Circuit Breaker Sizing Calculator
Find the correct circuit breaker rating for any electrical load by entering the full-load current and a safety factor. Use this during panel design, home wiring projects, or electrical code compliance checks.
Last updated: September 2026
Formula below · 2 sources (ieee.org, Wikipedia) · Updated Sep 2026
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About this calculator
Circuit breakers protect wiring from overcurrent damage, and correct sizing ensures they trip under fault conditions without nuisance tripping during normal operation. The sizing formula is: Breaker Size = the next standard rating at or above Load Current (A) × Safety Factor. The NEC requires a factor of 1.25 for continuous loads (NEC 210.20(A): the breaker must be rated at least 125% of a continuous load); 1.0 is the code minimum for non-continuous loads. The result is rounded up to the next standard breaker rating listed in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and so on. Undersizing a breaker causes nuisance trips, while a breaker larger than the conductor’s ampacity allows the wire to overheat. Size the conductor for the breaker (for copper, 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A per NEC 240.4(D)).
How to use
Suppose a circuit carries a continuous load current of 16 A and you apply the NEC factor of 1.25. Step 1 — Enter 16 as the load current. Step 2 — Enter 1.25 as the safety factor. Step 3 — Multiply: 16 × 1.25 = 20 A. Step 4 — 20 A is a standard rating, so the calculator returns a 20 A breaker. For a 22 A continuous load: 22 × 1.25 = 27.5 A, and the next standard rating is 30 A. The defaults (15 A × 1.25 = 18.75 A) give a 20 A breaker.
Frequently asked questions
What safety factor should I use when sizing a circuit breaker for continuous loads?
For continuous loads — those operating for three hours or more — the NEC requires a safety factor of 1.25, meaning the breaker must be rated at 125% of the load current. This accounts for heat buildup in the breaker and wiring during sustained operation. Non-continuous loads may use a safety factor of 1.0 in some jurisdictions, though a margin of 1.25 is still recommended for safety. Always consult your local electrical code, as requirements can vary by region.
Why do circuit breakers need to be rounded up to standard sizes after calculating the minimum rating?
Breakers are made only in the standard ratings listed in NEC 240.6(A) (15, 20, 25, 30, 35, 40, 45, 50, 60, 70 A and up), so the calculated minimum is rounded up to the next available size; the calculator does this for you. Selecting a breaker below the calculated minimum causes nuisance tripping. Choosing a breaker larger than the wire’s ampacity is dangerous because it may not trip before the wire overheats. Match the conductor to the breaker rating.
How does improper circuit breaker sizing affect electrical safety in a home or commercial building?
An undersized breaker trips repeatedly under normal load, causing inconvenience and potentially damaging sensitive equipment connected to that circuit. An oversized breaker is far more dangerous — it allows fault currents to flow through wiring beyond the wire's rated capacity, heating the insulation and potentially igniting surrounding materials. According to the NFPA, faulty electrical wiring and related equipment are among the leading causes of structure fires in the US. Proper breaker sizing, validated against the wire gauge ampacity, is one of the most critical steps in any electrical installation.