Built and fact-checked by the DocNectar team — see our editorial standards
Key Features
Instant Calculation
Get accurate results in real time with our optimized algorithm.
Mobile Friendly
Fully responsive design. Works on all devices & screen sizes.
Privacy Focused
Your data stays on your device. We don't store any inputs.
100% Free
No hidden costs. This tool is completely free forever.
Every wire has some resistance, so voltage drops slightly along its length as current flows through it.
This calculator estimates that voltage drop for a copper wire run, based on its gauge, the load current, the one-way distance, and whether the circuit is single-phase or three-phase.
How it works
Select a wire gauge (AWG), phase, and enter the current, one-way distance, and source voltage. The calculator looks up the wire's resistance per 1000 ft (NEC Chapter 9, Table 8), scales it to the actual distance, and applies the voltage drop formula.
- Enter wire gauge (AWG, copper).
- Enter circuit phase.
- Enter load current (A).
- Enter one-way distance (ft).
- Enter source voltage (V).
- Click Calculate to see your results.
Examples
12 AWG, 20A, 100 ft, 120V
A single-phase 120V circuit with 12 AWG copper wire, 20 amps of current, and a 100 ft one-way run has a voltage drop of about 7.9V (6.6% of source voltage).
Who should use it
- Sizing wire gauge for a long extension cord or sub-panel feeder run.
- Checking whether a planned circuit run will have excessive voltage drop before installation.
Industry applications
- Residential and commercial electrical wiring
- Electrical engineering and code compliance
Advantages
- Uses standard NEC copper conductor resistance values for accurate estimates.
- Supports both single-phase and three-phase circuits.
Limitations
- Copper conductors only — doesn't include an aluminum resistance table.
Common mistakes to avoid
- Entering the total (round-trip) wire length instead of the one-way distance to the load.
- Using the aluminum resistance table by mistake — this calculator's built-in values are for copper conductors only.
Best practices
- If your calculated drop exceeds recommended limits, try a lower AWG number (thicker wire) and recalculate.
Tips
- Voltage drop matters most on long runs — a run under 50 ft rarely has a meaningful drop even with lighter gauge wire.