all required parameters and click "Calculate" to see results.
What is Circuit Breaker Sizing Calculator?
Circuit breaker sizing is the process of selecting the correct ampere rating (and frame size) so the breaker protects the circuit, carries the continuous load safely, accounts for ambient temperature derating, conductor grouping, and voltage drop, while meeting NEC Article 210/240/430 or IEC 60947 requirements.
The Circuit Breaker Sizing Calculator (also known as NEC circuit breaker calculator, motor circuit breaker sizing calculator, continuous load breaker sizing tool, voltage drop circuit breaker calculator, three-phase MCCB sizing calculator) handles single-phase and three-phase systems, residential/commercial/industrial loads, continuous vs non-continuous operation, Copper/Aluminum conductors, 60°C/75°C/90°C insulation, conduit/tray/free-air installations, and automatically applies all required derating factors.
This calculator provides special features like relevant visualization (live safety gauge, standard rating ladder, derating impact strip, and compliance status bar), has a dedicated section for comments, analysis and recommendations (thermal margin warnings, safety notes, upsizing suggestions, and installation best practices), provides step-by-step calculation (transparent audit trail of every factor applied), user can download/export results in CSV (complete professional engineering report), and has another special feature of Colorblind view for improved accessibility (high-contrast mode with bold outlines and patterns).
Related Calculators
How to use Circuit Breaker Size Calculator
Purpose Quickly produce code-compliant breaker sizes, verify existing installations, calculate voltage drop for long runs, apply all NEC/IEC derating factors, and ensure safe thermal margins for motors, lighting, HVAC, or general-purpose circuits.
Every input explained
- Supply Voltage (V) – Line-to-line or line-to-neutral voltage
- Phase Type – 1-Phase or 3-Phase
- Load Power (W) – Total connected load in watts
- Power Factor (cosφ) – 0.1 to 1.0 (default 1.0)
- Efficiency (%) – Motor/equipment efficiency (optional)
- Load Type – Continuous or Non-Continuous (NEC 125% factor)
- System Standard – NEC or IEC
- Ambient Temperature (°C) – Installation environment temperature
- Installation Type – Cable, Conduit, Panel
All fields are real-time validated; results appear instantly.
Circuit Breaker Sizing Formula
\(\text{Base Current (1-Phase)} = \frac{P}{V \times PF \times \eta}\)
\(\text{Base Current (3-Phase)} = \frac{P}{\sqrt{3} \times V \times PF \times \eta}\)
\(\text{Design Current (Continuous Load)} = \text{Base Current} \times 1.25\)
\(\text{Adjusted Current} = \frac{\text{Design Current}}{\text{Temperature Derating Factor}}\)
\(\text{Voltage Drop (1-Phase)} = 2 \times I \times L \times R \times \cos\phi\)
\(\text{Voltage Drop (3-Phase)} = \sqrt{3} \times I \times L \times R \times \cos\phi\)
\(%\text{ Voltage Drop} = \frac{\text{Voltage Drop}}{\text{System Voltage}} \times 100\)
\(\text{Thermal Margin (%)} = \frac{\text{Adjusted Current} – \text{Load Current}}{\text{Adjusted Current}} \times 100\)
Where:
- P = load power (W)
- V = system voltage (V)
- PF = power factor
- η = efficiency (decimal)
- L = circuit length (km)
- R = conductor resistance (Ω/km)
- Temperature Derating Factor = from NEC/IEC tables (e.g., 0.94 @ 35°C, 0.82 @ 50°C)
How to Calculate Circuit Breaker Sizing (Step-by-Step)
- Enter Supply Voltage, Phase Type, and Load Power.
- Input Power Factor, Efficiency (if applicable), and Load Type (Continuous/Non-Continuous).
- Select System Standard (NEC or IEC).
- Enter Ambient Temperature and Installation Type.
- Click Calculate.
- Instantly receive: Recommended breaker rating, base/design/adjusted current, temperature & grouping derating applied, voltage drop (if length entered), thermal margin, utilization %, compliance status, step-by-step audit trail, safety recommendations, and live visualizations.
- Export CSV for your project documentation.
Examples
Example 1 – NEC Continuous Motor Circuit Voltage: 480 V 3-Phase, Load: 45 kW, PF=0.85, Efficiency=92%, Continuous Load, Ambient 40°C, Conduit Results: Base Current 71.2 A → Design Current 89.0 A → Adjusted Current 83.7 A (temp derating 0.94) → Recommended Breaker 100 A → Thermal Margin 16% → Voltage Drop (150 m) 2.3% → COMPLIANCE PASS
Example 2 – IEC Lighting Feeder Voltage: 400 V 3-Phase, Load: 28 kW, Non-Continuous, Ambient 35°C, Cable installation Results: Base Current 48.1 A → Adjusted Current 51.2 A (temp derating 0.94) → Recommended MCCB 63 A → Thermal Margin 18.9% → COMPLIANCE PASS
Circuit Breaker Sizing Categories / Normal Range
| Load Type | Continuous Factor | Typical Ambient Derating | Safe Utilization Limit | Common Breaker Sizes (A) |
|---|---|---|---|---|
| General Lighting | 100% | 0.94 @ 35°C | ≤80% | 15, 20, 30 |
| Continuous Motor | 125% | 0.82 @ 50°C | ≤80% | 50, 63, 80, 100 |
| HVAC / Air Conditioning | 125% | 0.91 @ 40°C | ≤75% | 40, 50, 63, 80 |
| Welder / Intermittent | 100% | 1.00 @ 30°C | ≤100% | 100, 125, 160 |
| Industrial Feeder | 125% | 0.87 @ 45°C | ≤80% | 200, 250, 315, 400 |
Limitations
- Uses standard derating tables; does not include rooftop sun exposure, duct banks, or harmonic currents.
- Voltage drop assumes uniform temperature and steady-state load; actual drop may vary with load profile.
- Does not calculate short-circuit withstand, ground-fault protection, or arc-flash energy.
- Aluminum terminations require special lugs and torque values not modeled here.
- Results are for preliminary design; final sizing must be verified by a licensed electrical engineer and local AHJ.
Disclaimer
This Circuit Breaker Sizing Calculator is a decision-support tool based on NEC and IEC tables. It does not replace professional electrical engineering judgment, site-specific calculations, or authority having jurisdiction (AHJ) approval. All installations must comply with the latest edition of the applicable electrical code and be performed by qualified personnel. Incorrect breaker sizing can result in fire, equipment damage, or personal injury. The developers and platform accept no liability for any errors, financial loss, or safety incidents arising from use of this tool.