WireGaugeCalc Free calculator · NEC cited · works offline
NEC Voltage Drop Calculations
NEC voltage drop calculations are fast with WireGaugeCalc, since the calculator is built right into this page.
You enter the voltage, the load in amps, the one-way run length, and the conductor material, and it returns the drop as a percentage.
That means you can size a longer run and stay under the 3% branch-circuit guideline before you pull any wire.
Open the calculatorUses Vd = (phase factor × K × I × L) / circular mils, K = 12.9 copper, 21.2 aluminum. Field aid only, verify against the current NEC and your local authority having jurisdiction.
Why run length and the 3% guideline matter
Voltage drop grows as the run gets longer, so a circuit that passes at 50 feet can fail at 150 feet.
The NEC recommends keeping branch-circuit drop at or under 3% so equipment sees enough voltage to work right.
WireGaugeCalc shows the percentage as you type, so you can bump up a wire size when the run is too long.
Copper vs aluminum and what the tool needs
Give the calculator a few inputs and it handles the rest:
- System voltage, such as 120V or 240V
- Load current in amps
- One-way run length in feet
- Conductor material, since copper has lower resistance than aluminum
- Wire size in AWG or kcmil
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
Is the 3% voltage drop limit a hard NEC rule?
It is a recommendation in the informational notes, not a strict requirement. Many designers and inspectors still treat 3% on the branch circuit as a target, so verify the current NEC and your local authority having jurisdiction.
Does aluminum wire drop more voltage than copper?
Yes, aluminum has higher resistance per AWG size, so it drops more voltage over the same run. The calculator lets you switch material and compare the two.
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.