Breaker Size Calculator
Calculate the correct circuit breaker size for single-phase and three-phase electrical loads using voltage, power, current, power factor, and continuous load requirements.
Breaker Size Calculator
Results shown are estimates. Circuit breaker sizing and installation should strictly comply with the National Electrical Code (NEC), local regulations, and equipment manufacturer standards. Conductor ampacities must match standard overcurrent device ratings.
* Note: Continuous loads normally require breaker sizing at 125% according to common electrical design practices.
How to Use Breaker Size Calculator
Follow these straightforward, step-by-step engineering guidelines to compute the recommended overcurrent protection size for distribution panels and individual machinery circuits:
- Step 1: Select phase configuration. Choose Single Phase (1Ф) for typical residential 120V/230V systems, or Three Phase (3Ф) for commercial and industrial 400V/415V/480V systems.
- Step 2: Enter operating voltage. Input the line-to-line operating voltage in Volts (V) suitable for your electrical grid configuration.
- Step 3: Input electrical load value. Input the total active load of your equipment using the numeric input field.
- Step 4: Choose unit selector. Toggle between entering Watts (W), Kilowatts (kW), or direct Current in Amps (A) using the segmented control.
- Step 5: Apply power factor (PF). Provide the load's power factor (cos φ). If entering load directly in Amps, the power factor and voltage fields will be disabled dynamically as they are not needed.
- Step 6: Toggle continuous load option. Check the continuous load option if the circuit operates for 3 hours or more to automatically apply the standard NEC 125% safety multiplier. Click "Calculate" to view standard breaker ratings.
Properly configuring these inputs ensures standard protective coordination, saving conductors from premature thermal aging and preventing catastrophic arc faults within custom electrical panels.
How to Calculate Breaker Size
Sizing overcurrent protection devices (OCPD) is a critical task in panelboard engineering. Circuit breakers are designed to trip when current exceeds rated thermal-magnetic thresholds, protecting downstream wires from overheating and fire hazards.
Primary Sizing Calculations
1. Single Phase Line Current (from Power):
Where Power (P) is in Watts. If using kW, multiply by 1,000.
2. Three Phase Line Current (from Power):
3. Circuit Breaker Sizing Rule:
Once the required current is computed, round up to the next higher standard commercial rating.
Practical Engineering Sizing Example
Let's compute the recommended circuit breaker rating for a three-phase continuous industrial load with the following specifications:
- Nominal Rated Power: 12 kW (12,000 W)
- System Operating Voltage: 415 V
- System Power Factor (PF): 0.90
- System Phase configuration: Three Phase (3Ф)
- Load Cycle: Continuous Load (NEC 125% Rule)
Step 1: Calculate Winding Full Load Current (FLC):
Apply the three-phase formula to extract system line current:
Step 2: Apply Continuous Load Safety Multiplier:
For continuous loads (operating over 3 hours), we apply the mandatory 1.25 (125%) safety coefficient:
Step 3: Select Standard Commercial Rating:
Standard low-voltage circuit breaker ratings include 15A, 20A, 25A, 32A, 40A, etc. The calculated required minimum is 23.19 A. Standard engineering rules enforce rounding up to the next higher standard rating, which is 25 Amps.
Breaker Size Calculator Chart
Use this convenient technical index to quickly coordinate standard load currents, safety limits, recommended standard breaker ratings, and typical panelboard applications:
| Load Current (Amps) | 125% Continuous Current | Recommended Breaker Rating | Typical Application Example |
|---|---|---|---|
| 10 A | 12.50 A | 15 A | Standard Residential Branch Lighting Circuit |
| 16 A | 20.00 A | 20 A | General Purpose Power Outlets / Small Heaters |
| 24 A | 30.00 A | 32 A | Heavy Duty Heat Pumps / Air Conditioners |
| 38 A | 47.50 A | 50 A | Commercial Kitchen Ranges & Water Boilers |
| 48 A | 60.00 A | 63 A | Sub-Distribution Panels / Large Motor Feeders |
| 64 A | 80.00 A | 80 A | Industrial Welding Stations / Heavy Machinery |
| 82 A | 102.50 A | 100 A | Main Panel Switchboards / High Capacity Feeders |
* All coordination metrics align with international engineering practices. Select copper conductor ratings rated above the nominal breaker rating to maintain full thermal integrity.
Breaker Size Calculator Frequently Asked Questions
To calculate breaker size, determine the full load current of the circuit, and then multiply by 1.25 (125%) if it is a continuous load. This sizing margin is required by electrical codes to prevent thermal fatigue in the switchgear. Finally, round up the calculated value to the nearest standard commercial circuit breaker rating, such as 15A, 20A, 25A, 32A, 40A, or 50A.
Under the National Electrical Code (NEC) and other global engineering standards, circuit breakers protecting continuous loads (operating 3 hours or more) must be oversized by 125% of the continuous current. This safety margin accounts for constant thermal loading, environmental factors, and prevents persistent high-temperature buildups that cause nuisance tripping.
Continuous loads demand breaker ratings evaluated at 125% of the nominal FLC. For instance, a continuous circuit carrying 16 A requires a breaker with an absolute minimum carrying capacity of 20 A (16 A × 1.25). Standard electrical design enforces this rule for panel feeds, lighting circuits, and industrial heaters to maintain optimal long-term reliability.
Yes, a circuit breaker rating must always exceed the nominal load current, typically by 125% for continuous loads. However, the breaker size must never exceed the maximum safe current-carrying capacity (ampacity) of the downstream circuit conductors. Oversizing the breaker beyond the wire's capacity exposes the conductors to high temperatures and combustion risks.
Three-phase electrical systems employ three-pole (or four-pole) gang-operated circuit breakers. These devices ensure that if an overcurrent event or a single-phase fault occurs on any single line, all three lines are disconnected simultaneously. Common three-phase configurations include 415V, 480V, and 600V systems, sized using three-phase equations.
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