Voltage Drop Calculator
Estimate voltage drop on a wire run — enter the voltage, current, one-way distance, wire size (AWG) and material to get the voltage dropped and the percentage.
Informational estimate only — not a substitute for a licensed electrician or NEC compliance. It uses standard conductor resistances and a simplified model; actual drop varies with temperature, power factor, conduit and terminations. The NEC recommends keeping voltage drop to about 3% on a branch circuit and 5% total. Verify against the current NEC and have a licensed electrician size conductors.
How to calculate voltage drop
Voltage drop is the voltage “lost” to a conductor’s resistance over a long run. For a single-phase circuit it is estimated as 2 × K × I × L ÷ CM, where K is the conductor’s resistivity (about 12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way distance in feet, and CM is the wire’s area in circular mils (from NEC Chapter 9, Table 8). Three-phase uses 1.732 instead of 2. The calculator looks up the circular mils for your AWG size and shows the drop in volts and percent.
voltage drop = 2 × K × I × length ÷ circular mils (×1.732 for three-phase)
For example, 20 A on 10 AWG copper (10,380 cmil) over a 100 ft one-way run at 120 V drops 2 × 12.9 × 20 × 100 ÷ 10,380 ≈ 4.97 V, or 4.1% — over the recommended 3%, so you would step up to 8 AWG. Going up a wire size or shortening the run lowers the drop.
How much voltage drop is acceptable?
The National Electrical Code recommends (in an informational note — it is guidance, not a hard rule) keeping voltage drop to about 3% on a branch circuit and 5% total from the service to the farthest outlet. Excessive drop dims lights, overheats and slows motors, and wastes energy. Long runs to garages, well pumps, EV chargers and RV hookups are where it matters most.
This is an informational estimate only — not a substitute for a licensed electrician or NEC compliance. Pairing it with conduit? Check the conduit fill calculator.
Frequently asked questions
How do I calculate voltage drop?
For a single-phase run, voltage drop ≈ 2 × K × I × L ÷ CM, where K is the resistivity (about 12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way distance in feet, and CM is the wire’s circular-mil area. The calculator applies this for your AWG size and shows the drop in volts and percent.
How much voltage drop is acceptable?
The NEC recommends (in an informational note, not a hard rule) keeping voltage drop to about 3% on a branch circuit and 5% from the service to the final outlet. More than that can dim lights, slow motors and waste energy. If your run exceeds it, go up a wire size.
How do I reduce voltage drop?
Use a larger conductor (lower AWG number), shorten the run, switch from aluminum to copper, or raise the system voltage. Long runs to outbuildings, well pumps or RV hookups are the usual culprits — upsizing the wire is the normal fix.