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Breaker Size Calculator

By · Updated Jul 2026

Estimate a standard breaker size from load current with optional continuous-load adjustment.

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Your Breaker Estimate

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Suggested Breaker Size
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Entered Load
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Adjusted Amps
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Utilization
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Assumption
What This Result Means
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How to Use
  1. Enter amps - Use the expected current draw in amps.
  2. Pick load type - Continuous loads often use a 125% planning adjustment.
  3. Calculate - The tool selects the next standard breaker size at or above the adjusted amps.
  4. Compare utilization - Utilization helps you see how close the breaker is to the adjusted load.
  5. Use wire sizing too - Breaker sizing and wire sizing work together, not independently.
Quick answer

Breakers protect the wire, so they come in fixed standard sizes - 15, 20, 30, 40, 50, 60 amps and up (NEC 240.6). For a continuous load, one that runs three hours or more, size the breaker at 125 percent of the load and round up: a 40 amp continuous load needs a 50 amp breaker, while a 40 amp non-continuous load takes a 40. One catch from the wire side: 14, 12, and 10 AWG copper are capped at 15, 20, and 30 amp breakers no matter what, so the conductor can hold the breaker below what the load math alone suggests. Enter your load amps above.

Load to Breaker in Two Steps

Loadx 1.25Breaker40 A continuous50 A adjusted50 A next standard sizeContinuous loads are sized at 125 percent, then rounded up
A continuous load is multiplied by 1.25, then rounded up to the next standard breaker size. A 40 amp continuous load becomes 50 amps adjusted, and 50 is already a standard size, so the breaker is 50 amps.

How Breaker Size Is Planned

A breaker is typically chosen to protect the wiring and equipment while allowing normal operating current. This calculator uses your load estimate and common breaker steps to suggest a planning-size breaker.

Some loads are treated as “continuous” in planning (expected to run for long periods), which can push the breaker size up to the next standard rating. Always confirm with local code rules and the equipment nameplate before installing anything.

Example: if your calculated load is close to a standard breaker rating, choosing the next size up may be appropriate when continuous duty or inrush is involved. Pair this with the Wire Size Calculator to make sure the conductor can support the selected breaker.

Standard Breaker Sizes (Common)

Small SizesMid SizesLarger Sizes
15, 20, 2530, 35, 4050, 60, 70
80, 90100, 110, 125150, 175, 200

This list is used by the calculator to select a “next standard size.”

Method Used

This tool applies:

Adjusted amps = A_adj = A_load × 1.25 (if continuous)
Breaker = Choose next standard breaker size ≥ A_adj

Code Notes & Sources

  • Overcurrent devices are made in fixed standard ampere ratings - 15, 20, 25, 30, 35, 40, 45, 50, 60 and up - so the calculator rounds your load up to the next one on the list.NFPA 70 (NEC) 240.6(A) - Standard Ampere Ratings
  • A continuous load must be served by an overcurrent device rated at least 125 percent of that load, which is why the continuous option raises the size. Non-continuous loads use the load current directly.NFPA 70 (NEC) 210.20(A) - Continuous Loads
  • Even when the load math allows a larger breaker, 14, 12, and 10 AWG copper are limited to 15, 20, and 30 amp overcurrent protection, so the conductor can cap the breaker. Check the wire size alongside this result.NFPA 70 (NEC) 240.4(D) - Small Conductors

Next Steps

A breaker is only right if the wire and load agree. Confirm both:

  1. Match the wireThe conductor must carry the breaker rating - and 240.4(D) can cap it.
  2. Total the loadSize a feeder or panel from the sum of its circuits.
  3. Check the runA long circuit may need a bigger wire even with the breaker settled.

Related Calculators

Watts/Amps/Volts CalculatorAmps from watts.Amp to Wire Size CalculatorGauge from amperage.Generator Size CalculatorBreaker for a generator circuit.Box Fill CalculatorBox cubic inches per NEC 314.16.

Always match breaker size to the wire gauge in the circuit. Browse the electrical calculator collection.

FAQ

What counts as a continuous load?
In planning discussions, continuous often means a load expected to run for extended periods. The 125% factor is a common planning adjustment in many contexts.
Does this pick the exact breaker I should buy?
This calculator applies NEC sizing rules and recommends the next standard breaker size up. The result is a strong planning reference, but always verify with a licensed electrician - local codes, panel compatibility, and load type all affect the final breaker selection.
What size breaker do I need for a 240V dryer?
Most residential electric dryers require a 30A double-pole breaker on a dedicated 240V circuit with 10 AWG wire. Heavy-duty or commercial dryers may require 40A. Always check the dryer nameplate for actual amperage - the nameplate rating determines the correct breaker size, not the outlet configuration.
Can I use a 20 amp breaker on 15 amp wiring?
No. The breaker must match the wire gauge - a 20A breaker on 14 AWG wire is a fire hazard because the wire can overheat before the breaker trips. 14 AWG requires a 15A maximum breaker. 12 AWG can use a 15A or 20A breaker. This is one of the most common and dangerous DIY electrical mistakes.
What is the difference between a single-pole and double-pole breaker?
A single-pole breaker controls one hot wire and protects a 120V circuit - used for outlets, lighting, and small appliances. A double-pole breaker controls two hot wires and protects a 240V circuit - used for dryers, ranges, EV chargers, water heaters, and air conditioners. Double-pole breakers take up two slots in the panel. For 240V circuits, breaker ampacity is based on one leg, not the sum.
Updated Jul 2026 · See our Methodology
This calculator uses standard electrical formulas and published reference tables. Local codes and amendments vary. Use for planning estimates only. See our Data Sources and Methodology.