Breaker Size Calculator

Your details

Enter the load as power (watts) or as a known design current (amps).
Most homes use single-phase. Commercial and industrial equipment often uses three-phase.
Total power consumption of the load (nameplate watts or calculated load).
W
Circuit voltage. Common values: 120V (US household outlet), 240V (dryer/oven), 208V (commercial three-phase), 480V (industrial).
V
Ratio of real power to apparent power. Use 1.0 for pure resistive loads (heaters, incandescents). Motors and fluorescent lighting are typically 0.80-0.95.
NEC specifies different sizing factors by equipment type. General/continuous loads use 125%, motors use 250% of full-load amps, air conditioners 175%, welders 200%.
NEC 210.20 requires that breakers for continuous loads be rated at no less than 125% of the load. If the load is non-continuous (cycles on/off), 100% applies for general loads.
Recommended breaker sizeResidential circuit
20A

Next standard ANSI/UL breaker size at or above the minimum rating

Design current (I)12.5A
Minimum breaker rating15.63A
Safety factor applied125%
Suggested wire gauge (Cu)12 AWG
20 A
Residential<20Large appliance20-60Subpanel / service60-100Main panel / commercial100+

Use a 20-amp breaker with 12 AWG copper wire.

  • Your design current is 12.50 A. After the 125% safety factor (General / Lighting (125%)), the minimum required breaker rating is 15.63 A.
  • The next standard breaker size is 20 A. A 20-amp breaker paired with 12 AWG copper wire satisfies NEC ampacity requirements for this load.

Next stepThis is a reference calculation. Always verify with a licensed electrician and confirm compliance with your local electrical code before any installation.

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