Circuit Breaker Size Calculator
Determine the correctly rated circuit breaker amperage for any electrical circuit based on total wattage and voltage. Essential for panel upgrades, new circuit installations, and NEC code compliance.
About this calculator
The correct breaker size is calculated with: Breaker Size (A) = ⌈(Load / Voltage) × 1.25⌉. First, Ohm's Law converts total wattage to amperes: Current (A) = Load (W) / Voltage (V). The NEC 80% rule then requires that a breaker operate at no more than 80% of its rated capacity for continuous loads, which means you must size the breaker at 125% (multiply by 1.25) of the calculated current. The ceiling function (⌈⌉) rounds up to the next standard breaker size. For example, on a 120 V circuit, you would round up to standard sizes of 15, 20, 25, 30 A, and so on. Using an undersized breaker creates a fire hazard; oversizing defeats the protection.
How to use
You're wiring a 240 V circuit for a 4,800 W electric water heater. Step 1 — find operating current: 4,800 / 240 = 20 A. Step 2 — apply the 125% NEC safety factor: 20 × 1.25 = 25 A. Step 3 — ceiling gives 25 A exactly, which is a standard breaker size. Result: install a 25 A double-pole breaker. If the load were 5,000 W instead: 5,000 / 240 = 20.83 A × 1.25 = 26.04 A → ⌈26.04⌉ = 27 A → round up to the next standard size, a 30 A breaker.
Frequently asked questions
Why do I need to multiply the calculated amperage by 1.25 when sizing a circuit breaker?
The National Electrical Code (NEC) Article 210.20 requires that continuous loads — those running for 3 hours or more — not exceed 80% of a breaker's rated ampacity. Multiplying by 1.25 is mathematically equivalent to dividing by 0.80, ensuring the breaker is rated for 125% of the actual load. This safety margin prevents nuisance tripping and reduces the risk of overheating at the breaker's contacts. Even for non-continuous loads, this margin provides a practical buffer for startup surges and future load additions.
What happens if I install a circuit breaker that is too large for the wiring?
An oversized breaker will not trip when current exceeds the wire's safe capacity, allowing the wire to overheat and potentially start a fire inside walls or conduit. The breaker's sole job is to protect the wiring, not the appliance. For example, putting a 30 A breaker on 14 AWG wire rated for only 15 A is a serious code violation and fire hazard. Always match the breaker size to the wire's ampacity rating, and use the load calculation only to ensure the wire and breaker together are adequate for the circuit.
How do I find the right breaker size for a 240-volt appliance like a dryer or range?
For 240 V appliances, divide the appliance's total wattage by 240 to get operating current, then multiply by 1.25 and round up to the nearest standard double-pole breaker size. Common sizes are 20, 30, 40, and 50 A. A typical electric dryer at 5,600 W needs 5,600 / 240 = 23.3 A × 1.25 = 29.2 A, rounded up to a 30 A double-pole breaker. Always check the appliance nameplate for its rated amperage, as that figure should also be used to verify your calculation.