Circuit Breaker Sizing Calculator
Calculate the correct circuit breaker size for your electrical load. Enter wattage or amperage to determine proper breaker sizing with NEC guidelines.
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How to Use This Calculator
Size your circuit breaker correctly for safety and code compliance:
- Determine the total wattage of devices that will be on this circuit.
- Enter the circuit voltage (120V for standard outlets, 240V for large appliances).
- Calculate or enter the expected amperage draw.
- Apply the 80% continuous load rule for circuits that run for 3+ hours.
- Click 'Calculate' to see the recommended breaker size.
Why This Matters
Circuit breakers are your home's primary defense against electrical fires. An undersized breaker trips constantly, while an oversized breaker won't trip before wires overheat - a serious fire hazard. The NEC requires breakers to be sized correctly for the wire gauge used, and continuous loads (operating 3+ hours) must not exceed 80% of breaker capacity. Proper sizing ensures safety while avoiding nuisance trips.
- Prevent electrical fires from overloaded circuits
- Avoid nuisance trips from undersized breakers
- Ensure code compliance for inspections and insurance
- Match breaker size to wire gauge for proper protection
- Account for continuous vs intermittent loads correctly
Worked Examples
Kitchen Small Appliance Circuit
Two 20A kitchen countertop circuits required by code. Maximum expected load: 1,800W.
Electric Dryer
Installing a 5,400W electric dryer on 240V.
Workshop Subpanel
Planning loads: table saw (15A), dust collector (12A), lights (5A), outlets (10A).
Common Mistakes to Avoid
- Oversizing breakers to stop tripping - this is extremely dangerous; find the actual problem instead.
- Forgetting the 80% rule for continuous loads - a 20A breaker should only carry 16A continuously.
- Not matching breaker size to wire gauge - the wire must be rated for the breaker, not the load.
- Using single-pole breakers for 240V circuits - large appliances need double-pole breakers.
- Ignoring NEC requirements for dedicated circuits - kitchens, bathrooms, and laundry have specific rules.