WireGaugeCalc Free calculator · NEC cited · works offline
Circuit Breaker Fault Currentc Calculator
Circuit breaker fault currentc calculator is quick with WireGaugeCalc, since you enter the voltage, transformer data, and conductor run and the app estimates available fault current.
The calculator uses Ohm's law and the conductor impedance, so the short-circuit current at the breaker reflects the real run length.
You get a number you can compare against the breaker interrupting rating before energizing.
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 Available Fault Current Is Estimated
Fault current depends on source voltage divided by total impedance, so a longer run lowers the available current at the breaker.
WireGaugeCalc adds the conductor resistance to the source impedance, then solves for the short-circuit value.
Since the breaker must clear that current, the result helps you confirm the interrupting rating is high enough.
Inputs For The Short-Circuit Estimate
The app works from the values that set fault current:
- Source voltage at the panel
- Transformer kVA and impedance
- Conductor size, material, and run length
- Single or three phase service
- Estimated available fault current for the breaker rating
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
Why does run length lower fault current?
A longer conductor adds resistance, so the total impedance rises and the available short-circuit current at the breaker drops. The estimate is a planning figure, so verify ratings against the current NEC and your local authority having jurisdiction.
How does fault current relate to the breaker rating?
The breaker interrupting rating must be at least the available fault current at its terminals, otherwise it cannot safely clear a short. Use this estimate to check that the rating has margin.
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.