Voltage Drop Calculator
Enter the circuit voltage, load current, one-way length and wire size. A 30 amp load on 100 feet of 10 AWG copper at 240 volts loses 7.46 V, a drop of 3.11%.
How the voltage drop calculator works
Wire has resistance, so some voltage is lost between the source and the load. The longer the run and the higher the current, the bigger the drop. Thicker wire (a lower AWG number) lowers it. This calculator uses the common approximate formula with a resistivity constant K of 12.9 ohm-circular-mils per foot for copper and 21.2 for aluminum.
The formulas
Example: 30 amps on 100 feet of 10 AWG copper (10,380 circular mils) at 240 volts: 2 x 12.9 x 30 x 100 / 10,380 = 7.46 V. That is 3.11% of 240 volts, leaving 232.5 V at the load, and about 224 W lost as heat.
Notes and limits
- A drop of about 3 percent or less is a commonly cited design goal for branch circuits. Your local code and the equipment maker set the real requirements.
- Results are estimates at about 75 C. Temperature, conduit and conductor type change resistance.
- Do not use this to size wire for safety on its own. Wire must also carry the current safely. Ask a licensed electrician.
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Frequently asked questions
What is a good voltage drop?
About 3 percent or less on a branch circuit is a commonly cited goal. Follow local code and equipment instructions.
How do I reduce voltage drop?
Use a larger wire (lower AWG number), shorten the run or raise the system voltage.
Is the length one-way or round trip?
Enter the one-way length. The formula already doubles it for single-phase circuits.
Why is aluminum different?
Aluminum has higher resistivity than copper, so it drops more voltage at the same size.
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