Breaker Size Calculator
Size circuit breakers from load amperage using NEC continuous load rules — apply the 125% multiplier for 3-hour loads and select the next standard breaker rating for 120 V and 240 V circuits.
Load Details
A
Live Results
Recommended Breaker Size
20A
Adjusted Load
20.00A
Load Amperage
20A
Voltage
120V
Continuous Load
No
Breaker sizing follows NEC 80% continuous load guidelines. Always verify final design with local electrical code and a licensed electrician.
How to Use This Breaker Size Calculator
- Enter the load amperage. Input the expected circuit current in amperes — use nameplate FLA for motors and appliances, or the calculated load for receptacle and lighting circuits. Common branch-circuit values are 15 A for lighting, 20 A for kitchen and bathroom receptacles, and 30 A or higher for dedicated appliance circuits.
- Select system voltage. Choose 120 V for standard single-pole branch circuits or 240 V for large appliances, HVAC equipment, and double-pole circuits. Voltage does not change the breaker ampere rating, but high loads on 120 V circuits may require larger conductors and panel capacity verification.
- Identify continuous vs non-continuous load. Check Continuous Load when the equipment is expected to run for 3 hours or more at full rated current — examples include water heaters, EV chargers, heat pumps, and commercial lighting. Non-continuous loads use 100% of the load current for breaker sizing.
- Review the adjusted load and recommended breaker. The calculator applies the NEC 125% rule for continuous loads to determine adjusted load amperage, then selects the smallest standard breaker size that meets or exceeds that value. Compare the result against wire gauge ampacity using the Wire Gauge Calculator.
- Verify warnings and local code. Review any high-load or large-breaker warnings before installation. Confirm conductor size, equipment ratings, and panel bus capacity with the current NEC edition and your local AHJ — this tool provides a planning estimate, not a final design.
Formulas & Example
Breaker sizing applies the NEC continuous load multiplier when the load runs 3 hours or more, then rounds up to the next standard breaker rating from the available size list.
Adjusted Load (A) = Load Amperage × (Continuous ? 1.25 : 1.00)
Recommended Breaker = smallest standard size ≥ Adjusted Load
Standard breaker sizes (A):
15, 20, 30, 40, 50, 60, 70, 80, 90, 100Worked Example
A 20 A non-continuous load on a 120 V circuit:
Load Amperage = 20 A
Continuous Load = No
Adjusted Load = 20 × 1.00 = 20 A
Standard sizes ≥ 20 A: 20, 30, 40 …
Recommended Breaker = 20 AIf the same 20 A load were continuous, adjusted load would be 25 A (20 × 1.25), requiring a 30 A breaker — the next standard size above 25 A. Pair this tool with the Wire Gauge (AWG) and Voltage Drop calculators to confirm conductor ampacity and circuit performance.
Frequently Asked Questions
What is the NEC 80% rule?â–¾
The NEC 80% rule (often discussed alongside continuous load requirements) means a circuit breaker should not be loaded to more than 80% of its rating for loads expected to run continuously — effectively requiring 125% sizing for continuous loads. For example, a 20 A continuous load needs a 25 A adjusted load, which rounds up to a 30 A breaker. On a 20 A breaker, the practical continuous limit is 16 A (80% of 20 A). This prevents overheating of conductors and breakers during extended operation.
What is a continuous load?â–¾
A continuous load is defined by the NEC as a load where the maximum current is expected to continue for 3 hours or more. Common examples include electric water heaters, baseboard heaters, heat pump compressors, EV supply equipment, commercial refrigeration, and rack-mounted IT equipment. Lighting in some commercial occupancies may also qualify. Non-continuous loads — such as a kitchen receptacle circuit or a power tool — use 100% of the load current when selecting breaker size.
Can I oversize a breaker?â–¾
You must not install a breaker larger than the ampacity of the conductors it protects — breaker size must match or be smaller than wire ampacity per NEC 240.4(D) and Table 310.16, with limited exceptions for specific equipment. Oversizing a breaker without upsizing wire creates a fire hazard because the conductors can overheat before the breaker trips. Always size the breaker to protect the conductor, not merely to accommodate the load.
Does voltage affect breaker size?â–¾
Breaker ampere rating is independent of system voltage — a 30 A breaker is 30 A whether the circuit is 120 V or 240 V. Voltage affects power delivery (watts = volts × amps) and conductor selection, but the NEC continuous load multiplier applies the same way at both voltages. High-amperage loads on 120 V circuits are less common in residential work and may warrant extra review of panel capacity and wire sizing.
What breaker sizes are standard?â–¾
Standard residential and light commercial circuit breakers are commonly available in 15, 20, 30, 40, 50, 60, 70, 80, 90, and 100 amp ratings. Branch circuits typically use 15 A and 20 A single-pole breakers; large appliances and HVAC equipment use 30 A to 60 A double-pole breakers. Loads above 100 A generally require feeder conductors, sub-panels, or main breaker/service equipment sized by a licensed electrician.
Related Tools
Explore more free calculators. Back to home