WireGaugeCalc Free calculator · NEC cited · works offline
Extension Cord Calculator
Extension cord calculator is simple with WireGaugeCalc, since you enter the tool amps, the cord length, and the cord gauge to see if the run holds up.
The calculator uses Ohm's law and the cord resistance to estimate voltage drop, so a long thin cord shows up as too much loss for the load.
Pick a cord that delivers full voltage to the tool instead of guessing at the hardware store.
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.
Matching cord gauge to amps and length
Longer cords add resistance, so the same tool needs a heavier gauge as the run grows.
Higher amp draw multiplies the loss, so a 15 amp saw needs a thicker cord than a 5 amp drill.
The tool reports the gauge that keeps voltage at the tool within a safe margin.
Inputs the cord check uses
Three values set the result:
- Tool or appliance load in amps
- Total cord length in feet
- Cord gauge from 16 down to 10 AWG
- Estimated voltage drop at the far end
- A pass or upsize flag against a 5 percent loss target
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
What gauge cord do I need for a 100 foot run at 15 amps?
At 100 feet and 15 amps a 12 AWG cord is the common choice, since lighter 14 or 16 AWG cords drop too much voltage at that length. The tool confirms the loss for your exact numbers.
Does daisy chaining two cords matter?
Yes, total length is what counts, so add both cords together when you enter the run. Joining cords also adds plug connections that can heat up, and any permanent wiring still needs to meet the current NEC and your local authority having jurisdiction.
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.