What Is the Breaker Sizing Calculator?
The Breaker Sizing Calculator is a free online tool designed for contractors and DIY builders who need quick, accurate calculations in the construction and building space. By entering your load type, load, system voltage, you get instant results including minimum breaker size, required wire, design current. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting minimum breaker size right can make the difference between success and costly mistakes. In construction and building, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by contractors and DIY builders worldwide, giving you confidence that your numbers are correct. Use it to plan your projects with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your load type and need to find the right minimum breaker size. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
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 to Use This Calculator
- Enter Your Load Type: Start by entering your load type — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: load, system voltage, phase. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your minimum breaker size, required wire, design current. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
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.
Tips & Considerations
- Double-check your load type before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both minimum breaker size and required wire — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to plan your projects.
- If you're unsure about your phase, start with a conservative estimate and adjust from there.
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).