Voltage Drop Calculator
Updated Formulas shown
Pick the wire, the load and the distance to see how many volts are lost in the conductors and what voltage reaches the load. A 16 A load on 100 ft of 12 AWG copper at 120 V drops 6.34 V (5.3%) — over the recommended 3% for a branch circuit. 10 AWG still loses 3.3%, so this run needs 8 AWG.
- Formula
- VD = 2 × L × R × I ÷ 1000L one-way ft, R Ω/1000 ft (NEC Ch. 9 Table 8)
Uses DC resistance, which slightly understates drop on large AC conductors with low power factor. Estimate only — confirm with a licensed electrician.
How to calculate voltage drop
- Choose copper or aluminum and the system: single-phase for 120/240 V homes, three-phase for commercial 208/480 V, or DC for solar, batteries, RVs and low-voltage lighting.
- Enter the voltage, the load current in amps and the one-way distance from source to load.
- Select the wire size. Use parallel sets only for large feeders with multiple conductors per phase.
- Compare the result with the 3% (branch circuit) and 5% (feeder plus branch) guidance. If it's over, try the suggested size or check the maximum run length.
Voltage drop formula
- Single-phase and DC: VD = 2 × L × R × I ÷ 1,000
- Three-phase: VD = √3 × L × R × I ÷ 1,000 (√3 ≈ 1.732)
- Percent drop = VD ÷ source voltage × 100
L is the one-way length in feet, I the current in amps and R the conductor resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8 (75 °C). The factor 2 accounts for current flowing out and back. Example: 12 AWG copper (1.98 Ω/kft), 20 A, 100 ft at 120 V → 2 × 100 × 1.98 × 20 ÷ 1,000 = 7.92 V, 6.6%.
Wire resistance table
| Size | Copper Ω/1000 ft | Aluminum Ω/1000 ft | Max run (Cu, 3%, 120 V) |
|---|---|---|---|
| 14 AWG | 3.14 | — | 38 ft at 15 A |
| 12 AWG | 1.98 | 3.25 | 45 ft at 20 A |
| 10 AWG | 1.24 | 2.04 | 48 ft at 30 A |
| 8 AWG | 0.778 | 1.28 | 58 ft at 40 A |
| 6 AWG | 0.491 | 0.808 | 67 ft at 55 A |
| 4 AWG | 0.308 | 0.508 | 83 ft at 70 A |
| 2 AWG | 0.194 | 0.319 | 98 ft at 95 A |
| 1/0 AWG | 0.122 | 0.201 | 118 ft at 125 A |
| 2/0 AWG | 0.0967 | 0.159 | 128 ft at 145 A |
| 4/0 AWG | 0.0608 | 0.1 | 152 ft at 195 A |
How much voltage drop is acceptable?
The NEC recommends — in an informational note to 210.19(A), not a mandatory rule — no more than 3% on a branch circuit and 5% total for feeder plus branch circuit. Some situations make it mandatory or stricter: fire pumps, sensitive electronics, and many solar designs aim for 1–2% on DC strings. Too much drop makes motors run hot, LED and halogen lights dim, and chargers slow down.
Low-voltage systems suffer most: a 3% drop at 12 V is only 0.36 V, so 12 V landscape lighting and RV runs need much heavier wire than a 120 V circuit carrying the same power.
Safety note: results are estimates using DC resistance. Consult a licensed electrician for design and installation.
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Frequently asked questions
What is an acceptable voltage drop?
3% or less on a branch circuit and 5% or less in total from the service to the farthest outlet, per the NEC's informational note in 210.19(A). On 120 V that's 3.6 V and 6 V.
How do I reduce voltage drop?
Use a larger wire (lower AWG number), shorten the run, raise the voltage (240 V instead of 120 V halves the current for the same power), or switch from aluminum to copper.
How far can I run 12 gauge wire on a 20 amp circuit?
At a full 20 A load on 120 V, about 45 ft before voltage drop exceeds 3%. At a more typical 16 A load, about 57 ft. Use 10 AWG for longer runs.
Does voltage drop use one-way or round-trip distance?
Enter the one-way distance. The single-phase formula multiplies by 2 to include the return conductor; three-phase uses √3.
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Estimates only. Check quantities with your supplier or contractor before ordering.