Circuit Breaker Size Calculator
How to Size a Circuit Breaker
Basic Formula
Current (Amps) = Power (Watts) / Voltage (Volts) Breaker Size = Current × 1.25 (for continuous loads)Continuous vs. Non-Continuous Loads
Continuous Load (3+ hours): Size breaker at 125% of calculated current. The NEC defines continuous load as operating for 3 hours or more.
Non-Continuous Load: Size breaker at 100% of calculated current. Examples include lighting, receptacles, and most general-purpose circuits.
Standard Breaker Sizes
Standard circuit breaker amperage ratings (NEC 240.6):
- 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A (common sizes)
- 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A (larger sizes)
- 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000A (commercial/industrial)
Breaker and Wire Size Relationship
The minimum wire size must be rated for at least the breaker size:
- 15A breaker: 14 AWG copper minimum
- 20A breaker: 12 AWG copper minimum
- 30A breaker: 10 AWG copper minimum
- 40A breaker: 8 AWG copper minimum
- 50A breaker: 6 AWG copper minimum
- 60A breaker: 6 AWG copper or 4 AWG aluminum
- 100A breaker: 4 AWG copper or 2 AWG aluminum
Common Applications
- Lighting circuits: 15A breaker, 14 AWG wire
- General receptacles: 20A breaker, 12 AWG wire
- Small appliances (kitchen): 20A breaker, 12 AWG wire
- Dryer: 30A breaker, 10 AWG wire (240V)
- Electric range: 40-50A breaker, 6-8 AWG wire (240V)
- AC unit: sized per nameplate, typically 30-60A
- Water heater: 30A breaker (4500W) or 38A breaker (5500W), 10 AWG
- EV charger: sized per manufacturer, typically 40-60A
About Circuit Breaker Calculator
Circuit Breaker Calculator - Calculate Breaker Size Instantly
Calculate the correct circuit breaker size for your electrical load instantly. Includes NEC compliant sizing for continuous loads and wire size recommendations.
How to Calculate Circuit Breaker Size
Step 1: Calculate Load Current
Current (Amps) = Power (Watts) / Voltage (Volts)Step 2: Apply Continuous Load Factor
For continuous loads (3+ hours): Multiply by 125%
Minimum Breaker = Load Current × 1.25 (for continuous loads)Step 3: Select Standard Breaker Size
Round up to the next standard size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A...
Standard Breaker Sizes (NEC 240.6)
Common residential sizes:
- 15A - Lighting, general receptacles
- 20A - Kitchen counter appliances, bathroom circuits
- 30A - Dryer, water heater (240V)
- 40A - Electric range, cooktop
- 50A - Large range, oven
- 60A - Sub-panel, feeder
Larger sizes for commercial: 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A
Continuous vs. Non-Continuous Loads
Continuous Loads (3+ hours):
- Must be sized at 125% of calculated current
- Examples: Lighting circuits, HVAC equipment, continuous process equipment
- Formula: Breaker Size = Load Amps × 1.25
Non-Continuous Loads:
- Sized at 100% of calculated current
- Examples: Receptacles, switches, intermittent motors
- Formula: Breaker Size = Load Amps
Related Calculators
Calculate concrete volume for slabs, footings, columns, and tubes. Get accurate estimates in cubic yards, cubic meters, and bags needed.
Calculate paint and primer needed for interior and exterior projects. Estimate gallons, coats, and costs for walls, ceilings, and trim.
Calculate roofing materials, shingles, and costs instantly. Estimates for gable, hip, and flat roofs with waste factor included.
Calculate tile needs for floors, walls, and backsplashes. Get accurate estimates of tiles needed including waste percentage and grout coverage.
Calculate concrete blocks and mortar needed for walls. Estimate blocks, mortar bags, and costs for standard and custom block sizes.
Calculate concrete volume for slabs, patios, and footings. Get accurate estimates in cubic yards, bags, and cost for concrete pours.
Calculate sand volume and weight for construction projects. Estimate sand needed for concrete, landscaping, and playgrounds.
Calculate gravel volume and weight for driveways, pathways, and landscaping. Estimate tons and cubic yards needed.
Calculate asphalt needed for driveways, parking lots, and roads. Estimate hot mix asphalt volume in tons and cubic yards.
Calculate wallpaper rolls needed for any room. Estimate wallpaper, account for pattern repeats, doors, and windows.
Calculate carpet needed for rooms and stairs. Estimate carpet in square yards and square feet with waste factor included.
Calculate fence materials including posts, panels, pickets, and concrete needed. Estimates for wood, vinyl, and chain link fences.
Calculate decking materials including deck boards, joists, and screws. Estimate composite, wood, and PVC decking needs.
Calculate siding materials needed for your home. Estimate vinyl, wood, fiber cement siding in squares and linear feet.
Calculate electrical load for your home or office. Determine circuit capacity and prevent overloads.
Calculate wire ampacity and proper electrical conductor sizing based on code requirements.
Calculate your GPA instantly. Support for weighted and unweighted GPA. Multiple courses, cumulative GPA, and grade tracking.
Calculate your final grade with our free grade calculator. Determine what you need on your final exam to achieve your desired course grade.