Breaker Size Calculator
Estimate breaker size from amps, watts, kW, VA, kVA, horsepower, voltage, phase, continuous-load rules, wire checks, or HVAC MCA/MOCP nameplate values.
Calculator is for informational purposes only. Terms and Conditions
Choose the breaker sizing setup
Select what you know first. The calculator shows only the inputs needed for that method.
Enter the known values
The result updates automatically as you change inputs, units, and options.
Breaker sizing visual
Shows load current, adjusted sizing current, breaker rating, and the 80% continuous-load threshold when relevant.
Solution
Live breaker result, selected-breaker check, wire guidance, assumptions, warnings, and solution walkthrough.
Quick checks
- Calculated load current—
- Adjusted sizing current—
- Minimum required breaker—
- Selected breaker check—
- Common wire guidance—
- Voltage drop check—
Source, standards, and assumptions
Educational NEC-style sizing method. This calculator estimates branch-circuit breaker sizing logic, but it does not claim code compliance.
- Standard breaker estimate: non-continuous load at 100% plus continuous load at 125% unless 100%-rated listed assembly mode is selected.
- Standard breaker values are rounded up to common ratings selected in Advanced Options.
- Wire-size guidance is a common educational branch-circuit check, not a substitute for NEC ampacity tables.
- Final sizing depends on conductor ampacity, insulation, terminal temperature ratings, equipment nameplate data, local code, and the authority having jurisdiction.
Show solution steps See formulas, substitutions, breaker rounding, selected-breaker checks, and assumptions
- Enter values to see the full breaker sizing steps and checks.
How to Use the Breaker Size Calculator
The Breaker Size Calculator helps estimate the breaker rating for a circuit based on amps, watts, kilowatts, VA, kVA, horsepower, voltage, phase, and whether the load is continuous. The goal is not just to get a number, but to understand whether the breaker, wire, and equipment limits make sense together.
Most users searching for a breaker size calculator want to know one of three things: what breaker size they need, how many watts a breaker can handle, or what wire size commonly matches a breaker. This guide explains those questions below the calculator so you can understand the result and avoid unsafe assumptions.
Choose what you already know
Select whether you know the load current, watts, kW, VA, kVA, motor horsepower, breaker size, wire size, or HVAC nameplate MCA/MOCP values.
Enter voltage and phase when needed
If the calculator must convert power to current, enter the voltage and choose DC, single-phase AC, or three-phase AC. For three-phase loads, use line-to-line voltage.
Check whether the load is continuous
A continuous load is commonly treated as a load expected to run for 3 hours or more. Standard breaker sizing usually applies a 125% factor to continuous load.
Review the breaker and wire guidance together
The breaker result is only useful if the conductors, terminals, equipment, and installation conditions are suitable for that breaker rating.
Important safety reminder
A breaker protects the circuit conductors from overcurrent. Do not increase breaker size to stop nuisance tripping unless the wire, terminals, equipment, and local code allow the larger breaker.
Breaker Size Formula
The basic breaker sizing method is to calculate the load current, apply the continuous-load adjustment when needed, and then round up to a common standard breaker size. For educational branch-circuit estimates, the most useful formula is:
General breaker sizing formula
The required breaker current is the non-continuous load plus 125% of the continuous load. After calculating this value, round up to the next common breaker rating and verify conductor ampacity.
Direct answer
To calculate breaker size, determine the circuit current, multiply continuous load by 125%, add non-continuous load at 100%, then round up to the next common breaker size. Always verify that the wire and equipment are rated for that breaker.
| Term | Meaning | How It Affects Breaker Size |
|---|---|---|
| Irequired | Required breaker current before rounding | Minimum calculated current the breaker must cover |
| Icontinuous | Continuous load current | Usually multiplied by 1.25 for standard breaker sizing |
| Inoncontinuous | Non-continuous load current | Usually counted at 100% of current |
| Standard breaker size | Next common breaker rating | The calculated current is rounded up to a common breaker size |
Continuous Load and the 125% Rule
A continuous load is commonly understood as a load expected to operate at its maximum current for 3 hours or more. For standard breaker sizing, continuous loads are commonly sized at 125% of the load current.
Continuous load breaker sizing
If a load draws 16 amps continuously, the estimated required breaker current is 16 × 1.25 = 20 amps.
Non-continuous load
Usually counted at 100% of current for this simplified sizing method.
Continuous load
Commonly multiplied by 125% before selecting the breaker.
Mixed load
Add 100% of non-continuous load plus 125% of continuous load.
This is why two circuits with the same running amps may need different breaker sizes. A short-duration load and a continuous load are not always sized the same way.
The 80% Breaker Rule
The 80% breaker rule is another way users describe the same continuous-load idea. For a standard breaker, the maximum continuous load is commonly estimated at 80% of the breaker rating.
Maximum continuous load on a standard breaker
A 20 amp standard breaker is commonly treated as suitable for about 16 amps of continuous load because 20 × 0.80 = 16.
100%-rated breakers are different
A 100%-rated breaker is not just a normal breaker used harder. It must be a listed 100%-rated assembly installed under the required conditions. When in doubt, use the standard breaker assumption.
Breaker Size Chart
This chart shows common breaker sizes and their approximate maximum continuous load using the 80% rule. It also shows estimated continuous watts at 120 V and 240 V for simple resistive single-phase loads.
| Breaker Size | Max Continuous Amps | 120 V Continuous Watts | 240 V Continuous Watts | Typical User Question |
|---|---|---|---|---|
| 15 A | 12 A | 1,440 W | 2,880 W | How many watts can a 15 amp breaker handle? |
| 20 A | 16 A | 1,920 W | 3,840 W | How many watts can a 20 amp breaker handle? |
| 25 A | 20 A | 2,400 W | 4,800 W | What is 80% of a 25 amp breaker? |
| 30 A | 24 A | 2,880 W | 5,760 W | How many watts on a 30 amp breaker? |
| 40 A | 32 A | 3,840 W | 7,680 W | What size breaker for a 32 amp continuous load? |
| 50 A | 40 A | 4,800 W | 9,600 W | How many watts can a 50 amp breaker handle? |
| 60 A | 48 A | 5,760 W | 11,520 W | What size breaker for a 48 amp EV charger? |
These watt values assume simple resistive loads and do not account for power factor, motor starting current, equipment nameplate rules, or special circuit requirements.
Breaker and Wire Size Chart
Breaker size and wire size must be checked together. The chart below shows common simple copper branch-circuit pairings, but it is not a substitute for code ampacity tables, terminal temperature limits, insulation type, ambient temperature corrections, conductor bundling, or equipment instructions.
| Breaker Size | Common Copper Wire Check | Common Use Case | Important Caution |
|---|---|---|---|
| 15 A | 14 AWG copper | Lighting, light receptacle circuits | Do not put a 20 A breaker on 14 AWG copper in typical branch-circuit use. |
| 20 A | 12 AWG copper | General receptacles, kitchen/bath circuits | Confirm receptacle, circuit purpose, and local code requirements. |
| 30 A | 10 AWG copper | Water heaters, dryers, equipment circuits | Do not use 12 AWG copper with a 30 A breaker in typical branch-circuit use. |
| 40 A | 8 AWG copper | Ranges, EV circuits, larger equipment | Verify conductor insulation and terminal temperature ratings. |
| 50 A | 6 AWG copper | Ranges, welders, EV circuits | Equipment instructions may require a different conductor or breaker. |
| 60 A | Verify conditions | Subpanels, EV chargers, larger equipment | Conductor sizing depends heavily on terminals, insulation, and installation conditions. |
Authority reference
For final electrical installation requirements, verify sizing against the applicable adopted code and local authority. The NFPA 70 National Electrical Code is the primary U.S. electrical code reference.
How to Calculate Breaker Size From Watts
Many users know the appliance wattage but not the amperage. For a single-phase load, calculate current from watts, voltage, and power factor. For simple resistive loads such as many heaters, power factor is often treated as 1.0.
Single-phase watts to amps
Once current is calculated, apply the continuous-load rule if the load runs for 3 hours or more.
Current
Continuous-load sizing
Result
Recommended common breaker: 20 A, assuming properly sized conductors and equipment.
How to Calculate Breaker Size From kW
Larger appliances, EV chargers, commercial loads, and equipment nameplates often use kilowatts instead of watts. Convert kW to watts by multiplying by 1000, then calculate current.
kW to amps
For DC loads, omit power factor. For three-phase loads, use the three-phase formula instead.
Current
Continuous-load sizing
Result
Recommended common breaker: 40 A, assuming the conductor and equipment are rated for the circuit.
Single-Phase Breaker Sizing
Single-phase breaker sizing is common for residential and light commercial circuits. For real power in watts, divide power by voltage and power factor to estimate current.
Single-phase AC current
Use this for single-phase AC loads where real power is known. For resistive loads, PF is commonly treated as 1.0.
120 V circuits
Common for general receptacles, lighting, and small plug loads.
240 V circuits
Common for water heaters, dryers, EV chargers, ranges, and larger loads.
Power factor
Important for non-resistive AC loads, motors, and some equipment.
Three-Phase Breaker Sizing
Three-phase breaker sizing is common for commercial and industrial loads. For three-phase real power, use the square-root-of-three relationship and line-to-line voltage.
Three-phase AC current
Use line-to-line voltage for three-phase loads. For a 480/277 V system, that usually means using 480 V for the three-phase calculation.
Current
Continuous-load sizing
Result
Recommended common breaker: 20 A, assuming the conductor and equipment ratings are suitable.
Common Breaker Size Examples
The examples below match the types of questions users often search before using a breaker size calculator. Treat them as educational examples, not installation instructions.
| Load | Current Calculation | Continuous Sizing | Common Breaker Result |
|---|---|---|---|
| 1500 W heater at 120 V | 1500 / 120 = 12.5 A | 12.5 × 1.25 = 15.625 A | 20 A |
| 4500 W water heater at 240 V | 4500 / 240 = 18.75 A | 18.75 × 1.25 = 23.44 A | 25 A or 30 A depending equipment and conductor sizing |
| 7.2 kW EV charger at 240 V | 7200 / 240 = 30 A | 30 × 1.25 = 37.5 A | 40 A |
| 40 A EV charging load | Known load = 40 A | 40 × 1.25 = 50 A | 50 A |
| 48 A EV charging load | Known load = 48 A | 48 × 1.25 = 60 A | 60 A, with conductor and equipment verification |
| 10 kW, 480 V, 3-phase, PF 0.90 | 10000 / (1.732 × 480 × 0.90) = 13.4 A | 13.4 × 1.25 = 16.8 A | 20 A |
Water heater and EV charger caution
Water heaters, EV chargers, HVAC equipment, welders, and motors may have equipment-specific requirements. Always compare the calculated result with the equipment nameplate and installation instructions.
Why Breaker Size Must Match Wire Size
Breakers and conductors are selected together. A breaker that is too large for the wire can allow the conductor to overheat before the breaker trips. This is why a breaker should not be upsized just because a circuit trips.
Unsafe Assumptions
- Increasing breaker size without checking wire size
- Putting a 20 A breaker on typical 14 AWG copper branch-circuit wiring
- Putting a 30 A breaker on typical 12 AWG copper branch-circuit wiring
- Assuming a larger breaker solves nuisance tripping safely
- Ignoring equipment nameplate maximum breaker size
Better Checks
- Confirm the actual conductor size and material
- Verify conductor ampacity with applicable code tables
- Check terminal temperature ratings and insulation type
- Use the equipment MCA/MOCP where provided
- Diagnose overloads instead of simply upsizing the breaker
Simple principle
The load tells you how much current the circuit needs. The wire tells you how much current the circuit can safely carry. The breaker must protect the wire.
When the Calculator Is Not the Final Answer
A breaker size calculator is useful for education and preliminary checks, but some circuit types should be finalized using equipment nameplates, manufacturer instructions, and applicable code requirements.
HVAC equipment
Use the nameplate MCA for conductor ampacity and MOCP for maximum breaker or fuse size.
Motors
Motor circuits may require full-load current tables, overload protection, short-circuit protection, and starting-current considerations.
Welders
Welder circuits may depend on duty cycle, manufacturer data, and special circuit rules.
Solar and inverter circuits
PV and inverter output circuits have equipment-specific and code-specific requirements.
Service or panel sizing
Do not use a branch-circuit breaker calculator to size an entire service, feeder, or panel load.
Existing circuits
If replacing a breaker, identify the existing conductor size, circuit purpose, and equipment requirements before making changes.
Common Breaker Sizing Mistakes
Many breaker sizing mistakes come from treating a circuit as only a math problem. A mathematically correct load calculation can still be unsafe if the wire, terminals, equipment, or installation conditions are not checked.
Common Mistakes
- Using watts-to-amps but forgetting the continuous-load adjustment
- Using 277 V for a 480/277 V three-phase load instead of line-to-line voltage
- Using horsepower-to-amps as the final motor breaker answer
- Ignoring HVAC MCA and MOCP nameplate values
- Assuming a 100%-rated breaker option applies to a standard breaker
- Treating a wire chart as a substitute for final code ampacity verification
Better Practice
- Separate continuous and non-continuous load where possible
- Use line-to-line voltage for three-phase loads
- Use motor nameplate and applicable motor rules for final design
- Use HVAC MCA/MOCP when the equipment provides it
- Assume standard breaker behavior unless a listed 100%-rated assembly is confirmed
- Verify conductor ampacity, terminals, insulation, and local code requirements
Frequently Asked Questions
How do I calculate breaker size?
Calculate the load current, multiply continuous load by 125%, add non-continuous load at 100%, and round up to the next common breaker size. Then verify that the wire and equipment are rated for that breaker.
What is the 125% rule for breaker sizing?
The 125% rule means a continuous load is commonly multiplied by 1.25 before selecting a standard breaker. For example, a 16 amp continuous load commonly requires a 20 amp breaker because 16 × 1.25 = 20.
What is the 80% rule for breakers?
The 80% rule means a standard breaker is commonly treated as suitable for a continuous load up to 80% of its rating. A 20 amp breaker is commonly limited to about 16 amps of continuous load.
How many watts can a 20 amp breaker handle?
At 120 volts, a 20 amp breaker has a theoretical maximum of 2400 watts. For continuous loads using the 80% rule, the practical continuous-load estimate is about 1920 watts.
What size breaker do I need for a 1500 watt heater?
A 1500 watt heater at 120 volts draws 12.5 amps. If treated as a continuous load, 12.5 × 1.25 = 15.625 amps, so the next common breaker size is usually 20 amps with properly sized conductors.
What size breaker do I need for a 4500 watt water heater?
A 4500 watt water heater at 240 volts draws 18.75 amps. If treated as continuous, 18.75 × 1.25 = 23.44 amps. The next common breaker may be 25 or 30 amps depending on the equipment and conductor sizing.
What wire size goes with a 20 amp breaker?
In common copper residential branch-circuit guidance, 12 AWG copper is commonly paired with a 20 amp breaker. Final conductor sizing still depends on code ampacity tables, terminals, insulation, and installation conditions.
Can I use a 20 amp breaker with 14 gauge wire?
In typical branch-circuit use, 14 AWG copper is commonly protected by a 15 amp breaker, not a 20 amp breaker. Do not increase breaker size unless the conductor and installation are rated for it.
Can I use a 30 amp breaker with 12 gauge wire?
In typical branch-circuit use, 12 AWG copper is commonly associated with 20 amp circuits, not 30 amp circuits. A 30 amp breaker commonly points to larger conductor sizing, subject to code and installation requirements.
Does voltage affect breaker size?
Yes. For the same wattage, higher voltage means lower current. Since breakers are sized in amps, voltage affects breaker size when you are converting watts or kW into current.
How do I size a three-phase breaker?
For three-phase real power, calculate current with I = P / (√3 × V × PF), using line-to-line voltage. Then apply continuous-load sizing if needed and round up to a common breaker size.
What are MCA and MOCP on HVAC equipment?
MCA means minimum circuit ampacity and is used for conductor sizing. MOCP means maximum overcurrent protection and is used to limit the maximum breaker or fuse size for the equipment.
Can I install a larger breaker if my breaker keeps tripping?
No, not without verifying the circuit conductors, equipment, and installation are rated for the larger breaker. A tripping breaker may indicate overload, a fault, defective equipment, or a circuit that needs troubleshooting.