Free tool · for electricians, solar installers and DIYers
Voltage Drop Calculator: Find the Right Wire Size for 3%
Single-phase, three-phase or DC. Enter the voltage, load and one-way distance. See the voltage drop, the voltage at the load, and the smallest copper or aluminum wire that keeps the drop within 3% or 5% and can carry the current.
Voltage Drop Calculator
Uses NEC Chapter 9 Table 8 DC resistance at 75°C; single-phase and DC: VD = 2 × I × R × L ÷ 1,000, three-phase: VD = 1.732 × I × R × L ÷ 1,000. For large AC conductors, Table 9 (AC impedance and power factor) gives a slightly more exact result. Ampacity is from Table 310.16 before any adjustment for temperature or bundling.
How Do You Calculate Voltage Drop?
Multiply 2 × amps × the wire’s resistance per 1,000 feet × the one-way length, then divide by 1,000; use 1.732 instead of 2 for three-phase. Twenty amps over 100 feet of 12 AWG copper at 120 volts drops about 7.9 volts, or 6.6%, so 8 AWG is needed to stay within 3%.
Checked against the sources below · How we build and test our calculators
Maximum Wire Length for 3% Voltage Drop
Copper, single-phase, one-way distance in feet, at the full rated load of each size.
| Wire | Load | 120 V | 240 V |
|---|---|---|---|
| 14 AWG | 15 A | 38 ft | 76 ft |
| 12 AWG | 20 A | 45 ft | 90 ft |
| 10 AWG | 30 A | 48 ft | 96 ft |
| 8 AWG | 40 A | 57 ft | 115 ft |
| 6 AWG | 55 A | 66 ft | 133 ft |
Stranded copper resistance at 75°C from NEC Chapter 9 Table 8. At half the load, the distance doubles.
How to Reduce Voltage Drop
- Use a bigger wire: each step up in size cuts resistance by roughly a fifth to a third.
- Run at a higher voltage: the same load on 240 V draws half the current of 120 V.
- Shorten the run or move the panel or subpanel closer to the load.
- Choose copper over aluminum where size or space is tight.
Which Length Do I Enter?
Enter the one-way distance from the panel or source to the load. The formula already doubles it for the out-and-back path in single-phase and DC circuits.
For long feeders to a detached garage, barn or EV charger, check the feeder and branch circuit together against 5%.
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Questions
How do I calculate voltage drop?
For single-phase and DC circuits: VD = 2 × I × R × L ÷ 1,000, where I is the current in amps, R the conductor resistance in ohms per 1,000 ft (NEC Chapter 9, Table 8) and L the one-way length in feet. For three-phase circuits, use 1.732 instead of 2. Divide VD by the supply voltage for the percent.
What is the maximum allowed voltage drop?
The NEC recommends, in informational notes, no more than 3% on a branch circuit or feeder and 5% total from the service to the farthest outlet. These are recommendations for efficiency, not mandatory rules, except where a code section or local amendment requires them.
What size wire do I need for 100 feet at 20 amps?
On a 120-V circuit, 12 AWG copper drops about 7.9 V (6.6%) over 100 ft at 20 A. To stay within 3%, you need 8 AWG copper. At 240 V the same load and distance needs 10 AWG for 3%.
Is voltage drop worse on aluminum wire?
Yes. Aluminum has about 1.6 times the resistance of copper, so you generally need a wire about two sizes larger in aluminum for the same drop.
Why does DC voltage drop matter so much for solar, RV and boat wiring?
At 12 V, a 1-V drop is over 8%, so low-voltage DC circuits need much bigger wire than 120-V circuits for the same power. Many marine and solar guides aim for 3% or less on critical circuits.
Does this check ampacity too?
Yes. The recommended size must both meet your voltage-drop target and carry the load at the 75°C rating in NEC Table 310.16, with the small-conductor limits of 15, 20 and 30 A for 14, 12 and 10 AWG copper. Adjust further for high ambient temperature or more than three current-carrying conductors.