Circuit Breaker Calculator
Determine the correct circuit breaker size for any electrical load using the NEC 80% rule. Get wire gauge recommendations and standard breaker sizes in seconds.
How to Use This Calculator
- Select your load type — the wattage will auto-fill with a typical preset. Adjust if needed.
- Set the voltage — 120V for standard outlets and lighting; 240V for dryers, ranges, HVAC, and EV chargers.
- Choose continuous load — select "Yes" if the load runs for 3 or more hours at a time (lights, HVAC).
- Click Calculate — the recommended breaker size and wire gauge appear instantly.
How Breaker Size Is Calculated
- Step 1 — Load Amps:
Load Amps = Watts ÷ Voltage - Step 2 — Continuous Load (NEC 210.20):
Min Breaker = ceil(Load Amps × 1.25) - Step 2 — Standard Load (NEC 80% Rule):
Min Breaker = ceil(Load Amps ÷ 0.80) - Step 3 — Round Up to Standard Size:
Use: 15, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, 200A
NEC 240.6 defines standard ampere ratings for fuses and circuit breakers. Your breaker must be at least as large as the minimum calculated amperage, rounded up to the next standard size.
Pro Tips
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Recommended Breakers & Tools
Square D 20A Single-Pole Breaker
QO120CP — plug-on, fits QO panels
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QO230CP — 240V for dryers, A/C units
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LB250-00S — suitable for EV chargers
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Verify circuits are de-energized before work
View on Amazon →Frequently Asked Questions
What size breaker do I need for kitchen outlets?
Kitchen counter outlets require at least two dedicated 20A small appliance circuits per NEC 210.11(C)(1). These circuits must be 20A and use 12 AWG wire. They serve only counter receptacles — no lighting or other loads can share these circuits.
What size breaker do I need for a bedroom?
Standard bedroom circuits are typically 15A, which is sufficient for lighting and general receptacles. However, if you plan to run high-draw electronics, a home office setup, or space heaters, a 20A circuit (12 AWG wire) is a better choice and costs little extra during construction.
What size breaker do I need for an electric dryer?
Electric dryers require a dedicated 30A, 240V double-pole breaker with 10 AWG wire. This is a 240V circuit, meaning it uses both hot legs from the panel. Never attempt to run a dryer on a 120V circuit or a shared circuit.
What size breaker do I need for an EV charger?
A Level 2 EV charger (240V) typically requires a 50A dedicated circuit with 6 AWG wire, though a 40A circuit suffices for many home chargers. Check your charger's nameplate amperage and apply the 80% rule: a 40A continuous load needs a 50A breaker. Always verify with the charger manufacturer.
What is the NEC 80% rule for circuit breakers?
The NEC 80% rule (NEC 210.19 and 210.20) states that the continuous load on a circuit must not exceed 80% of the breaker's ampere rating. A load is "continuous" if it operates for 3 or more hours. So a 20A breaker can only carry 16A continuously. This rule prevents heat buildup in wiring and breaker components over extended operation.