WireGaugeCalc Free calculator · NEC cited · works offline
Breaker Size Calculator
breaker size calculator is simple with WireGaugeCalc, since you enter the load amps and the app returns a standard breaker rating.
It applies the continuous load factor and rounds up to the next standard NEC overcurrent device size.
That gives you a breaker that protects the conductor without nuisance trips.
Open the calculatorEnter any two values and read the rest.
Ohm’s law: V = I × R and P = V × I. Field aid only, verify against the current NEC and your local authority having jurisdiction.
How The AC Circuit Breaker Calculator Sizes The Device
The ac circuit breaker calculator starts with your load amps and multiplies a continuous load by 125 percent, since the NEC requires that headroom.
It then rounds up to a standard rating such as 15, 20, 30, 40, or 50 amps.
The breaker must protect the wire, so the conductor ampacity has to equal or exceed the device rating.
What The Breaker Sizing Math Needs
To size the overcurrent device correctly, the calculator looks at:
- Total load amps on the circuit
- Whether the load is continuous, which adds the 125 percent factor
- Conductor ampacity, since the breaker cannot exceed what the wire carries
- The next standard breaker rating above the calculated value
- Any motor or HVAC rule that allows a larger device
One app for every NEC calculation
WireGaugeCalc keeps the calculations you reach for most in one place:
- Voltage drop and wire sizing for any run
- Conduit fill for EMT, PVC, and RMC
- Box fill and junction box sizing
- Ampacity and temperature derating
- Motor circuit and load calculations
- Conduit bend offsets and saddles
Built for the field, works offline
The whole app runs on your phone, so it keeps working in a basement, an attic, or a job site with no signal.
There is no account to create and nothing to load. Open it, run the number, and get back to work.
- No signup and no signal needed
- Answers in a tap, not a spreadsheet
- Same tool on phone, tablet, and desktop
Every result cites the NEC article
Each answer shows the table or formula it came from, so you can check the method and learn the code as you go.
That makes the app useful on the job and during exam prep, since the reasoning is right next to the number.
Switch the code year your job runs on
Jurisdictions adopt the NEC at different times, so you can match the calculation to the code in force:
- NEC 2017, 2020, and 2023 tables
- Copper and aluminum conductors
- 60, 75, and 90 degree C terminations
- Single-phase and three-phase systems
Run the number, then get back to work
Stop flipping through a paper book or hopping between calculator sites. Enter your values, read the code-cited answer, and move on. Free to use, no signup.
Open the calculatorFrequently asked questions
Can a breaker be bigger than the wire?
Generally no, because the breaker protects the conductor. Motors and a few specific loads have exceptions, so verify the rule against the current NEC and your local authority having jurisdiction.
Why add 125 percent for a continuous load?
A continuous load runs three hours or more, so the NEC requires the breaker and wire to be sized at 125 percent of that load to avoid overheating.
Is WireGaugeCalc free to use?
Yes. Every calculator is free to run with no signup. A paid tier adds saved projects, PDF reports, and extra code years, but the core math stays free.
Does it work without internet?
Yes. The app runs on your device, so it keeps working with no signal on a job site, in a basement, or in an attic.
Are the results code accurate?
Results follow published NEC tables and standard formulas, and each answer shows the article it came from. It is a field aid, not a stamp of approval, so verify against the current code and your local authority before you wire anything.
Which NEC code year does it use?
You can switch between NEC 2017, 2020, and 2023, since jurisdictions adopt the code at different times. Pick the year your job runs on.