Skip to content
ENExplore Calculators

Breaker Size

Free Breaker Size calculator with clear step-by-step results.

About Breaker Size

Converts a connected load in watts to circuit current, applies the continuous-load sizing factor that codes require for loads running three hours or more, then rounds up to the next standard breaker rating rather than down. Conductor ampacity must be selected to match or exceed that breaker.

Formulas

Load current

Amps = watts / (volts x power factor)

Required ampacity

Required = load amps x continuous-load factor (typically 125%)

Worked example

With the default values below, Breaker Size returns Next standard breaker size of 20. Change any input above and every figure here is recalculated in your browser.

Inputs used

Connected loadminimum 1 W
3,600 W
Circuit voltageminimum 12 V
240 V
Power factoraccepts 0.3–1
1
Continuous-load sizing factoraccepts 100–200 %
125 %

Results

Next standard breaker size
20 A
Actual load current
15.00 A
Required ampacity after sizing factor
18.75 A
Load as a share of breaker rating
75.0% %

Step by step

  1. 1Values used
  2. 2Load current
  3. 3Required ampacity
  4. 4Next standard breaker size= 20 A
  5. 5Actual load current= 15.00 A
  6. 6Required ampacity after sizing factor= 18.75 A
  7. 7Load as a share of breaker rating= 75.0 %

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Not a substitute for design to code. Overcurrent protection, conductor sizing, derating for ambient temperature and conduit fill, and motor or HVAC loads all have specific code rules. Work must be designed and inspected by a licensed electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Not a substitute for design to code. Overcurrent protection, conductor sizing, derating for ambient temperature and conduit fill, and motor or HVAC loads all have specific code rules. Work must be designed and inspected by a licensed electrician.

Frequently Asked Questions

Why 125% and not 100%?

Thermal-magnetic breakers and conductor insulation are rated for a load that is not continuous. A load drawing full current for three hours or more must be sized at 125% so the assembly does not run at its thermal limit.

Does the breaker protect the appliance?

No - it protects the conductor. The wire gauge must be chosen for the breaker rating, and appliance protection comes from its own internal fusing or overload device.

Related Calculators

Breaker Size – Calculate Next standard breaker size