What voltage drop is
Every wire has some resistance, so part of the supply voltage is used up pushing current through the wire itself before it reaches the load. That loss is the voltage drop. It grows with the current, the length of the run and the resistance of the wire, and it shrinks when you use a thicker gauge. Too much drop makes motors and compressors run hot, lights dim and electronics misbehave, and the lost energy becomes heat in the wire.
Single-phase or DC: VD = 2 × L × R × I ÷ 1,000
Three-phase: VD = 1.732 × L × R × I ÷ 1,000
Percent drop = VD ÷ supply voltage × 100
L is the one-way length in feet, I the current in amps and R the conductor's resistance in ohms per 1,000 ft. The factor 2 counts the trip out and back on a two-wire circuit; three-phase circuits use √3 ≈ 1.732.
Resistance values used
| AWG | Copper (Ω per 1,000 ft) | Aluminum (Ω per 1,000 ft) |
|---|---|---|
| 14 | 3.14 | – |
| 12 | 1.98 | 3.25 |
| 10 | 1.24 | 2.04 |
| 8 | 0.778 | 1.28 |
| 6 | 0.491 | 0.808 |
| 4 | 0.308 | 0.508 |
| 2 | 0.194 | 0.319 |
| 1/0 | 0.122 | 0.201 |
| 2/0 | 0.0967 | 0.159 |
| 4/0 | 0.0608 | 0.100 |
These are the DC resistances of stranded, uncoated conductors at 75 °C from NEC Chapter 9, Table 8. For ordinary household circuits at 60 Hz the AC result is very close; for large conductors or poor power factor, an engineer would use the AC impedance in Table 9.
Worked example 1: a 120 V circuit 100 ft long
- 16 A on 12 AWG copper, 100 ft one way: VD = 2 × 100 × 1.98 × 16 ÷ 1,000 = 6.34 V.
- Percent drop = 6.34 ÷ 120 = 5.28%: only about 113.7 V reaches the load, above the 3% recommendation.
- 10 AWG gives 3.97 V (3.31%) and 8 AWG gives 2.49 V (2.07%), the smallest copper size within 3%.
Worked example 2: three-phase 208 V
30 A on 8 AWG copper, 200 ft one way: VD = 1.732 × 200 × 0.778 × 30 ÷ 1,000 = 8.09 V, or 3.89% of 208 V. Moving up to 6 AWG brings it to 5.10 V (2.45%).
How much voltage drop is acceptable?
Informational notes in the National Electrical Code suggest no more than 3% on a branch circuit and no more than 5% overall for feeder plus branch circuit. These notes are recommendations for efficiency rather than enforceable rules in most jurisdictions, but some equipment instructions and local codes require them. Low-voltage DC systems such as 12 V solar, RV and boat wiring are much more sensitive: a 0.6 V loss is already 5% of 12 V.
Ways to reduce voltage drop
- Use a larger wire. Each step of about three AWG sizes halves the resistance.
- Raise the voltage. The same power at 240 V draws half the current of 120 V, so the percentage drop falls to a quarter.
- Shorten the run by moving the panel, adding a subpanel or choosing a more direct route.
- Choose copper over aluminum at the same gauge, or go up about two sizes in aluminum.
Common mistakes
- Doubling the length twice. Enter the one-way distance: the formula already counts the return path.
- Using the breaker size instead of the real load when checking drop on a lightly loaded circuit; the drop depends on the current that actually flows.
- Forgetting the ampacity check. A wire with low drop can still be too small for the current. Use the wire size calculator to check both.
Frequently asked questions
How do you calculate voltage drop?
For single-phase or DC: VD = 2 × one-way length (ft) × resistance (Ω per 1,000 ft) × current (A) ÷ 1,000. For three-phase, replace 2 with 1.732.
What is the maximum voltage drop allowed by the NEC?
The NEC's informational notes recommend 3% for a branch circuit and 5% total for feeder plus branch circuit. They are recommendations in most places, not mandatory limits.
What is the voltage drop for 12 gauge wire at 100 feet?
At 16 A and 120 V, 12 AWG copper over 100 ft one way drops about 6.3 V, or 5.3%. At 10 A it drops about 4 V, or 3.3%.
Does voltage drop depend on the voltage?
The volts lost depend only on current, length and wire, but the percentage depends on the supply. The same 6 V drop is 5% at 120 V but only 2.5% at 240 V.
Is aluminum wire worse for voltage drop?
Aluminum has about 1.6 times the resistance of copper at the same gauge, so it drops more voltage. Use roughly two sizes larger in aluminum.
Sources
- NFPA 70, National Electrical Code (2023), Chapter 9, Table 8: Conductor properties (DC resistance at 75 °C).
- NFPA 70, informational notes to 210.19(A) and 215.2(A) on voltage drop for branch circuits and feeders.
Last reviewed 31 October 2026. Results are estimates; see our disclaimer.