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Breaker Size Calculator

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

This Breaker Size Calculator page is designed as a practical planning resource for electricians, apprentices, and estimators.

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

  • Confirm load assumptions and conductor lengths before selecting hardware.
  • Compare drop and ampacity impacts together for balanced decisions.
  • Validate final circuit design against NEC/local requirements.

Worked Example

Worked example: use your real project inputs, run at least one conservative scenario, and compare outputs against one related calculator before finalizing direction.

Detailed Explanation

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

  • Checking only one electrical constraint instead of ampacity + voltage-drop together.
  • Using approximate run lengths that miss conduit routing reality.
  • Skipping final code checks before procurement or install.

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 final circuit, safety, and compliance decisions are required.

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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 this estimate to improve planning speed and communication, then validate final values against code and manufacturer data.

About This Calculator

About this calculator: Electricians, apprentices, and estimators.

Educational and planning use only. Verify final values with licensed professionals.

Written by BuildCalcTools Team - Electrical planning guidance.