How to use the voltage drop calculator
- Pick single-phase/DC or three-phase and enter the source voltage.
- Enter the load current and the one-way length from the panel to the load.
- Choose copper or aluminum and the wire size. Use parallel sets for large feeders.
- Read the drop in volts and percent. If it's over your target, the calculator suggests the smallest size that passes.
Voltage drop formula
Single-phase / DC: VD = 2 × I × R × L ÷ 1000
Three-phase: VD = 1.732 × I × R × L ÷ 1000
VD % = VD ÷ source voltage × 100
Three-phase: VD = 1.732 × I × R × L ÷ 1000
VD % = VD ÷ source voltage × 100
R is the conductor resistance in ohms per 1,000 feet from NEC Chapter 9, Table 8 (for example 1.98 Ω for 12 AWG stranded copper at 75°C). L is the one-way length in feet.
Worked example
A 20 A load, 100 feet from the panel, on 12 AWG copper at 120 V single-phase: VD = 2 × 20 × 1.98 × 100 ÷ 1000 = 7.92 V, or 6.6%. That's more than double the 3% guideline, so the calculator recommends upsizing. With 8 AWG (0.778 Ω) the drop falls to 3.11 V — 2.6%.
Voltage drop table: copper, 120 V single-phase, 20 A
| Wire | Ω/1000 ft | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|---|
| 14 AWG | 3.14 | 5.2% | 10.5% | 15.7% | 20.9% |
| 12 AWG | 1.98 | 3.3% | 6.6% | 9.9% | 13.2% |
| 10 AWG | 1.24 | 2.1% | 4.1% | 6.2% | 8.3% |
| 8 AWG | 0.778 | 1.3% | 2.6% | 3.9% | 5.2% |
| 6 AWG | 0.491 | 0.8% | 1.6% | 2.5% | 3.3% |
| 4 AWG | 0.308 | 0.5% | 1.0% | 1.5% | 2.1% |
| 3 AWG | 0.245 | 0.4% | 0.8% | 1.2% | 1.6% |
| 2 AWG | 0.194 | 0.3% | 0.6% | 1.0% | 1.3% |
| 1 AWG | 0.154 | 0.3% | 0.5% | 0.8% | 1.0% |
| 1/0 AWG | 0.122 | 0.2% | 0.4% | 0.6% | 0.8% |
| 2/0 AWG | 0.0967 | 0.2% | 0.3% | 0.5% | 0.6% |
| 3/0 AWG | 0.0766 | 0.1% | 0.3% | 0.4% | 0.5% |
| 4/0 AWG | 0.0608 | 0.1% | 0.2% | 0.3% | 0.4% |
| 250 kcmil | 0.0515 | 0.1% | 0.2% | 0.3% | 0.3% |
| 350 kcmil | 0.0367 | 0.1% | 0.1% | 0.2% | 0.2% |
| 500 kcmil | 0.0258 | 0.0% | 0.1% | 0.1% | 0.2% |
Conductor resistance (NEC Table 8, 75°C)
| Size | Circular mils | Copper Ω/kft | Aluminum Ω/kft |
|---|---|---|---|
| 14 AWG | 4,110 | 3.14 | 5.17 |
| 12 AWG | 6,530 | 1.98 | 3.25 |
| 10 AWG | 10,380 | 1.24 | 2.04 |
| 8 AWG | 16,510 | 0.778 | 1.28 |
| 6 AWG | 26,240 | 0.491 | 0.808 |
| 4 AWG | 41,740 | 0.308 | 0.508 |
| 3 AWG | 52,620 | 0.245 | 0.403 |
| 2 AWG | 66,360 | 0.194 | 0.319 |
| 1 AWG | 83,690 | 0.154 | 0.253 |
| 1/0 AWG | 105,600 | 0.122 | 0.201 |
| 2/0 AWG | 133,100 | 0.0967 | 0.159 |
| 3/0 AWG | 167,800 | 0.0766 | 0.126 |
| 4/0 AWG | 211,600 | 0.0608 | 0.1 |
| 250 kcmil | 250,000 | 0.0515 | 0.0847 |
| 350 kcmil | 350,000 | 0.0367 | 0.0605 |
| 500 kcmil | 500,000 | 0.0258 | 0.0424 |
Ways to reduce voltage drop
- Use a larger conductor — each three-AWG step roughly halves resistance.
- Raise the voltage: the same load at 240 V draws half the current and loses a quarter of the percentage.
- Shorten the route or move the panel closer to the load.
- Run conductors in parallel for big feeders (1/0 and larger, NEC 310.10(G)).
Bigger wire takes more room, so re-check your raceway with the conduit fill calculator and confirm ampacity with the wire size calculator.