Voltage Drop Calculator
Estimate voltage drop on a copper circuit from the current, one-way wire length, and gauge, and check it against the common 3% guideline.
What your result means
Voltage drop is the voltage lost to wire resistance over the length of the run, there and back. Percent drop compares it to your source voltage; many electricians aim to keep branch circuits under 3%. Voltage at the load is what's actually left at the device. Too much drop means dim lights, weak motors, and wasted energy. If your drop is high, step up a gauge with our wire size calculator.
How to use this calculator
- Enter the circuit voltage (120 or 240 for most homes).
- Enter the load current in amps.
- Enter the one-way distance from the panel to the load.
- Select the copper wire gauge.
- Read the voltage drop, percent, and voltage at the load, and compare to 3%.
The formula
For a single-phase circuit, current travels out and back, so the length is doubled. Each AWG has a resistance per 1,000 feet; multiply by current and length to get the drop. This uses copper values.
Worked example
A 120 V circuit pulling 15 A through 50 ft of 12 AWG copper: resistance is 1.93 ohms per 1,000 ft, so drop is 2 × 15 × 1.93 × 0.05 = 2.90 V. That's 2.41% of 120 V, leaving 117.1 V at the load — within the 3% guideline.
Copper resistance by gauge
| AWG | Ohms / 1000 ft | Typical use |
|---|---|---|
| 14 | 3.07 | 15 A lighting circuits |
| 12 | 1.93 | 20 A general circuits |
| 10 | 1.21 | 30 A dryer, AC |
| 8 | 0.764 | 40-50 A range, subpanel |
| 6 | 0.491 | 55-65 A subpanel feeds |
Tips & gotchas
- Aim for under 3% on branch circuits and under 5% total including the feeder.
- Long runs are the usual culprit — go up a wire size before the distance bites you.
- Voltage drop wastes energy as heat and can shorten motor life.
- Aluminum has higher resistance than copper; this tool uses copper values.
- This is a planning estimate — follow the NEC and a licensed electrician for real installs.
Frequently asked questions
What is an acceptable voltage drop?
The NEC recommends no more than 3% on a branch circuit and 5% for the combined feeder and branch. Beyond that, equipment underperforms and energy is wasted.
How do I reduce voltage drop?
Use a larger wire gauge, shorten the run, or raise the voltage (240 V instead of 120 V). Increasing wire size is the most common fix.
Why double the wire length?
Current flows out to the load and back, so the total conductor length is twice the one-way distance in a single-phase circuit.
Does wire material matter?
Yes. Aluminum has more resistance than copper for the same gauge, so it drops more voltage. This calculator uses copper figures.
Is this accurate enough to wire from?
It's a planning estimate. Always follow the National Electrical Code and have a licensed electrician verify conductor sizing for real work.
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