Breaker Size Calculator

Calculate Breaker Size

Enter the details of your electrical load to determine the appropriate breaker size.

Chart: Required Breaker Size vs. Power

What is a Breaker Size Calculator?

A breaker size calculator is a tool used to determine the minimum amperage rating for a circuit breaker needed to protect an electrical circuit. It takes into account the voltage of the system, the total power (or current) drawn by the loads on the circuit, whether the load is continuous or non-continuous, and if the system is single-phase or three-phase. Using an appropriately sized breaker is crucial for preventing electrical fires and equipment damage by ensuring the circuit is interrupted if the current exceeds a safe level for the wiring.

Electricians, engineers, and even DIY homeowners use a breaker size calculator when planning new circuits or modifying existing ones. It helps ensure compliance with electrical codes (like the NEC – National Electrical Code in the US) which mandate specific safety factors, especially for continuous loads.

Common misconceptions include thinking you can just use any breaker or that a bigger breaker is always better. However, an oversized breaker will not protect the wires from overheating, while an undersized one will trip unnecessarily.

Breaker Size Calculator Formula and Mathematical Explanation

The calculation for the required breaker size involves a few steps:

1. Calculate Full Load Current (I):
   • For Single-Phase: Current (I) = Power (P) / Voltage (V)
   • For Three-Phase: Current (I) = Power (P) / (Voltage (V) * √3), where √3 ≈ 1.732
2. Apply Safety Factor: Electrical codes often require a safety factor, especially for continuous loads (those operating for 3 hours or more). A common safety factor is 1.25 (125%), meaning the circuit and breaker should be rated for 125% of the continuous load current. For non-continuous loads, the factor is typically 1.0 (100%).
   Required Ampacity = Full Load Current * Safety Factor
3. Select Standard Breaker Size: You must then choose the next standard breaker size that is greater than or equal to the Required Ampacity. Breakers come in standard sizes (e.g., 15A, 20A, 25A, 30A, 40A, 50A, etc.).

Variables Used in Breaker Size Calculation

Variable	Meaning	Unit	Typical Range
P	Total Power	Watts (W)	100 – 100,000+
V	System Voltage	Volts (V)	120, 208, 240, 277, 480
I	Full Load Current	Amps (A)	0.1 – 1000+
Safety Factor	Multiplier for continuous loads	Dimensionless	1.0 or 1.25
Required Ampacity	Minimum current capacity	Amps (A)	0.1 – 1000+
√3	Square root of 3 (for three-phase)	Dimensionless	~1.732

Practical Examples (Real-World Use Cases)

Example 1: Kitchen Appliance Circuit

You are installing a dedicated circuit for kitchen appliances expected to draw a total of 1800 Watts at 120V (single-phase). You expect these might run for extended periods (continuous load).

• Voltage (V) = 120V
• Power (P) = 1800W
• Load Type = Continuous
• Phase = Single-Phase

Using the breaker size calculator:

1. Current (I) = 1800W / 120V = 15A
2. Safety Factor = 1.25
3. Required Ampacity = 15A * 1.25 = 18.75A
4. Next Standard Breaker Size: 20A. You would use a 20A breaker and appropriate wiring (e.g., 12 AWG copper).

Example 2: Three-Phase Motor

A small industrial machine uses a three-phase motor rated at 5000 Watts on a 208V system, running continuously.

• Voltage (V) = 208V
• Power (P) = 5000W
• Load Type = Continuous
• Phase = Three-Phase

Using the breaker size calculator:

1. Current (I) = 5000W / (208V * 1.732) ≈ 5000 / 360.256 ≈ 13.88A
2. Safety Factor = 1.25
3. Required Ampacity = 13.88A * 1.25 ≈ 17.35A
4. Next Standard Breaker Size: 20A.

How to Use This Breaker Size Calculator

Using our breaker size calculator is straightforward:

1. Enter Voltage: Input the voltage of your electrical system (e.g., 120, 240).
2. Enter Total Power: Input the total wattage of all devices that will be on the circuit.
3. Select Load Type: Choose ‘Continuous’ if the load will run for 3 hours or more, otherwise ‘Non-Continuous’.
4. Select Phase: Choose ‘Single-Phase’ or ‘Three-Phase’ based on your power supply.
5. View Results: The calculator will instantly show the Calculated Full Load Current, Safety Factor, Required Ampacity, and the recommended standard Breaker Size.

The primary result is the recommended breaker size in Amps. Ensure the wire gauge used for the circuit is also rated for this amperage or higher, according to electrical codes.

Key Factors That Affect Breaker Size Calculator Results

• Voltage: Higher voltage results in lower current for the same power, potentially allowing for a smaller breaker.
• Total Power (Wattage): Higher power draw directly increases the current and thus the required breaker size.
• Load Type (Continuous/Non-Continuous): Continuous loads require a 1.25 safety factor, increasing the required breaker size compared to non-continuous loads of the same power.
• Phase (Single/Three): Three-phase systems are more efficient for the same power, resulting in lower current per phase and potentially a smaller breaker than a single-phase system delivering the same total power.
• Ambient Temperature: While not directly in this basic calculator, high ambient temperatures can de-rate the capacity of wires and breakers, sometimes requiring a larger size. See our Wire Ampacity Guide for more.
• Wire Size (Gauge) and Material: The breaker protects the wire. The wire must be adequately sized for the breaker. A 20A breaker typically requires 12 AWG copper wire, while a 15A breaker uses 14 AWG. Consult our Wire Gauge Chart.
• Voltage Drop: For long wire runs, voltage drop can become significant, potentially requiring larger wires and affecting breaker choice indirectly. Use a Voltage Drop Calculator for long runs.
• Local Electrical Codes: Always adhere to local and national electrical codes (like the NEC), which may have specific requirements beyond this basic calculation. Our NEC Code Finder can help.

Frequently Asked Questions (FAQ)

What happens if I use a breaker that is too small?

An undersized breaker will trip frequently even under normal load conditions, interrupting power unnecessarily. It indicates the circuit is drawing more current than the breaker is rated for, even if the wiring could handle it.

What happens if I use a breaker that is too large?

This is dangerous. A breaker that is too large for the wire gauge will not trip when the wire is overloaded and overheating, creating a fire hazard. The breaker must be sized to protect the wire.

How does the 80% rule relate to the 125% rule for continuous loads?

The 80% rule states that a breaker should not be loaded to more than 80% of its rating for continuous loads. This is the inverse of the 125% rule (1/1.25 = 0.80). So, sizing a breaker at 125% of the continuous load ensures the load is only 80% of the breaker rating.

Can I add more outlets or fixtures to a circuit after using this breaker size calculator?

Only if the total load, including the new additions, does not exceed the capacity calculated (and allowed by the breaker and wire size). It’s best to recalculate with the new total load.

Does this breaker size calculator work for both AC and DC?

The fundamental power formulas (P=VI) are similar, but breakers designed for AC and DC are different in construction and interrupting capacity. This calculator is primarily for AC systems as used in homes and most businesses. Ensure your breaker is rated for the correct voltage and AC/DC application.

What are standard breaker sizes?

Common standard breaker sizes in North America include 15A, 20A, 25A, 30A, 35A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, and larger sizes for main panels or heavy equipment.

Why is there a safety factor for continuous loads?

Continuous loads generate heat in wires and breakers for extended periods. The 1.25 safety factor ensures components don’t overheat under prolonged use, reducing the risk of fire and equipment failure.

Should I use a breaker size calculator for a main breaker?

Yes, but calculating the load for a main breaker involves a load calculation for the entire building, summing up various loads with demand factors according to electrical codes. It’s more complex than a single circuit. See our Whole House Load Calculator.

Related Tools and Internal Resources

• Wire Size Calculator: Determine the correct wire gauge based on amperage and voltage drop.
• Voltage Drop Calculator: Calculate voltage drop over long wire runs.
• Ohm’s Law Calculator: Calculate voltage, current, resistance, and power.
• Power Calculator (Watts/Amps/Volts): Convert between power, voltage, and current.
• Whole House Load Calculator: Estimate the total electrical load for a building to size the main service.
• Electrical Wire Ampacity Charts: View ampacity ratings for different wire gauges and materials.