Voltage Drop Calculator
Enter the current, the one-way run length and the wire size to see the voltage drop over a cable, the percentage lost, and whether it clears the usual 3% branch and 5% total limits.
This free voltage drop calculator shows how much voltage a wire loses over its run. Set the system voltage, the load current in amps, the one-way length, the conductor material and the AWG size, and it returns the cable voltage drop in volts, the percentage, and the voltage left at the load. Switch between DC, single-phase and three-phase to match your circuit. Use it as a quick wire voltage drop check before you pull a long run or size up a conductor.
Wire run
Voltage drop
4 V
3.33% of 120 V
- Voltage at load
- 116 V
- Percent drop
- 3.33%
- Branch limit (3%)
- Over
Using 1 Ω/1000 ft for copper AWG 10. Over the 3% branch guideline but within 5% total. Consider a thicker wire.
A planning estimate using DC resistance at room temperature. Real drop shifts with conductor temperature, power factor and conduit type. Follow the NEC and your local code for final sizing.
How it works
- 1
Enter the circuit
Type the system voltage and the load current in amps, then pick DC, single-phase or three-phase.
- 2
Set the run and wire
Enter the one-way distance from the source to the load and choose the conductor material and AWG size.
- 3
Read the drop
See the volts lost, the percentage drop, the voltage at the load and a pass or fail against the 3% branch limit.
Instant & 100% private — nothing is uploaded
Every calculation runs locally in your browser. The income, balances and goals you enter stay on your own device and are never sent to a server — nothing is stored, logged or shared.
Frequently asked questions
- How do I calculate voltage drop on a wire?
- Multiply the current by the conductor resistance per foot and by the run length, then double it for the return conductor on DC and single-phase, or use √3 for three-phase. This vdrop calculator does that from the AWG resistance table and shows the result as both volts and a percentage.
- What is an acceptable voltage drop?
- The common rule of thumb is 3% or less on a branch circuit and 5% or less from the service to the far end of the load. The calculator flags whether your run clears the 3% mark, which is where most electricians draw the line.
- Do I enter the one-way length or the round trip?
- Enter the one-way distance from the source to the load. The formula already accounts for the return conductor, so you do not double the length yourself.
- Why does aluminum drop more than copper?
- Aluminum has about 1.64 times the resistance of copper for the same wire size, so an aluminum run of the same gauge and length loses more voltage. That is why aluminum feeders are often one or two sizes larger than the copper equivalent.
- How do I reduce voltage drop on a long run?
- Step up to a thicker wire (a lower AWG number), shorten the run, raise the system voltage, or split the load. Increasing the wire size is usually the practical fix; drop a size or two and watch the percentage fall.
- How accurate is this cable voltage drop estimate?
- It uses DC resistance at room temperature, which is close for most residential and light commercial runs. Hot conductors, poor power factor and conduit type all move the real figure, so treat it as a planning number and follow the NEC and your local code for final sizing.
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