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Breaker Size Calculator

Who it's for: Electricians, apprentices, and estimators.

Breaker Size Calculator helps electricians, apprentices, and estimators. turn load amps (a) or load watts (w), system voltage (v), phase (single/three) into calculated load amps, required amps with factor, suggested breaker size without hiding the assumptions behind the result.

Calculator Inputs

Enter load amps directly, or enter load watts with voltage and phase to convert to amps.

MVP note: this flag is informational only.

Next Steps

  • If both amps and watts are available, compare outputs to catch bad assumptions before field decisions.
  • Continuous loads can materially increase required breaker size when planning circuits.
  • Review conductor selection with the Wire Size Estimator after selecting a breaker target.

Worked Example

Example: enter realistic values for load amps (a) or load watts (w), system voltage (v), review calculated load amps, required amps with factor, then compare the result with field conditions before using it for a material or equipment decision.

How This Calculator Works

Uses direct load amps input when provided, otherwise converts watts to amps using voltage, phase, and power factor assumptions. Applies a configurable continuous-load factor for planning-level breaker demand. Selects the smallest breaker in the standard list that meets or exceeds required amps.

  • Uses direct load amps input when provided, otherwise converts watts to amps using voltage, phase, and power factor assumptions.
  • Applies a configurable continuous-load factor for planning-level breaker demand.
  • Selects the smallest breaker in the standard list that meets or exceeds required amps.

Common Mistakes

  • Entering load amps (a) or load watts (w) in the wrong unit or using an unverified field estimate.
  • Reading calculated load amps without checking the assumptions listed on the page.
  • Skipping manufacturer data, local code, or field verification when the result affects final installation.

FAQ

Yes. If load amps is provided, the calculator uses that value directly and bypasses the watts-to-amps conversion path.

Continuous-load factors increase the required planning amperage so that sustained current assumptions are reflected in the suggested breaker range.

Not in this MVP version. It is informational only; motor and HVAC-specific protective-device rules still require dedicated code checks.

No. Use this as a planning estimate only and verify final breaker and conductor sizing with adopted code and manufacturer documentation.

When to Call a Professional

Use a licensed electrician or engineer when the Breaker Size Calculator result affects final circuit sizing, equipment protection, or code compliance.

Related Calculators

Wire Size Estimator

Estimate conductor size starting points from amperage, run length, and basic installation assumptions.

Water Heater Size Calculator

Estimate recommended storage water heater gallons from occupants, bathrooms, and simultaneous hot-water demand for replacement and planning decisions.

Conclusion

Use the Breaker Size Calculator result to understand calculated load amps, required amps with factor, suggested breaker size, method explanation, planning disclaimer from load amps (a) or load watts (w), system voltage (v), phase (single/three). Keep the input assumptions with the result so another person can reproduce the same estimate.

About This Calculator

About this calculator: Electricians, apprentices, and estimators.

Educational planning estimate. Verify final electrical work against applicable code, equipment data, and qualified professional judgment.

Written by BuildCalcTools Team - Breaker Size Calculator methodology and field-use notes.

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