WireGaugeCalc Free calculator · NEC cited · works offline
12vdc Voltage Drop Calculator
12vdc voltage drop calculator is fast with WireGaugeCalc, since it finds the volts lost on a 12V run from current, length, and wire size.
You enter the load amps, the one-way distance, and the gauge, so the tool returns both the volts dropped and the percentage.
The result tells you whether the load gets enough voltage or whether you need a larger conductor.
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 Voltage Drop Matters On A 12V Run
At 12V even a small loss is a big share of the supply, so drop limits matter more than on higher voltage systems.
The 3 percent branch-circuit guideline leaves only about 0.36 volts on a 12V circuit, since that is 3 percent of 12.
The calculator uses round-trip distance, since both conductors carry current and add resistance.
What Drives The Volts Lost
Voltage drop on a DC run depends on a few factors:
- Load current, since drop rises directly with amps
- One-way run length, because the round trip doubles the resistance
- Conductor gauge, where a larger size lowers resistance
- Copper versus aluminum, since aluminum has higher resistance
- Connection quality, because loose terminals add loss
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
How much voltage drop is acceptable on a 12V circuit?
The common guideline is 3 percent for a branch circuit, which is about 0.36 volts at 12V. Sensitive electronics may need a tighter limit, so size accordingly.
Why do I enter one-way distance instead of total wire length?
The calculator doubles the one-way length internally because current travels out and back through two conductors. Entering the one-way run keeps the input simple while the math covers the round trip.
Does copper or aluminum change the drop?
Aluminum has higher resistance than copper, so it drops more voltage at the same size. Check the conductor choice and the percentage against 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.