Breaker Sizing Calculator
Standard Breaker Sizes & Wire Pairings
Minimum copper wire gauge and maximum continuous load per NEC.
| Breaker Size | Min Wire (Cu) | Max Continuous | Typical Use |
|---|---|---|---|
| 15A | 14 AWG | 12A | Lighting, bedroom outlets |
| 20A | 12 AWG | 16A | Kitchen, bath, garage outlets |
| 30A | 10 AWG | 24A | Water heater, dryer (small) |
| 40A | 8 AWG | 32A | Range, cooktop |
| 50A | 6 AWG | 40A | Range, EV charger, sub-panel |
| 60A | 6 AWG | 48A | Sub-panel, large HVAC |
| 100A | 3 AWG | 80A | Sub-panel, service entrance |
How We Calculate This
This breaker sizing calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
Breaker sizing is about matching protection to the wire and load. The breaker protects the wire, not the appliance — so it must be sized correctly for both the load type and conductor ampacity.
The basic rule:
- Continuous loads (3+ hours): breaker must be rated for 125% of the load current per NEC 210.20(A). A 16A continuous load needs a 20A breaker minimum
- Non-continuous loads: breaker must be rated at or above the actual load current. Use the next standard size up
- Motor loads: size at 125% of the motor's full-load amps per NEC 430.52, then select the next standard breaker size
Standard residential breaker sizes are 15, 20, 25, 30, 40, 50, 60, 70, 80, and 100 amps. The wire gauge must match or exceed the breaker rating — never put a 20A breaker on 14 AWG wire.
When Would You Use This Calculator?
This breaker sizing calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
What is the NEC 80% rule for breakers?
The NEC 80% rule means continuous loads (running 3+ hours) can only use 80% of a breaker's rating. A 20A breaker can handle only 16A continuously. This is really a 125% sizing rule — multiply the continuous load by 1.25 to find the minimum breaker size.
What size breaker do I need for a 240V water heater?
A typical 4,500W water heater at 240V draws 18.75A. Since it's a continuous load, multiply by 1.25 = 23.4A minimum. The next standard breaker size is 25A or 30A. Most installations use a 30A double-pole breaker with 10 AWG wire.
Can I use a bigger breaker if my circuit keeps tripping?
Never upsize a breaker without also upsizing the wire. The breaker protects the wire from overheating. If a 15A breaker trips, the fix is to reduce the load on the circuit or run a new circuit — not install a 20A breaker on 14 AWG wire, which creates a fire hazard.
What size wire goes with each breaker size?
Common pairings (copper THHN): 15A breaker = 14 AWG, 20A = 12 AWG, 30A = 10 AWG, 40A = 8 AWG, 50A = 6 AWG, 60A = 6 AWG, 100A = 3 AWG. These are minimums — always check voltage drop for long runs.
How do I size a breaker for a motor?
Motor circuits are special under NEC Article 430. Size the breaker at 125% of the motor's full-load amps (from NEC Table 430.248, not the nameplate). Use the next standard size up. Motor branch circuits also need overload protection sized differently from the breaker.
What is the difference between single-pole and double-pole breakers?
Single-pole breakers provide 120V and take one panel slot. Double-pole breakers provide 240V and take two slots. For 240V loads like dryers, ranges, water heaters, and AC units, you need a double-pole breaker. Some 120V circuits use double-pole for MWBC (multi-wire branch circuits).