Circuit Breaker Calculator
The National Electrical Code (NEC) Section 210.20(A) requires that a branch circuit overcurrent device (breaker) be rated at not less than 125% of the continuous load plus 100% of the non-continuous load. This rule exists because circuit breakers are typically listed for continuous operation at only 80% of their rated amperage, ensuring the breaker does not run at or near its trip point during normal long-duration use. Enter the load current in amperes and whether the load is continuous (running for three hours or more) or non-continuous. The calculator shows the minimum required breaker amperage, the next standard breaker size per industry convention, and the applicable NEC note. Standard breaker sizes follow the ranges listed in NEC Table 240.6(A). All electrical work must be performed by or supervised by a licensed electrician and inspected as required by your local jurisdiction. This calculator is for educational reference only.
NEC circuit breaker sizing formula
Continuous load: Minimum breaker (A) = Load current (A) x 1.25
Non-continuous load: Minimum breaker (A) = Load current (A) x 1.00
Then round up to the next standard breaker size from NEC Table 240.6(A)
Worked example
A 16 A continuous load (e.g. commercial lighting on for 8 hours a day):
- Continuous load multiplier = 1.25
- Minimum breaker = 16 x 1.25 = 20 A
- 20 A is a standard size, so the recommended breaker is 20 A
- NEC reference: NFPA 70 Section 210.20(A)
Standard NEC breaker sizes (NEC Table 240.6(A))
15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A. For loads requiring a breaker above 800 A, NEC allows the next higher standard size above the calculated minimum.
Frequently asked questions
What is a continuous load?
Under NEC Article 100, a continuous load is one where the maximum current is expected to continue for three hours or more. Examples include lighting in commercial buildings left on all day, industrial process equipment running continuously, and HVAC equipment in most applications. Residential loads like refrigerators and water heaters may also qualify. A load that operates for less than three hours is non-continuous.
Why must a continuous load breaker be rated at 125% of the load current?
Circuit breakers are rated for continuous operation at 80% of their trip rating by default (per NEC 210.20(A) and NFPA 70 Table 310.16 notes). This means a 20 A breaker is designed for a maximum continuous load of 16 A. To find the breaker size needed for a given continuous load current, divide by 0.80, which is the same as multiplying by 1.25. The 125% rule is essentially building in the 80% derating factor required by the breaker's listing.
What are the standard circuit breaker sizes?
Standard single-pole residential breaker sizes in amperes: 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, and 200 A. Commercial and industrial applications use larger sizes up to 6,000 A. The most common residential sizes are 15 A (lighting and general outlets), 20 A (kitchen and bathroom circuits), and 30 A (electric dryers, water heaters). Double-pole breakers are used for 240 V circuits.
When are AFCI and GFCI breakers required?
Arc-fault circuit interrupter (AFCI) breakers are required by the 2023 NEC for most habitable spaces in dwelling units, including bedrooms, living rooms, hallways, and kitchens. Ground-fault circuit interrupter (GFCI) breakers are required in areas where moisture is present: bathrooms, kitchens (within 6 feet of a sink), garages, outdoor outlets, crawl spaces, unfinished basements, and boat docks. Many circuits now require both AFCI and GFCI protection. Check your local adopted NEC edition for exact requirements.
What happens if a circuit breaker is undersized?
An undersized breaker trips repeatedly under normal operating loads. Repeated nuisance tripping is a sign that either the breaker is too small for the load, the load has increased beyond design, or there is a fault. Never replace a breaker with a larger one to stop nuisance tripping without first investigating the cause. Oversizing a breaker to stop tripping is dangerous: it allows the circuit wiring to carry more current than it was designed for, which can cause insulation damage and fire.
Sources
- NFPA 70 (National Electrical Code) 2023, Section 210.20(A): NFPA 70 Standard. Branch-circuit overcurrent protection requirements.
- NFPA 70, Table 240.6(A): Standard ampere ratings for fuses and inverse time circuit breakers.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Educational reference only. All electrical work must comply with your local adopted NEC edition and must be performed by a licensed electrician.