This voltage drop calculator works out how much voltage is lost along a cable run, using conductor resistance from NEC Chapter 9, Table 8. It handles single-phase, three-phase and DC circuits, copper and aluminum wire, and suggests the smallest wire size that keeps the drop at or below 3%.
Voltage drop formulas
Single-phase and DC: VD = 2 × L × I × R ÷ 1,000
Three-phase: VD = 1.732 × L × I × R ÷ 1,000
L is the one-way length in feet, I is the current in amps, and R is the conductor resistance in ohms per 1,000 ft. The factor of 2 accounts for current flowing out and back; three-phase uses √3 instead.
Conductor resistance (NEC Chapter 9, Table 8, 75°C)
| Size | Copper (Ω/1,000 ft) | Aluminum (Ω/1,000 ft) |
|---|---|---|
| 14 AWG | 3.07 | 5.04 |
| 12 AWG | 1.93 | 3.17 |
| 10 AWG | 1.21 | 1.99 |
| 8 AWG | 0.778 | 1.28 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
| 2 AWG | 0.194 | 0.319 |
| 1/0 AWG | 0.122 | 0.201 |
| 2/0 AWG | 0.0967 | 0.159 |
| 3/0 AWG | 0.0766 | 0.126 |
| 4/0 AWG | 0.0608 | 0.100 |
Worked example
A 120 V single-phase circuit carries 16 A over a 100 ft one-way run of 12 AWG copper. The drop is 2 × 100 × 16 × 1.93 ÷ 1,000 = 6.18 V, or 5.15%. Moving to 10 AWG cuts it to 3.23%, and 8 AWG brings it down to 2.07%, inside the 3% recommendation.
How much voltage drop is acceptable?
An informational note in the NEC recommends no more than 3% on a branch circuit and 5% in total for feeders and branch circuits together. Too much drop makes motors run hot, lights dim and electronics misbehave.
Frequently asked questions
What is the maximum allowed voltage drop?
The NEC recommends, in an informational note, no more than 3% on a branch circuit and 5% combined for feeder and branch circuit. Some local codes and specifications make this mandatory.
Should I use one-way or round-trip length?
Enter the one-way length from the panel to the load. The formula doubles it for single-phase and DC circuits.
Why is aluminum wire larger for the same job?
Aluminum has about 60% more resistance than copper of the same size, so it needs a larger gauge to carry the same current with the same voltage drop.
Does a larger wire mean I can use a bigger breaker?
No. Breaker size is set by the circuit and the ampacity rules. Upsizing wire for voltage drop does not change the breaker rating for the load.
Does this include power factor or reactance?
No. It uses DC resistance, which is accurate for most building circuits up to about 1/0 AWG. Large conductors on AC have extra reactance; NEC Table 9 values are used for those.
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Last reviewed: October 2026.