Breaker Size Calculator
Calculate a recommended circuit breaker size from amps, watts, kW, VA, kVA, voltage, phase, and load duration, then check breaker capacity and wire-size assumptions.
Calculator is for informational purposes only. Terms and Conditions
Choose what to calculate
Select whether you want a recommended breaker size or the usable capacity of an existing breaker.
Enter the known values
Use amps, watts, kW, VA, or kVA. Three-phase calculations use line current.
Visual Check
Compare the calculated current against the recommended or selected breaker capacity.
Solution
Live breaker result, capacity checks, warnings, and full solution steps.
Quick checks
- Load current—
Show solution steps See current conversion, sizing factor, standard breaker rounding, assumptions, and checks
- Enter values to see the full calculation steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
This calculator uses standard AC power relationships and a simplified continuous-load sizing method. It does not certify code compliance.
- Final breaker and conductor sizing must be verified against local code, manufacturer data, equipment nameplates, and qualified electrical judgment.
On this page
Calculator Guide
How to Use the Breaker Size Calculator
The Breaker Size Calculator above helps estimate the circuit breaker rating from amps, watts, kW, VA, kVA, horsepower, voltage, phase, continuous-load assumptions, wire checks, or HVAC MCA/MOCP values. To calculate breaker size, convert the load to amps, apply the continuous-load adjustment when needed, then round up to a suitable standard breaker size.
The important safety idea is that the breaker protects the circuit conductors and equipment wiring path. A larger breaker is not automatically better. The final breaker must be allowed by the conductor ampacity, terminal ratings, panel, equipment nameplate, manufacturer instructions, and local electrical code.
Quick Answer
For a standard breaker estimate, size the circuit at \(100\%\) of the non-continuous load plus \(125\%\) of the continuous load. Then round up to the next suitable standard breaker size and verify the conductor and equipment ratings. For example, a \(16 \, A\) continuous load usually needs a \(20 \, A\) breaker because \(16 \times 1.25 = 20 \, A\).
Important safety limit
This calculator estimates breaker size; it does not verify conductor ampacity, conductor derating, interrupting rating, panel compatibility, AFCI/GFCI requirements, local amendments, permit requirements, or approval by the authority having jurisdiction.
Inputs and Outputs Used by the Calculator
The calculator supports common ways people know a load: amps, watts, kilowatts, VA, kVA, horsepower, breaker size, wire size, or HVAC nameplate values. If you are starting from watts, use voltage and phase to convert the load to current first. If you are sizing a whole service or panel instead of a single branch circuit, use an Electrical Load Calculator before choosing individual breaker sizes.
| Type | Value | What It Means | Common Unit |
|---|---|---|---|
| Input | Load value | The known load entered as amps, watts, kW, VA, kVA, or horsepower depending on the selected mode. | A, W, kW, VA, kVA, hp |
| Input | Voltage and phase | Used to convert power into current for DC, single-phase AC, or three-phase AC circuits. | V, DC, 1-phase, 3-phase |
| Input | Power factor and efficiency | Used for AC power and horsepower estimates when current is not already known. | decimal, percent |
| Input | Load duration | Determines whether the continuous-load \(125\%\) sizing factor should be applied. | non-continuous, continuous, mixed |
| Input | Wire size or HVAC MCA/MOCP | Used for practical checks against common conductor pairings or equipment nameplate limits. | AWG, A |
| Output | Recommended breaker size | The calculated sizing current rounded up to a suitable standard breaker rating. | A |
Breaker Size Formula
The main breaker sizing formula is based on current. Non-continuous load is counted at \(100\%\), while continuous load is commonly counted at \(125\%\) for a standard breaker estimate.
General breaker sizing current
After finding \(I_{size}\), round up to the next suitable standard breaker rating and confirm the conductor and equipment can be protected by that rating.
Watts to amps for DC or simple single-phase loads
For a simple resistive load, divide watts by volts to estimate current. If the load is continuous, multiply the resulting current by \(1.25\).
Three-phase real power to current
For balanced three-phase AC loads, use line-to-line voltage and include power factor when converting real power in watts to current.
Why the 80% rule appears
For standard breakers, sizing a continuous load at \(125\%\) is equivalent to limiting the continuous load to \(80\%\) of the breaker rating because \(1 \div 1.25 = 0.80\). A \(20 \, A\) breaker therefore commonly corresponds to a \(16 \, A\) maximum continuous load in simplified estimating.
What the Variables Mean
Breaker sizing becomes much easier when each variable is understood before entering values. The key is to get to load current in amps, then decide whether any part of that load should be treated as continuous.
\(I_{size}\)
The adjusted sizing current used to choose the breaker. It includes the continuous-load adjustment and any extra design margin selected in the calculator.
\(I_{continuous}\)
The portion of the load expected to run at maximum current for 3 hours or more. EV chargers, heaters, and some lighting loads are common examples.
\(I_{noncontinuous}\)
The portion of the load not expected to run continuously at full current. It is commonly counted at \(100\%\) in the simplified sizing formula.
\(P\)
Real power. Use watts in the formula. Convert kilowatts to watts with \(1 \, kW = 1000 \, W\).
\(V\)
Voltage at the load. Use line-to-line voltage for three-phase calculations and the actual circuit voltage for DC or single-phase calculations.
\(PF\)
Power factor. Use \(1.0\) for simple resistive loads, or the nameplate value when available for AC equipment.
How to Use the Calculator
Use the calculator by selecting the input mode that matches what you know, entering voltage and phase when needed, choosing the load duration, and then checking the recommended breaker against the wire and equipment limits.
Select what you know
Choose amps, watts, kW, VA, kVA, horsepower, breaker size, wire check, or HVAC MCA/MOCP mode. If you already know the load current, amps mode is the cleanest option.
Enter voltage, phase, and power factor
For power-based inputs, voltage and phase determine the current. Three-phase loads use a different formula than single-phase loads, so do not leave the phase selection on the wrong setting.
Choose continuous or non-continuous load
If the load may run for 3 hours or more, use the continuous-load setting. When you are unsure and the load is safety-relevant, the conservative choice is usually to treat it as continuous until verified.
Check breaker, wire, and equipment together
Use the output as a sizing estimate, then verify wire size with a Cable Sizing Calculator, check long-run voltage drop if needed, and follow the equipment nameplate.
How to Interpret the Result
The result is the breaker rating that meets the simplified current requirement after the load assumptions are applied. A valid result still needs practical review because standard breaker sizes, conductor ampacity, terminal ratings, and equipment requirements can control the final answer.
What to do with the result
Use the recommended breaker as a starting point for a circuit check. Confirm that the wire and equipment are rated for that breaker before using it in a design or installation.
What changes the result most?
Voltage, phase, and continuous-load setting usually have the largest effect. Doubling voltage often cuts current roughly in half for the same wattage.
Sanity check
A standard breaker serving a continuous load should usually have continuous load no higher than about \(80\%\) of its rating unless a listed 100%-rated assembly is being used.
A suspicious result to investigate
If the calculator recommends a breaker larger than the common breaker for the selected wire size, do not simply install the larger breaker. That result means you need to revisit the conductor size, wiring method, equipment nameplate, and applicable code requirements.
Input Checklist Before You Trust the Answer
Most wrong breaker sizes come from entering the wrong load unit, using the wrong voltage, skipping the continuous-load adjustment, or treating equipment nameplate values like ordinary load current.
Confirm the load unit
Do not enter \(7.2\) as watts when you mean \(7.2 \, kW\). Kilowatts must be converted to watts if the mode expects watts.
Use the correct voltage
A \(240 \, V\) load draws half the current of a \(120 \, V\) load at the same wattage. For three-phase, use line-to-line voltage.
Check load duration
EV charging, heating, and commercial lighting are common cases where the continuous-load setting can change the recommended breaker.
Verify nameplate data
For HVAC equipment, use MCA and MOCP from the nameplate instead of guessing from running amps alone.
Worked Example: Breaker Size for a 7.2 kW EV Charger
This example follows a common user search: sizing a breaker from watts or kilowatts. EV charging is commonly treated as a continuous load, so the \(125\%\) factor matters.
Step 1: Convert watts to amps
Step 2: Apply continuous-load sizing
Final answer
The next common breaker size is typically \(40 \, A\), assuming the conductor, terminals, charger instructions, and local code allow it. The result is reasonable because \(40 \, A \times 0.80 = 32 \, A\), which is above the \(30 \, A\) continuous charging current.
What the Breaker Sizing Logic Represents
Breaker sizing is best visualized as a sequence rather than a picture: first convert the load to amps, then adjust for continuous operation, then round to a breaker size, then check the wire and equipment. Skipping the last step is the most dangerous mistake.
Calculation path: known load \( \rightarrow \) load current \( \rightarrow \) continuous-load adjustment \( \rightarrow \) standard breaker size \( \rightarrow \) wire and nameplate verification.
For example, a \(30 \, A\) continuous load does not stay at a \(30 \, A\) breaker in a standard estimate. The adjusted sizing current is \(37.5 \, A\), so the next common breaker is typically \(40 \, A\), provided the rest of the circuit is suitable.
Common Breaker Size Examples
These examples are useful reference checks for common search cases. They are educational estimates only. Final breaker size still depends on conductor ampacity, equipment instructions, available breaker ratings, local code, and inspection requirements.
| Load | Calculation | Continuous Check | Typical Educational Result |
|---|---|---|---|
| 1500 W heater at 120 V | \(1500 \div 120 = 12.5 \, A\) | \(12.5 \times 1.25 = 15.625 \, A\) | Often \(20 \, A\), if wire and equipment allow it |
| 4500 W water heater at 240 V | \(4500 \div 240 = 18.75 \, A\) | \(18.75 \times 1.25 = 23.44 \, A\) | Often \(25\) to \(30 \, A\), depending on equipment, wire, and code |
| 7.2 kW EV charger at 240 V | \(7200 \div 240 = 30 \, A\) | \(30 \times 1.25 = 37.5 \, A\) | Often \(40 \, A\), if the circuit is rated for it |
| 9.6 kW EV charger at 240 V | \(9600 \div 240 = 40 \, A\) | \(40 \times 1.25 = 50 \, A\) | Often \(50 \, A\), if the circuit is rated for it |
| 10 kW at 480 V, 3-phase, PF 0.90 | \(10000 \div (1.732 \times 480 \times 0.90) = 13.4 \, A\) | Apply \(125\%\) only if continuous | Often \(15\) to \(20 \, A\), depending on duration and equipment |
Do not use examples as final installation approval
Example breaker sizes help check whether the calculator result is plausible. They do not replace the NEC, local amendments, conductor ampacity tables, manufacturer instructions, or approval by the authority having jurisdiction.
Design Notes and Practical Ranges
A breaker that is too small may nuisance trip or fail to support the load. A breaker that is too large may fail to protect the conductor properly. The practical target is not the biggest breaker possible; it is the breaker that coordinates with the load, wire, panel, and equipment.
Standard breaker assumption
Under the standard continuous-load sizing assumption, a \(30 \, A\) breaker is commonly used for up to about \(24 \, A\) of continuous load, unless a listed 100%-rated assembly or a more specific code/equipment rule applies.
100%-rated breakers
Only use a 100%-rated assumption when the breaker and assembly are specifically listed and installed for that use. Do not assume a normal breaker can carry full rating continuously.
Long wire runs
Breaker size does not solve voltage drop. If the run is long, check the conductor run separately with a Voltage Drop Calculator.
Units and Conversions
Breaker size is selected in amps, so every power-based input must eventually be converted to current. The most common unit trap is confusing watts, kilowatts, VA, and kVA.
Power unit conversions
Apparent power to current
VA and kVA are often easier for AC circuit sizing because apparent power already includes the voltage-current relationship. For real power in watts, power factor may be needed.
Hidden unit trap
A \(4500 \, W\) load at \(240 \, V\) draws \(18.75 \, A\), but a \(4500 \, W\) load at \(120 \, V\) draws \(37.5 \, A\). Same wattage, very different breaker sizing current.
Breaker Size from Amps vs Watts vs HVAC MCA/MOCP
The best calculation method depends on what information you have. Use amps when the load current is known, watts when you know power and voltage, and HVAC MCA/MOCP when the equipment nameplate provides those values.
Use the direct amp method when…
- The nameplate gives load current in amps.
- You are checking a known circuit load.
- You already separated continuous and non-continuous current.
Use the power method when…
- The equipment rating is listed in watts, kW, VA, or kVA.
- You know the correct voltage and phase.
- You can estimate or enter power factor where needed.
HVAC is different
For HVAC equipment, MCA usually guides the minimum conductor ampacity, while MOCP or MOP usually limits the maximum breaker or fuse. For example, if an HVAC nameplate lists MCA \(= 24 \, A\) and MOCP \(= 40 \, A\), the conductor must be sized for at least the MCA value, while the breaker or fuse must generally not exceed the MOCP value unless manufacturer instructions and code allow otherwise.
Common Breaker Sizing Mistakes
The most common mistakes are not math mistakes. They are assumption mistakes: wrong voltage, wrong phase, ignored continuous load, oversized breaker for the wire, or misunderstood nameplate values.
Do
- Convert the load to amps before choosing a breaker.
- Apply \(125\%\) to continuous loads when using the standard-breaker assumption.
- Check the breaker against the conductor ampacity and equipment nameplate.
- Use power factor for AC loads when real power in watts is converted to current.
Don’t
- Do not upsize a breaker just because it trips.
- Do not put a breaker on a wire size that cannot be protected by that breaker.
- Do not confuse MCA with breaker size or MOCP with running current.
- Do not ignore voltage drop on long runs or low-voltage circuits.
Troubleshooting Unrealistic Breaker Results
If the breaker result looks too high, too low, or unsafe, pause before changing the circuit. Recheck the input mode, units, voltage, phase, and continuous-load selection first.
Result looks too high
Check whether kW was entered as W, whether voltage was set too low, whether single-phase was selected instead of three-phase, or whether unnecessary design margin was added.
Result looks too low
Check whether a continuous load was entered as non-continuous, whether power factor was ignored, or whether the wrong voltage was selected.
Breaker fails wire check
The load may require a larger conductor, a different circuit design, or a more detailed code review. Do not solve this by using the larger breaker on the existing wire.
HVAC result seems strange
HVAC nameplates can allow breaker or fuse values that do not look like simple wire-size pairings. Follow MCA/MOCP and manufacturer instructions instead of guessing.
Assumptions and Limitations
This calculator is an educational sizing tool. It estimates breaker size from simplified electrical relationships, but it does not verify complete code compliance, manufacturer requirements, available fault current, interrupting rating, selective coordination, conductor derating, panel compatibility, or permit requirements.
Standard breaker assumption
The common continuous-load method assumes standard breaker behavior unless a listed 100%-rated breaker assembly is specifically selected and allowed.
Conductor limitations
Wire ampacity depends on more than gauge. Insulation, terminals, raceway fill, ambient temperature, and conductor material all matter.
Equipment nameplate control
Equipment instructions can control the allowable breaker size. HVAC, motors, welders, and transformers often have special rules.
Professional review
Use qualified electrical review for installations, code-regulated work, safety-critical circuits, or any case where failure could create fire, shock, or equipment hazards.
Standards and source note
Final electrical installations should be checked against the applicable edition of NFPA 70, the National Electrical Code, local amendments, manufacturer instructions, and the authority having jurisdiction. For equipment-specific circuits, nameplate values and listing instructions may control the final breaker and conductor selection.
Key Terms
These terms help connect the calculator inputs, formulas, and safety checks.
Circuit breaker
An overcurrent protective device intended to open a circuit when current exceeds allowed limits under defined conditions.
Continuous load
A load expected to run at maximum current for 3 hours or more. These loads commonly require a \(125\%\) sizing adjustment in standard estimates.
Conductor ampacity
The current a conductor can carry under specified conditions without exceeding its temperature rating.
MCA
Minimum Circuit Ampacity. HVAC nameplate value used to help determine minimum conductor ampacity.
MOCP or MOP
Maximum Overcurrent Protection. The maximum breaker or fuse size allowed by the equipment nameplate or manufacturer instructions.
Power factor
The ratio of real power to apparent power in an AC circuit. It affects current when converting watts to amps.
Breaker Size Calculator FAQ
How do I calculate breaker size?
Calculate the load current, count non-continuous load at \(100\%\), count continuous load at \(125\%\), then round up to the next suitable standard breaker size. Always verify the wire, terminals, equipment nameplate, and local code.
What is the 80 percent rule for breakers?
For a standard breaker serving a continuous load, the maximum continuous load is commonly treated as \(80\%\) of the breaker rating. This is the same idea as multiplying the continuous load by \(125\%\) when sizing the breaker.
Is breaker size based on watts or amps?
Breaker size is based on current in amps. If you know watts, convert watts to amps using voltage, phase, and power factor before selecting a breaker.
How many watts can a 20 amp breaker handle?
At \(120 \, V\), a \(20 \, A\) breaker can supply up to \(2400 \, W\) for a non-continuous resistive load, but a standard continuous-load estimate is \(20 \times 120 \times 0.80 = 1920 \, W\). At \(240 \, V\), those values double to \(4800 \, W\) non-continuous or \(3840 \, W\) continuous, assuming the circuit and equipment allow it.
Can I use a bigger breaker to stop tripping?
Do not install a larger breaker just to stop tripping. The breaker must be coordinated with the conductor ampacity, equipment rating, terminal limits, and applicable code. A tripping breaker may indicate overload, short circuit, ground fault, damaged equipment, or an undersized circuit.
What is the difference between MCA and MOCP?
MCA is the minimum circuit ampacity used for conductor sizing. MOCP or MOP is the maximum overcurrent protection value, often the largest breaker or fuse allowed by the equipment nameplate.