Breaker Size Calculator
Determine the appropriate circuit breaker size for your electrical load.
Breaker Size Calculator
Load in Amps: –
Adjusted Load (with safety factor): –
Calculated Required Amps: –
Formula Used (Single Phase): Amps = Watts / Volts. Required Amps = Amps * Safety Factor (1.25 for continuous, 1.0 for non-continuous). The breaker size is the next standard size up from Required Amps.
Formula Used (Three Phase): Amps = Watts / (Volts * √3 * Power Factor). Required Amps = Amps * Safety Factor.
Breaker Size vs. Load (120V & 240V Single Phase)
Common Breaker Sizes and Max Continuous Loads (Single Phase)
| Standard Breaker Size (Amps) | Max Continuous Load at 120V (Watts) | Max Continuous Load at 240V (Watts) |
|---|---|---|
| 15 | 1440 | 2880 |
| 20 | 1920 | 3840 |
| 25 | 2400 | 4800 |
| 30 | 2880 | 5760 |
| 40 | 3840 | 7680 |
| 50 | 4800 | 9600 |
| 60 | 5760 | 11520 |
| 70 | 6720 | 13440 |
| 100 | 9600 | 19200 |
What is a Breaker Size Calculator?
A breaker size calculator is a tool used to determine the correct amperage rating for a circuit breaker needed to protect an electrical circuit. It takes into account the electrical load (in watts or amps), the circuit voltage, whether the load is continuous or non-continuous, and the phase (single or three-phase) to recommend a standard breaker size that will prevent overcurrents while avoiding nuisance tripping. Using the correct breaker size is crucial for electrical safety and preventing fires.
Electricians, engineers, and DIYers working on electrical installations should use a breaker size calculator to ensure compliance with electrical codes like the National Electrical Code (NEC) and to safeguard the wiring and connected devices. A common misconception is that you can simply use a breaker that matches the appliance’s amperage; however, factors like continuous load require a higher rated breaker (typically 125% of the continuous load).
Breaker Size Calculator Formula and Mathematical Explanation
The calculation for the required breaker size involves a few steps:
- Determine the Load in Amps: If the load is given in Watts, convert it to Amps using the formula:
- Single Phase: `Amps = Watts / Volts`
- Three Phase: `Amps = Watts / (Volts * √3 * Power Factor)` (where √3 ≈ 1.732)
If the load is already in Amps, use that value directly.
- Apply the Safety Factor: For continuous loads (those operating for 3 hours or more), the load is multiplied by 125% (a factor of 1.25). For non-continuous loads, the factor is 100% (or 1.0). `Adjusted Load Amps = Load Amps * Safety Factor`
- Determine the Breaker Size: The circuit breaker must have an amperage rating equal to or greater than the Adjusted Load Amps. You select the next standard breaker size that is available above the calculated Adjusted Load Amps. For instance, if you calculate 18 amps for a continuous load, you would use a 20-amp breaker.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Load Value | The power consumed or current drawn by the device(s) | Watts or Amps | 1 – 10000+ |
| Voltage | The electrical potential of the circuit | Volts (V) | 120, 208, 240, 277, 480 |
| Load Type | Continuous (3+ hrs) or Non-Continuous | – | Continuous, Non-Continuous |
| Phase | Electrical system phase | – | Single, Three |
| Power Factor | Ratio of real to apparent power (for 3-phase) | – | 0.7 – 1.0 |
| Safety Factor | Multiplier for continuous loads (1.25) or non-continuous (1.0) | – | 1.0, 1.25 |
| Load Amps | Calculated or given load current | Amps (A) | Calculated |
| Adjusted Load Amps | Load Amps multiplied by Safety Factor | Amps (A) | Calculated |
| Breaker Size | Standard breaker rating selected | Amps (A) | 15, 20, 25, 30… |
Practical Examples (Real-World Use Cases)
Example 1: Kitchen Appliance Circuit
You are installing a dedicated circuit for a powerful kitchen appliance that draws 1800 Watts and will run for less than 3 hours at a time on a 120V single-phase supply.
- Load Value: 1800 Watts
- Voltage: 120V
- Load Type: Non-Continuous
- Phase: Single Phase
Using the breaker size calculator:
Load Amps = 1800W / 120V = 15 Amps.
Safety Factor for non-continuous = 1.0.
Adjusted Load = 15A * 1.0 = 15 Amps.
The next standard breaker size is 15 Amps. However, for a dedicated appliance circuit, especially in a kitchen, it’s often wise to use a 20A breaker with 12-gauge wire to allow for slight overages or future upgrades, provided the appliance’s plug and cord are compatible and local codes allow. If the load was continuous, it would be 15 * 1.25 = 18.75A, requiring a 20A breaker.
Example 2: Electric Water Heater
An electric water heater is rated at 4500 Watts and operates on a 240V single-phase circuit. It is considered a continuous load.
- Load Value: 4500 Watts
- Voltage: 240V
- Load Type: Continuous
- Phase: Single Phase
Using the breaker size calculator:
Load Amps = 4500W / 240V = 18.75 Amps.
Safety Factor for continuous = 1.25.
Adjusted Load = 18.75A * 1.25 = 23.44 Amps.
The next standard breaker size is 25 Amps or even 30 Amps depending on wire size and local regulations. A 30A breaker with 10-gauge wire is commonly used for this load. You can learn more about wiring for water heaters on our site.
How to Use This Breaker Size Calculator
- Enter Load Value: Input the total load connected to the circuit.
- Select Load Unit: Choose whether you entered the load in Watts or Amps.
- Enter Voltage: Input the circuit voltage (e.g., 120, 240).
- Select Load Type: Specify if the load is continuous (runs for 3 or more hours) or non-continuous.
- Select Phase: Choose between Single Phase or Three Phase. If Three Phase, the Power Factor input will appear.
- Enter Power Factor (if Three Phase): Input the power factor (between 0 and 1, typically 0.8-0.95) if you selected Three Phase.
- View Results: The calculator automatically updates, showing Load in Amps, Adjusted Load Amps, and the recommended standard Breaker Size.
The “Required Breaker Amps” is the calculated minimum after the safety factor, and the “Recommended Standard Breaker Size” is the next commercially available size you should use. Always pair the breaker with the appropriate wire gauge for safety.
Key Factors That Affect Breaker Size Calculator Results
- Total Load (Watts or Amps): Higher load requires a larger breaker.
- Voltage: For a given wattage, lower voltage means higher amperage, thus a larger breaker.
- Continuous vs. Non-Continuous Load: Continuous loads require a breaker rated for 125% of the load, increasing the breaker size.
- Phase (Single or Three): Three-phase calculations involve the square root of 3 and power factor, affecting the amperage for a given wattage. More info on three-phase power here.
- Power Factor (for Three Phase): A lower power factor increases the amperage needed for the same wattage in a three-phase system.
- Ambient Temperature: Although not in this basic calculator, very high ambient temperatures can de-rate a breaker’s capacity, potentially requiring a larger size.
- Wire Size: The wire size must be adequate for the breaker size to prevent overheating; a larger breaker needs thicker wire. Check our ampacity charts.
- NEC and Local Codes: Always adhere to the National Electrical Code and local regulations, which may have specific requirements.
Frequently Asked Questions (FAQ)
A: Generally, no. The breaker is sized to protect the wire. Using a breaker much larger than the wire’s ampacity can lead to the wire overheating before the breaker trips, causing a fire hazard. Only go to the next standard size up from the calculated *adjusted* load, ensuring the wire is rated for that breaker size.
A: If the breaker is too small for the load, it will trip frequently (nuisance tripping), interrupting power. While safer than an oversized breaker, it’s inconvenient and indicates an undersized breaker or overloaded circuit.
A: A continuous load is one that operates at its maximum current for three hours or more, like electric heating or lighting in a commercial space. These require a 125% safety factor for the breaker and wire sizing according to the NEC.
A: The load in Watts or Amps is usually listed on the appliance’s nameplate or in its user manual.
A: This basic breaker size calculator does not directly account for voltage drop over long wire runs. For long distances, you may need to use larger wire and potentially adjust breaker size based on voltage drop calculations.
A: Common standard breaker sizes in North America include 15A, 20A, 25A, 30A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, and larger.
A: This is a requirement from the NEC to prevent overheating from sustained loads, as breakers and wires can get hotter when current flows for extended periods. The breaker size calculator incorporates this.
A: This breaker size calculator is primarily designed for AC circuits (single or three-phase). DC circuit calculations are similar (Amps = Watts/Volts), but breaker types and ratings for DC can differ. Consult DC-specific guidelines.
Related Tools and Internal Resources
- Wire Gauge Calculator: Determine the correct wire size based on amperage and distance.
- Water Heater Wiring Guide: Specifics on wiring electric water heaters.
- Understanding Wire Ampacity: Learn about how much current different wire sizes can carry.
- Three-Phase Power Explained: Details about three-phase electrical systems.
- Ampacity and Temperature Correction: How ambient temperature affects wire capacity.
- Voltage Drop Calculator: Calculate voltage drop over long wire runs.