Circuit Breaker Size Calculator
Determine the correct circuit breaker size for your home or project. This tool automatically applies NEC continuous load rules to recommend a safe, standard breaker size for your exact load.
Will this run for 3 hours or more? (e.g., EV chargers, large water heaters, commercial lighting)
How this calculator sizes your circuit breaker
This tool follows the safety standards set by NEC Article 210.20(A) for overcurrent protection. According to the code, circuit breakers must be sized at 100% of non-continuous loads, plus 125% of all continuous loads.
What is a continuous load? The NEC defines a continuous load as any load where the maximum current is expected to continue for three hours or more. Common examples include EV chargers, large water heaters, space heating, and commercial lighting.
By applying this 125% multiplier, the calculator ensures that a continuous load doesn't cause the breaker to slowly overheat and trip prematurely. Once the "design current" is calculated, it rounds up to the next standard breaker size.
The risks of undersizing (or oversizing) a breaker
A circuit breaker's primary job is to protect the wiring inside your walls from overheating, melting, or causing a fire. If a breaker is undersized for your load (or if the 125% continuous rule is ignored), the breaker will run hot and frequently trip. While annoying, this means the breaker is doing its job and cutting power before the wires can become dangerous.
Conversely, if a breaker is oversized relative to the wire gauge used in the circuit, it creates a severe fire hazard. The wire could draw more current than it is safely rated to handle, melting its insulation and causing an electrical fire long before the oversized breaker ever trips. This is why breaker sizing and wire sizing must always be paired correctly.
Methodology: Standard Breaker Sizes
The National Electrical Code (NEC 240.6) establishes standard ampere ratings for fuses and inverse time circuit breakers. Once this calculator determines your continuous or non-continuous design current, it automatically rounds up to the nearest standard size from this series:
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400.
For example, if your calculated design current is 22A, the tool will round up and recommend a 25A circuit breaker.
Frequently Asked Questions
It depends on whether the load is continuous. If it runs for less than 3 hours (non-continuous), a 30 amp breaker is sufficient. If it runs for 3 hours or more, you must apply the 125% rule: 30A × 1.25 = 37.5A. You would then round up to the next standard size, which is a 40 amp breaker.
The "80% rule" is just the reverse math of the 125% continuous load rule. It states that a standard circuit breaker should only be loaded to 80% of its rated capacity for continuous loads. For example, a 20A breaker should only carry a 16A continuous load (because 16 is 80% of 20, and 16 × 1.25 = 20).
Standard breaker sizes are defined in NEC 240.6(A). The common residential sizes are 15A, 20A, 25A, 30A, 40A, 50A, 60A, and up to 200A for main service panels. You can use this calculator as an interactive chart to instantly snap your design current to the correct standard size.