Breaker Size Calculator
Determine the correct circuit breaker size based on wire gauge and load current per NEC 240.6.
Results
How to Use This Calculator
Enter the continuous load in amps, which includes any load expected to operate for three hours or more, such as lighting circuits. Enter the non continuous load in amps for intermittent loads like receptacle circuits. Select the conductor size and type to verify the breaker does not exceed the conductor ampacity. Choose the breaker type: standard, GFCI, or AFCI, as this may affect available sizes. The calculator applies the NEC 210.20(A) rule that the breaker must be rated at 125% of the continuous load plus 100% of the non continuous load. For example, with a 16A continuous lighting load and a 10A non continuous receptacle load, the minimum breaker rating is (16 x 1.25) + 10 = 30A. The calculator selects the next standard breaker size, which in this case is 30A.
Understanding the Concept
Proper breaker sizing protects conductors from overcurrent while avoiding nuisance tripping during normal operation. A breaker that is too small trips frequently, disrupting power to the circuit. A breaker that is too large fails to protect the conductor, allowing dangerous overheating before the breaker opens. NEC 240.4 is the primary rule: conductors must be protected against overcurrent in accordance with their ampacity. For small conductors, NEC 240.4(D) sets absolute limits: 14 AWG at 15A, 12 AWG at 20A, and 10 AWG at 30A. NEC 210.20(A) adds the requirement that breakers protecting circuits with continuous loads must be sized at 125% of the continuous load plus 100% of the non continuous load. This 125% rule ensures the breaker operates within its 80% continuous duty rating, since standard breakers are only rated to carry 80% of their trip rating continuously. Listed 100% rated breakers do not require this 125% calculation, but they are significantly more expensive and less commonly used in residential and light commercial work.
The Formula Explained
The breaker sizing formula per NEC 210.20(A) is Minimum Breaker Size = (Continuous Load x 1.25) + Non Continuous Load. Continuous loads are those expected to operate for three hours or more, as defined in NEC Article 100. The 1.25 multiplier compensates for the fact that standard breakers are rated for 80% continuous duty. After calculating the minimum, round up to the next standard breaker size from the series: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 amps. The selected breaker must not exceed the conductor ampacity per NEC 240.4. For conductors larger than 10 AWG, NEC 240.4(B) permits using the next standard breaker size above the conductor ampacity. For 14, 12, and 10 AWG, the maximums of 15A, 20A, and 30A per NEC 240.4(D) are absolute.
Frequently Asked Questions
Can I use a 20 amp breaker on 14 gauge wire?
No. NEC 240.4(D) strictly limits 14 AWG conductors to 15A overcurrent protection. Even though 14 AWG copper has an ampacity of 15A at 60C and 20A at 75C, the NEC does not permit using the higher temperature rating to justify a 20A breaker on 14 AWG wire. This rule exists because 14 AWG connections at receptacles and switches are most commonly rated for 60C, and overloading them creates fire risk at terminal points.
What is the 80% rule for circuit breakers?
Standard circuit breakers are designed for a maximum continuous load of 80% of their rating. A 20A breaker should carry no more than 16A continuously (loads lasting three hours or more). This is why NEC 210.20(A) requires sizing breakers at 125% of the continuous load. The inverse of 1.25 is 0.80, so a 20A breaker at 80% handles 16A continuously. Breakers listed and marked for 100% continuous duty can be loaded to their full rating but are more expensive and typically found in commercial switchgear.
What size breaker do I need for a 240V water heater?
A standard residential water heater draws 4500W at 240V, which equals 18.75A (4500 / 240). Since a water heater is a continuous load (it can operate for more than three hours), apply the 125% rule: 18.75 x 1.25 = 23.4A. The next standard breaker size is 25A or 30A. Most electricians install a 30A double pole breaker with 10 AWG copper wire, which provides some headroom and matches the common 30A disconnect switch required by many jurisdictions.
Do I need AFCI or GFCI breakers?
NEC 210.12 requires AFCI protection for most 15A and 20A branch circuits in dwelling units, including bedrooms, living rooms, kitchens, hallways, closets, and laundry areas. NEC 210.8 requires GFCI protection for receptacles in bathrooms, kitchens within 6 feet of a sink, garages, outdoors, basements, crawl spaces, and near laundry sinks. Dual function AFCI/GFCI breakers are available and satisfy both requirements. Some jurisdictions have additional or different requirements, so check your local amendments.