WireGaugeCalc Free calculator · NEC cited · works offline
Oy Not Voltage Drop Calculator
Oy not voltage drop calculator searches land you on WireGaugeCalc, which is a simple tool that returns the percentage of voltage lost on a run.
You enter the circuit voltage, the load in amps, the one-way length, and the conductor size, and the calculator returns the drop and the volts at the far end.
The payoff is a clear read on whether the run stays inside the 3 percent branch-circuit target before you pull the 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.
How the Voltage Drop Percentage Is Found
Voltage drop rises as the run gets longer, since resistance adds up over the full length of the conductor.
Copper loses less than aluminum at the same size, so the material you pick changes the result.
The calculator compares the loss to the 3 percent guideline for branch circuits so you know if a larger conductor is needed.
What Moves the Drop on a Long Run
A handful of inputs set the percentage:
- One-way run length, since longer runs add resistance
- Load current, where more amps mean a larger drop
- Conductor size, as a bigger conductor lowers the loss
- Material, with copper holding voltage better than aluminum
- System voltage, since 240V runs tolerate longer pulls than 120V
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 target should a branch circuit hit?
The NEC recommends keeping branch-circuit voltage drop at or below 3 percent, with a combined feeder and branch total near 5 percent. These are guidelines, so confirm them against the current NEC and your local authority having jurisdiction.
Why does my long run fail even with the correct breaker size?
A conductor can be rated for the load yet still drop too much voltage over a long run. The fix is usually to upsize the conductor one or two gauges to bring the percentage back under target.
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.