WireGaugeCalc Free calculator · NEC cited · works offline
Calculate Voltage Drop Across Wire
Calculate voltage drop across wire is fast with WireGaugeCalc, since you enter the length, load, and conductor and read the loss.
It uses the conductor resistance and your one-way run length to return both the volts lost and the percentage.
So you can check the result against the 3% branch-circuit guideline.
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.
See The Volts Lost Over The Run
You enter the one-way distance, the current, and the wire size, and the calculator returns the voltage dropped along the run.
Since a longer run adds resistance, the drop grows with distance, so the calculator shows when a larger size helps.
It reports the loss as a percentage of the supply, which makes the 3% target easy to read.
What Affects The Drop
Four inputs set how much voltage you lose across the wire:
- One-way run length in feet
- Load current in amps
- Conductor size and copper or aluminum
- Single-phase or three-phase
- Result in volts and percent of supply
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 voltage drop is acceptable on a branch circuit?
The NEC recommends keeping branch-circuit voltage drop at or below 3 percent, with a 5 percent total across feeder and branch. These are guidelines, so check the current NEC and your local authority having jurisdiction for any required limits.
Does aluminum drop more voltage than copper?
Yes, aluminum has higher resistance per foot, so the same run and load drop more voltage on aluminum. You often need to go up a size or two to match copper performance.
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.