Why do power tools slow down on a long, thin extension cord? Wire resists current, so some voltage is lost on the way. The Voltage Drop Calculator shows how many volts a copper wire run loses at a given current.
It is built for homeowners, students, and installers checking a feeder to a garage, shed, or pump. You pick a wire gauge, enter the one-way distance, and read three results.
Table of Contents
- What the Calculator Does
- How to Use It, Step by Step
- The Voltage Drop Formula
- Worked Example
- How to Read the Results
- What Changes Voltage Drop
- Limitations and Assumptions
- FAQs About the Voltage Drop Calculator
What the Calculator Does
The tool models a two-wire copper circuit. It uses a fixed resistance for each wire gauge and gives three results.
- Voltage Drop (Volts): the volts lost along the wire, shown with two decimals.
- Voltage Loss Percentage: the drop as a share of the source voltage.
- Voltage at Load Terminal: the source voltage minus the drop.
- A Calculate Voltage Drop button. Results also update as you type, change the gauge, or press Enter.
How to Use It, Step by Step
- Enter Source Voltage (V), from 1 to 1,000. The sample value is 120.
- Enter Current Draw (Amps), from 0.1 to 1,000. The sample value is 15.
- Choose Wire Gauge (AWG). Options run from 14 AWG to 2 AWG copper. The sample is 12 AWG.
- Enter One-Way Wire Distance (feet). Allowed values run from 1 to 10,000. Use the distance from panel to load, not the round trip. The sample is 100 feet.
- Read the three result cards, or press Calculate Voltage Drop to refresh them.
The Voltage Drop Formula
Current flows out on one wire and back on the other. So the tool doubles your one-way distance. It then multiplies by the current and the wire resistance per 1,000 feet.
2×Distance (ft)×Current (A)×Ohms per 1,000 ft÷1,000=Voltage Drop
- Voltage drop (V) = 2 × distance × current × resistance per 1,000 ft ÷ 1,000.
- Loss percentage = voltage drop ÷ source voltage × 100.
- Load voltage = source voltage minus voltage drop.
- Resistance, in ohms per 1,000 feet: 14 AWG 3.07, 12 AWG 1.93, 10 AWG 1.21, 8 AWG 0.764, 6 AWG 0.491, 4 AWG 0.308, 2 AWG 0.194.
Worked Example
Priya is running a 240 V circuit to a well pump. The pump draws 30 amps. The wire is 10 AWG copper, and the pump sits 80 feet from the panel.
| Result | Calculation | Output |
|---|
| Voltage Drop | 2 × 80 × 30 × 1.21 ÷ 1,000 | 5.81 V |
| Voltage Loss Percentage | 5.808 ÷ 240 × 100 | 2.42% |
| Voltage at Load Terminal | 240 - 5.808 | 234.19 V |
Now take the sample inputs: 120 V, 15 A, 12 AWG, and 100 feet. The tool shows 5.79 V, 4.83%, and 114.21 V. Switching the same run to 10 AWG cuts the drop to 3.63 V, or 3.03%.
How to Read the Results
Voltage Drop is the voltage lost along the wire. The lost power turns into heat, so a larger drop means a warmer wire and less power for the load.
Voltage Loss Percentage is the easiest number to judge. A common guideline keeps a branch circuit near 3% or lower. The tool does not color or flag a high percentage, so you compare it yourself.
Voltage at Load Terminal is what the device actually receives. Motors and compressors can run hot or stall when this number sits too low.
What Changes Voltage Drop
Four things move the result: distance, current, wire gauge, and source voltage. Doubling the distance or the current doubles the drop in volts. A thicker wire has lower resistance, so the drop falls.
Higher voltage helps in percentage terms. The same 3,600 watt load draws half the current at 240 V as at 120 V. That cuts the volt drop in half and the percentage to a quarter.
To pick a gauge for a new run, try the wire size calculator. For the basics behind volts, amps, and ohms, see the Ohm's law calculator. To turn amps into watts, use the watt calculator.
Backup power adds its own wiring runs. The generator size calculator and the battery backup calculator help you size the source. You can browse more tools in the math and science calculators hub.
Limitations and Assumptions
- The tool covers copper wire only, in seven gauges from 14 AWG to 2 AWG. It has no aluminum option.
- Resistance values are fixed. Wire temperature, stranding, and coating are not modeled.
- The math assumes a two-wire single-phase circuit. It has no three-phase option and no reactance for long AC runs.
- It does not check wire ampacity. A gauge can show a small drop and still be too small for the current.
- Blank, zero, negative, or out of range entries show a message, and the results read N/A. Nothing is assumed for you.
- Local electrical codes apply, and a licensed electrician should do or approve any wiring.
FAQs About the Voltage Drop Calculator
What is a safe voltage drop percentage?
A common guideline keeps a branch circuit at about 3% or lower. Feeders and branch circuits together are often held near 5%. Your local code and the equipment maker have the final say.
Why does the tool double the distance?
Current needs a full loop. It travels out on one wire and returns on the other. So the wire length that counts is twice the one-way distance.
How do I reduce voltage drop?
Use a thicker wire, shorten the run, or lower the current. Raising the circuit voltage also helps, since the same load then draws fewer amps.
Does the tool work for DC circuits?
The math is plain resistance, so a two-wire DC run follows the same formula. Enter the DC voltage and current. The tool has no AC or DC switch, and its note says single-phase or DC.
Why do the percent and load voltage show N/A?
The drop has reached the source voltage, so the wire cannot carry that load. A message appears, and only the drop in volts is shown. Try 12 V, 50 A, 14 AWG, and 500 feet to see it.
Can I use meters instead of feet?
No. Distance is in feet only, from 1 to 10,000, and the resistance table is per 1,000 feet. Convert meters to feet first by multiplying by 3.281.
This calculator gives general estimates only and is not professional advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.