Breaker Size Calculator
Find a practical starting breaker size from watts, voltage, phase, and load type
Use this breaker size calculator when you know the load wattage and want a practical starting breaker size for planning. Enter the watts, voltage, load type, and phase to estimate the minimum breaker amperage. For loads expected to run 3 or more hours, the calculator applies the 125% continuous-load rule, then rounds up to the next common breaker size. This is a planning tool only. Final breaker and wire sizing still need to match the conductor, equipment instructions, and local code. For the underlying current calculation, see the watts to amps calculator.
Minimum breaker size: 20A
Calculated current: 15 A
How to size a circuit breaker
A circuit breaker must carry the full load current without tripping under normal operation, while tripping fast enough to protect wiring from overloads and faults. The two key factors are the calculated load current and whether the load is continuous, meaning it runs for 3 or more hours at a time.
The NEC continuous load rule (Section 210.20(A)) requires that for continuous loads, the overcurrent protection device must be rated at no less than 125% of the continuous load current. This means a 16A continuous load needs a minimum 20A breaker, not a 15A breaker, even though a 15A breaker is technically above the running current.
After calculating the minimum required amperage, always round up to the next standard breaker size. Common residential sizes are 15, 20, 30, 40, 50, and 60A. Never round down. And never dramatically oversize, a 200A breaker on 14 AWG wire will not protect the wire from overload.
- Load watts: total wattage of all devices on the circuit
- System voltage: 120V for standard outlets, 240V for large appliances, 480V for industrial
- Load type: continuous (3+ hours) triggers the 125% rule; intermittent does not
- Phase: single-phase (residential/light commercial) vs three-phase (commercial/industrial)
Round up only to the next standard breaker size that the conductor and circuit design support. A larger breaker is not automatically safer if the wire or equipment is not rated for it.
How to convert
Step 1: Calculate load current
Determine the full load current drawn by the circuit.
Single-phase: I = Watts / Volts
Three-phase: I = Watts / (1.732 × Volts)
Step 2: Apply continuous load factor
If the load runs 3 or more hours continuously, multiply the current by 1.25 per NEC 210.20(A).
Continuous: I_required = I × 1.25
Non-continuous: I_required = I
Step 3: Round up to standard breaker size
Always round up to the next available standard overcurrent device rating.
Standard sizes: 15, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, 200A
Worked examples
Question: What breaker size for a 1,800W electric space heater at 120V (continuous load)?
Solution: I = 1,800 / 120 = 15A. Continuous: 15 × 1.25 = 18.75A. Round up → 20A breaker.
Question: What breaker for a 5,000W central A/C unit at 240V (continuous)?
Solution: I = 5,000 / 240 = 20.83A. Continuous: 20.83 × 1.25 = 26.04A. Round up → 30A breaker.
Question: What breaker for a 7,500W three-phase 480V motor (continuous)?
Solution: I = 7,500 / (1.732 × 480) = 9.02A. Continuous: 9.02 × 1.25 = 11.28A. Round up → 15A breaker.
Common mistakes and notes
- The 125% continuous rule applies to loads on for 3 or more hours, HVAC, water heaters, EV chargers, always-on commercial equipment.
- Conductor size must also be rated for the load. A 20A breaker requires 12 AWG or larger copper in most residential applications.
- Never upsize a breaker without also upgrading the wire, the breaker protects the wire, not the device.
- This calculator gives the minimum breaker size for planning. Always verify final circuit design with a licensed electrician and local code.
Assumptions
- Based on NEC 210.19(A) and 210.20(A) continuous load rules.
- Single-phase: I = W / V. Three-phase: I = W / (1.732 × V).
- Results are planning estimates only, final circuit design must be verified by a licensed electrician against local code.
Worked example
Example: 1,800 W ÷ 120 V = 15 A × 1.25 (continuous) = 18.75 A → minimum 20A breaker.
FAQ
What is the 80% rule for breakers?
The 80% rule is the practical side of the 125% continuous load requirement. A 20A breaker should carry no more than 16A of continuous load (20 × 0.80 = 16A). Running a breaker near its trip threshold for extended periods causes thermal degradation and nuisance tripping over time.
What counts as a continuous load?
Per NEC Article 100, a continuous load is one where the maximum current is expected to continue for 3 or more hours. Common examples: electric heating, HVAC equipment, EV chargers, commercial refrigeration, and permanently installed lighting that stays on during business hours.
Can I use a larger breaker than the minimum calculated?
You can go one standard size larger if the conductor is also rated for the larger breaker. However, dramatically oversizing, for example a 60A breaker on a 15A load, provides poor protection because the breaker will not trip until far above the wire's safe carrying capacity.
Does breaker size determine wire gauge?
Yes, breaker and wire must be matched. In North American residential wiring: 15A breaker = 14 AWG copper; 20A = 12 AWG; 30A = 10 AWG; 40–50A = 8 AWG; 60A = 6 AWG. Using wire that is too small for the breaker is a fire hazard.
Does this calculator handle motor starting current?
No. Motors draw 5–8 times their running current at startup. NEC Article 430 provides specific motor branch circuit protection rules that allow breakers larger than this calculator would suggest, sometimes up to 250% of full load current, to handle motor inrush. Use this calculator for resistive and general loads, and consult NEC Article 430 or a licensed electrician for motor circuits.
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⚠️ Sanity Check Only
This tool provides informational estimates only. It is not professional engineering advice. Electrical work is dangerous and governed by strict local codes.
Before you start:
- Verify these results with a licensed electrician.
- Cross-reference with the latest Electrical Code (NEC/CEC).
- Never work on live circuits.