How do I determine what size circuit breaker I need?

Calculate the load current: I = Watts ÷ Volts (single-phase) or I = Watts ÷ (√3 × Volts × PF) (3-phase). For continuous loads (running 3+ hours), apply the NEC 80% rule: Breaker = Load Amps ÷ 0.8 (or Load × 125%). Round up to the next standard breaker size (15, 20, 25, 30, 40, 50, 60, 70, 80, 100A). Then size wire to match the breaker rating per NEC Table 310.16.

What size wire for a 30 amp breaker?

10 AWG copper wire for a 30A breaker (NEC Table 310.16, 60°C column for residential). Common wire-to-breaker pairings: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, 6 AWG = 50-60A, 4 AWG = 70A, 3 AWG = 80A, 2 AWG = 100A. Always use copper in residential. For long runs, upsize wire to compensate for voltage drop.

What breaker size for common appliances?

Electric dryer: 30A/240V (10 AWG). Electric range/oven: 40-50A/240V (8-6 AWG). Water heater: 30A/240V (10 AWG). Central AC: 30-60A/240V (varies by unit). Dishwasher: 20A/120V (12 AWG). Microwave: 20A/120V (12 AWG). Garbage disposal: 15-20A/120V (14-12 AWG). EV charger (Level 2): 40-60A/240V (8-6 AWG). General lighting/outlets: 15-20A/120V.

What is voltage drop and when does it matter?

Voltage drop is the loss of voltage along wire length due to resistance. NEC recommends max 3% drop for branch circuits, 5% total (feeder + branch). It matters for long runs: at 100 feet, a 20A circuit on 12 AWG loses ~3.2% at 120V. Fix by upsizing wire. Formula: Vdrop = (2 × Length × Current × Resistance/ft) ÷ 1000. Our calculator checks this automatically.

Single pole vs double pole breaker?

Single pole: 120V circuits, one hot wire + neutral. Used for lighting, outlets, small appliances. Standard sizes: 15A, 20A. Double pole: 240V circuits, two hot wires. Used for dryers, ovens, AC, water heaters, EV chargers. Standard sizes: 20-60A+. A double-pole breaker trips both poles together for safety. GFCI and AFCI breakers add fault protection.

How do I calculate total electrical panel load?

Sum all circuit loads: lighting (3 VA per sq ft), small appliance circuits (1,500W each, minimum 2 required), laundry circuit (1,500W), then add each dedicated appliance at nameplate rating. Apply NEC demand factors: first 10,000VA at 100%, remainder at 40%. Add AC and heating at largest of the two (not both). Divide total VA by 240V for panel amperage. Most homes need 200A service.

Circuit Breaker Sizing Calculator — Free Amp Rating & Wire Size Tool | AllInOneTools

⚡ Electrical & Energy

Circuit Breaker Sizing Calculator

Determine the correct breaker amp rating and wire gauge for any electrical load. NEC compliant with 80% continuous load rule and voltage drop check.

30A

Recommended Breaker

10 AWG

Wire Size (Copper)

Load Information

Load Power

watts

Voltage

Wire Run Length (one-way)

feet

Power Factor (AC motors)

Continuous Load (3+ hours — applies 125% / 80% rule)

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Sizing Details

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Wire & Breaker Reference Chart

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Frequently Asked Questions

How do I determine breaker size?▼

I = Watts ÷ Volts. For continuous loads: multiply by 1.25 (NEC 80% rule). Round up to next standard size (15, 20, 25, 30, 40, 50, 60A). Then match wire gauge to breaker per NEC Table 310.16.

What is the NEC 80% rule?▼

Continuous loads (3+ hours) can't exceed 80% of breaker rating. So: breaker = load amps × 1.25. A 24A continuous load needs 30A breaker (24 × 1.25 = 30). Prevents overheating.

What wire size for each breaker?▼

Copper wire (60°C): 14 AWG=15A, 12 AWG=20A, 10 AWG=30A, 8 AWG=40A, 6 AWG=50-55A, 4 AWG=70A, 3 AWG=80A, 2 AWG=95A, 1 AWG=110A, 1/0=125A. Upsize for long runs (voltage drop).

What breaker for common appliances?▼

Dryer: 30A/240V (10 AWG). Oven: 40-50A/240V (8-6 AWG). Water heater: 30A/240V. AC: 30-60A/240V. EV charger: 40-60A/240V. Microwave: 20A/120V. General outlets: 15-20A/120V.

What is voltage drop?▼

Voltage loss along wire length. NEC recommends max 3% for branch circuits. Matters for long runs: at 100ft, 20A on 12 AWG loses 3.2% at 120V. Fix by using larger wire gauge. Our calculator checks this automatically.

Single vs double pole breaker?▼

Single pole: 120V circuits (lights, outlets). Double pole: 240V circuits (dryer, oven, AC, water heater, EV charger). Double pole trips both legs simultaneously for safety.

How to calculate panel load?▼

Sum all loads with NEC demand factors: first 10,000VA at 100%, rest at 40%. Add largest of AC or heat (not both). Divide total by 240V. Most homes need 200A service. Consult an electrician for panel upgrades.