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Breaker Sizing Calculator

Enter watts or amps to see the next standard breaker size, design current after continuous-load sizing, and a typical copper wire pairing. Everything runs locally in your browser.

This calculator provides a general US NEC-based estimate. Final circuit, conductor, breaker and equipment selection must be verified against the applicable NEC edition, local amendments, equipment instructions and by a qualified electrician where required.

What size breaker do I need?

Breaker sizing depends on load current, whether the load is continuous, the next standard breaker rating, and a conductor the breaker can protect. It is not a single number for every appliance.

Example — not universal advice: 5,500 W ÷ 240 V = 22.92 A. For a continuous load, 22.92 A × 1.25 = 28.65 A. The next standard rating is 30 A.

Check the wire size with our Wire Gauge Calculator, add multiple loads with the Circuit Load Calculator, or review a long run in the Voltage Drop Calculator.

By Muhammad Abdullah Rauf · Founder, EverydayTools.proUpdated 2026-09-20· Reviewed by EverydayTools Editorial Team

What Is a Breaker Sizing Calculator?

A breaker sizing calculator converts an electrical load into current, applies continuous-load sizing when it applies, then selects the next standard circuit breaker rating. This page also shows a simplified copper wire pairing so you can see why breaker size and conductor size are linked.

It is a planning tool for common US 120V and 240V branch-circuit questions — not a substitute for the adopted NEC edition, equipment instructions, or a qualified electrician.

Find load current first, apply 125% only where the load is continuous, then choose the next standard breaker and check the conductor.

How to Calculate Breaker Size

Determine the load

Use the nameplate watts or amps, not a guess from a similar appliance.

Convert watts to amps if needed

Single-phase: I = P / V. Three-phase: I = P / (√3 × V × PF).

Determine continuous vs non-continuous

A continuous load is generally one expected to run at maximum current for 3 hours or more.

Apply the applicable factor

Continuous current is multiplied by 1.25. Mixed circuits add non-continuous current to that result.

Select the next standard breaker

Do not round to the nearest 5 A. Use the next listing that is at or above design current.

Verify conductor and equipment requirements

Check wire conditions, voltage drop, listing instructions, and the locally adopted code.

Breaker Size Formula

The calculator keeps full precision internally and only rounds values for display. Special circuits can have additional rules that this general branch-circuit estimate does not apply.

Formula

Watts to amps (single-phase): I = P / V
Single-phase power: I = P / V
Three-phase power: I = P / (√3 × V × PF)
Continuous load: Design current = Load × 1.25
Mixed load: Design current = Non-continuous + (Continuous × 1.25)
Breaker = next standard rating ≥ design current

Assumptions

  • General US branch-circuit planning using watts or nameplate amps.
  • Power factor defaults to 1.00 unless you change it.
  • Standard breaker list from 15 A through 400 A.
  • Copper pairing uses a simplified 60°C ampacity column.

Limitations

  • Not NEC certification and not a substitute for the adopted code edition.
  • Motors, HVAC MCA/MOCP, welders, transformers, generators, and solar equipment are out of scope.
  • Does not apply ambient derating, bundling, aluminum, or voltage-drop design.
  • EV charging and some appliances have equipment-specific rules beyond watts math.

Sources

Real-world breaker sizing examples

Example 1 — 1,800 W at 120 V

Input

1,800 W · 120 V · single-phase · non-continuous

Output

Load 15.00 A · Design 15.00 A · Next standard breaker 15 A · Typical copper 14 AWG

1,800 ÷ 120 = 15 A. This is an example of a general 120 V load treated as non-continuous. A 15 A breaker with 14 AWG copper is the common pairing, but receptacle, kitchen, and laundry circuits often use 20 A / 12 AWG instead.

Example 2 — 5,500 W at 240 V

Input

5,500 W · 240 V · single-phase · continuous example

Output

Load 22.92 A · Design 28.65 A · Next standard breaker 30 A · Typical copper 10 AWG

5,500 ÷ 240 = 22.92 A. For a continuous load, 22.92 × 1.25 = 28.65 A, so the next standard rating is 30 A. This matches a common electric-dryer planning example. Confirm the appliance nameplate before installing anything.

Example 3 — Continuous 16 A load

Input

16 A · continuous

Output

Load 16.00 A · Design 20.00 A · Next standard breaker 20 A · Typical copper 12 AWG

16 × 1.25 = 20 A, so the next standard breaker is 20 A. The 125% factor is a design-current rule for standard breakers, not a claim that every breaker simply “trips at 80%.”

Example 4 — Mixed load

Input

10 A non-continuous + 16 A continuous

Output

Load 26.00 A · Design 30.00 A · Next standard breaker 30 A · Typical copper 10 AWG

Design current = 10 + (16 × 1.25) = 30 A. Mixed-load sizing is more useful than a single yes/no continuous toggle when both load types share one circuit.

Example 5 — Three-phase load

Input

10,000 W · 480 V · three-phase · PF 1.00 · non-continuous

Output

Load ≈ 12.03 A · Design ≈ 12.03 A · Next standard breaker 15 A

I = 10,000 ÷ (√3 × 480 × 1.00) ≈ 12.03 A. Three-phase watts math is not the same as 120/240 V single-phase math. If power factor is below 1.00, current rises and the breaker may step up.

Breaker size from watts, amps, 120V, and 240V

Breaker size from watts

Divide watts by volts to get load current. Apply 125% if the load is continuous, then choose the next standard breaker. A 5,500 W / 240 V continuous example becomes 22.92 A, then 28.65 A, then 30 A.

Breaker size from amps

If the nameplate already lists current, skip the watts conversion. Enter amps, mark continuous or mixed, and size from design current. A 48 A continuous EV charge rate is a 60 A breaker example.

120V breaker sizing

At 120 V, the same wattage produces twice the current of a 240 V circuit. Many general residential branch circuits are 15 A or 20 A. This is context, not a claim that every 120 V load belongs on those ratings.

240V breaker sizing

At 240 V, current is half the 120 V value for the same watts. That is why a 5,500 W dryer-style example is about 23 A at 240 V, not 46 A.

Continuous-load breaker sizing

Continuous loads are commonly sized at 125% of current for standard breakers. That is the same relationship people describe as an 80% continuous loading idea, but the calculator applies the 125% factor to the load rather than saying the breaker “trips at 80%.”

Breaker and wire size

The breaker protects the conductor. A 20 A breaker typically pairs with 12 AWG copper, and a 30 A breaker with 10 AWG copper, under simplified 60°C assumptions. Check the Wire Gauge Calculator when material, temperature, or installation method may change the size.

Common breaker and copper pairings

Simplified 60°C copper reference for common US breakers. Conductor size can change with insulation, terminals, ambient temperature, conductor count, aluminum, voltage drop, and local code.

BreakerTypical copper pairingCommon context
15 A14 AWGGeneral 120 V lighting or receptacle planning
20 A12 AWGCommon 120 V general-purpose circuit
30 A10 AWGMany 240 V dryer-style examples
40 A8 AWGLarger 240 V appliance planning
50 A6 AWGRange or large appliance planning
60 A4 AWGLarger feeder or EV-style continuous examples

This is a simplified reference, not a universal installation table. Never put a larger breaker on a smaller conductor just because a breaker keeps tripping.

Common Breaker Sizing Mistakes

Choosing a breaker from wattage alone

Convert watts to amps at the actual voltage, then apply load type and standard ratings.

Ignoring continuous-load requirements

If the load can run at maximum current for three hours or more, include the 125% factor.

Installing a larger breaker because the existing one trips

A trip usually means overload, a fault, or an undersized circuit. Upsizing the breaker without checking the wire is a fire hazard.

Assuming breaker size automatically determines wire size

The pairing table is a starting point. Insulation, terminals, ambient temperature, and voltage drop can require a larger conductor.

Ignoring voltage

The same wattage at 120 V draws twice the current of 240 V.

Ignoring phase

Three-phase watts math includes √3 and power factor. Do not reuse the single-phase shortcut.

Ignoring conductor temperature and installation conditions

Ampacity depends on insulation rating, terminals, bundling, and environment.

Applying general branch-circuit rules to motors or HVAC

Use the equipment MCA/MOCP or the applicable motor article. This calculator does not replace those rules.

Ignoring voltage drop on long runs

A longer circuit may need a larger conductor even when the breaker rating looks correct. Check the Voltage Drop Calculator.

Treating an online calculator as a substitute for code or equipment rules

Use this page for planning, then verify the adopted NEC edition, local amendments, and listing instructions.

How to Use the Breaker Sizing Calculator

  1. Choose Watts or Amps

    Use watts when you know the load power. Use amps when the nameplate already lists current.

  2. Enter the load

    Type the watts or amps. For mixed loads, enter the continuous and non-continuous portions separately.

  3. Select voltage

    Pick 120 V, 208 V, 240 V, 277 V, 480 V, or enter a custom voltage.

  4. Select phase if it applies

    Single-phase uses I = P / V. Three-phase watts math uses I = P / (√3 × V × PF).

  5. Choose the load type

    Mark the load as non-continuous, continuous, or mixed.

  6. Review calculated current

    Check load current and design current before you look at the breaker.

  7. Review the recommended standard breaker

    The result is the next listing from the standard ampere ratings, not the nearest 5 A.

  8. Review the conductor guidance

    Use the suggested copper pairing as a starting point, then confirm installation conditions.

  9. Read the limitations

    Do not treat the result as a permit package, especially for motors, HVAC, EV charging, or other special equipment.

When this tool isn't the right choice

Motors, HVAC, compressors, or welders

Those loads can require starting-current, MCA, or maximum-overcurrent rules that this general estimate does not apply.

EV charging, generators, solar, or transformers

Special-purpose equipment has additional listing and code requirements. Use the nameplate and the applicable article.

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Frequently Asked Questions

What size breaker do I need?

It depends on load current, whether the load is continuous, the next standard breaker rating, and a conductor the breaker can protect. A 5,500 W / 240 V continuous example is 22.92 A × 1.25 = 28.65 A, so the next standard rating is 30 A. Confirm the nameplate and local rules before you install anything.

How do I calculate breaker size from watts?

Divide watts by volts for single-phase current. For three-phase watts, use I = P / (√3 × V × PF). Multiply continuous current by 1.25, then choose the next standard breaker at or above that design current.

How do I calculate breaker size from amps?

Start with the nameplate current. If the load is continuous, multiply by 1.25. If the circuit is mixed, add non-continuous current to 125% of the continuous current. Then select the next standard breaker.

What is the 125% rule for continuous loads?

A continuous load is generally treated as a load expected to operate at maximum current for three hours or more. For standard breakers, design current is often Load × 1.25. That is related to the idea of not loading a standard breaker above 80% continuously, but it is not the same as saying the breaker trips at 80%.

What breaker size do I need for a 120V load?

Convert the load to amps at 120 V, apply continuous-load sizing if needed, then choose the next standard rating. Many general 120 V branch circuits are 15 A or 20 A, but the correct size is the one that fits your calculated current, wire, and device rating.

What breaker size do I need for a 240V load?

Use the 240 V current, not the 120 V current. The same wattage produces half the amps at 240 V. A 5,500 W example is about 23 A at 240 V and can land on a 30 A breaker when treated as continuous.

Does breaker size determine wire size?

They are linked because the breaker protects the conductor, but breaker selection alone does not fully size the wire. Material, insulation, terminals, ambient temperature, conductor count, installation method, voltage drop, and equipment rules all matter.

What wire size goes with a 20 amp breaker?

The common copper pairing is 12 AWG. That is a simplified 60°C reference. Confirm the wiring method and any conditions that would require a larger conductor. For reverse wire checks, use the Wire Gauge Calculator.

Can I use a larger breaker if my circuit keeps tripping?

Not as a first fix. A larger breaker on the same wire can leave the conductor unprotected. Find the overload, loose connection, fault, or undersized circuit first. Only change the breaker as part of a correctly sized circuit.

Does this calculator work for motors and HVAC?

Only as a rough watts/amps comparison. Motors, compressors, and HVAC equipment often require MCA and maximum overcurrent protection from the nameplate. Do not use this general estimate as the final motor or HVAC design.

Does this calculator work for three-phase power?

Yes for a general watts-to-amps estimate. Three-phase current is I = P / (√3 × V × PF). Enter the power factor when the load is not a simple resistive 1.00 PF case.

Does this calculator account for voltage drop?

No. It sizes a standard breaker from current and load type. For long runs, check the Voltage Drop Calculator. A voltage-drop result can force a larger conductor even when the breaker rating stays the same.

Related electrical calculators

This page sizes a breaker from a known load. Use the sibling tools when the question is wire ampacity, an existing multi-load circuit, or voltage drop.

Related toolUse this tool whenUse related tool when
Wire Gauge CalculatorYou need the next standard breaker from watts or amps.You need deeper AWG or ampacity sizing, including reverse wire-for-breaker checks.
Circuit Load CalculatorYou are sizing a new breaker for a known load.You are adding several loads on an existing breaker.
Voltage Drop CalculatorYou already have a breaker and a typical copper pairing.The run is long enough that voltage drop may require larger wire.

What to do next

Continue the workflow with the right follow-up tool.

Safety, privacy, and limits

Privacy

This tool is free, requires no signup, and runs in your browser. Watts, amps, voltage, and other inputs are not uploaded to EverydayTools servers.

Accuracy

Uses I = P / V or I = P / (√3 × V × PF), mixed-load design current = non-continuous + (continuous × 1.25), the next standard breaker from 15 A to 400 A, and a simplified 60°C copper pairing.

How this tool works

Breaker, current, and wire estimates are calculated locally on your device. There is no account, API, or saved circuit on EverydayTools servers.

Limitations: This calculator provides a general US NEC-based estimate. Final circuit, conductor, breaker and equipment selection must be verified against the applicable NEC edition, local amendments, equipment instructions and by a qualified electrician where required.

Electrical work can involve shock, fire and equipment hazards. This calculator is for estimation and educational use, not a substitute for the applicable electrical code, equipment instructions or professional verification. Special circuits such as motors, HVAC, EV chargers and generators may require additional calculations.

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Reviewed by EverydayTools Editorial Team on 2026-09-20.

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