Circuit Breaker Sizing Calculator
Determine the correct circuit breaker amperage for any electrical circuit using NEC load classification rules. Use it when designing new circuits or upgrading panels for continuous, non-continuous, and motor loads.
About this calculator
The National Electrical Code (NEC) requires that breakers be sized to handle 125% of continuous loads (loads energized for 3+ hours) plus 100% of non-continuous loads, and 125% of motor loads due to their high running current demands. The sizing formula is: Breaker Size = ceil((continuous_load × 1.25 + non_continuous_load + motor_load × 1.25) / 5) × 5. The result is rounded up to the nearest standard breaker increment of 5 amps, since breakers are manufactured in standard sizes (15, 20, 25, 30 A, etc.). Motors require the 125% multiplier because they draw sustained current throughout operation. Selecting an undersized breaker causes nuisance tripping, while an oversized breaker fails to protect the wiring from overheating and fire.
How to use
Suppose a workshop circuit has a 16-amp continuous load (lighting and tools running all day), a 10-amp non-continuous load (occasional equipment), and a 5-amp motor load. Apply the formula: (16 × 1.25) + 10 + (5 × 1.25) = 20 + 10 + 6.25 = 36.25 amps. Divide by 5: 36.25 / 5 = 7.25. Ceiling to 8, then multiply by 5 = 40 amps. The calculator recommends a 40-amp breaker. Verify this against the ampacity of your selected wire gauge — the wire must be rated for at least 40 amps.
Frequently asked questions
Why do continuous loads require a 125% multiplier when sizing a circuit breaker?
Continuous loads — defined by the NEC as loads expected to remain energized for three hours or more — generate sustained heat in the wiring and breaker. Over time, this heat degrades insulation and can trip a breaker that is running near its rated limit. The 125% safety factor ensures the breaker and conductors operate at no more than 80% of their rated capacity during continuous use, providing a thermal margin that extends equipment life and prevents nuisance trips or hazardous overheating.
What is the difference between a continuous and non-continuous electrical load?
A continuous load is one that is expected to be on for three or more hours at a stretch — examples include HVAC systems, commercial lighting, and server room equipment. A non-continuous load operates for shorter intervals, such as a microwave, toaster, or power tool. The distinction matters for breaker and conductor sizing because sustained current generates more cumulative heat than brief spikes. Most residential circuits contain a mix of both types, so the NEC formula applies each multiplier only to the appropriate portion of the total load.
How do I determine the motor load amps to enter into the circuit breaker sizing calculator?
Motor load amperage is typically found on the motor's nameplate as 'Full Load Amps' (FLA) or 'Rated Current.' For three-phase motors, NEC Table 430.250 lists standard FLA values by horsepower and voltage. If you have the motor's horsepower rating, you can estimate FLA: for a 1 HP, 120 V single-phase motor, FLA is approximately 16 amps. Always use the nameplate FLA rather than a calculated estimate when available, since actual motor efficiency and power factor affect the real current draw.