Voltage drop calculator
Voltage drop in volts and percent for copper or aluminum wire on single-phase, three-phase and DC circuits.
Voltage drop
7.92 volts (6.60%)
- Voltage at the load
- 112.1 V
- Wire resistance
- 1.98 Ω per 1,000 ft
- Longest run within 3%
- 45 ft
- Smallest size within 3% (drop only)
- 8 AWG
- Over your 3% target: use a larger wire or a shorter run
Voltage drop is only one part of sizing wire
The wire must also be rated to carry the current (its ampacity, NEC Table 310.16), matched to the breaker, and suited to the insulation, temperature and installation. A size that keeps voltage drop low can still be too small to carry the load safely. Follow your local electrical code, and have a licensed electrician design and check the circuit.
Voltage drop formula
Current flowing through wire loses voltage to the wire's resistance. The calculator uses the resistance of the conductor from NEC Chapter 9, Table 8 (direct-current resistance at 75 °C).
- Single-phase and DC: VD = 2 × I × R × L ÷ 1,000 (the current goes out and back)
- Three-phase: VD = 1.732 × I × R × L ÷ 1,000
- Percent drop = VD ÷ source voltage × 100
I is the current in amps, R the resistance in ohms per 1,000 feet, and L the one-way length in feet.
Worked example
A 20 A load on 100 ft of 12 AWG copper at 120 V: R = 1.98 Ω per 1,000 ft, so VD = 2 × 20 × 1.98 × 100 ÷ 1,000 = 7.92 V, or 7.92 ÷ 120 = 6.6%. That's over the 3% guideline; 8 AWG copper would keep it within.
Results are estimates. Resistance rises with temperature, and for large conductors on AC the effective resistance in NEC Chapter 9, Table 9 is a little different.
Wire resistance and voltage drop chart
Resistance per 1,000 feet from NEC Chapter 9, Table 8 (stranded, 75 °C), and the longest one-way run that stays within 3% at 120 V and 15 A (voltage drop only).
| Size | Copper Ω/1,000 ft | Aluminum Ω/1,000 ft | Max run, copper | Max run, aluminum |
|---|---|---|---|---|
| 14 AWG | 3.14 | 5.17 | 38 ft | 23 ft |
| 12 AWG | 1.98 | 3.25 | 60 ft | 36 ft |
| 10 AWG | 1.24 | 2.04 | 96 ft | 58 ft |
| 8 AWG | 0.778 | 1.28 | 154 ft | 93 ft |
| 6 AWG | 0.491 | 0.808 | 244 ft | 148 ft |
| 4 AWG | 0.308 | 0.508 | 389 ft | 236 ft |
| 3 AWG | 0.245 | 0.403 | 489 ft | 297 ft |
| 2 AWG | 0.194 | 0.319 | 618 ft | 376 ft |
| 1 AWG | 0.154 | 0.253 | 779 ft | 474 ft |
| 1/0 AWG | 0.122 | 0.201 | 983 ft | 597 ft |
| 2/0 AWG | 0.0967 | 0.159 | 1,240 ft | 754 ft |
| 3/0 AWG | 0.0766 | 0.126 | 1,566 ft | 952 ft |
| 4/0 AWG | 0.0608 | 0.1 | 1,973 ft | 1,199 ft |
| 250 kcmil | 0.0515 | 0.0847 | 2,330 ft | 1,416 ft |
| 300 kcmil | 0.0429 | 0.0707 | 2,797 ft | 1,697 ft |
| 350 kcmil | 0.0367 | 0.0605 | 3,269 ft | 1,983 ft |
| 400 kcmil | 0.0321 | 0.0529 | 3,738 ft | 2,268 ft |
| 500 kcmil | 0.0258 | 0.0424 | 4,651 ft | 2,830 ft |
Voltage drop questions
How much voltage drop is acceptable?
Informational notes to NEC 210.19(A) and 215.2(A) recommend sizing conductors so the drop is no more than 3% on a branch circuit and 5% for the feeder and branch circuit together. These are recommendations, not requirements, but following them keeps motors, lights and electronics working properly. Some equipment makers ask for less.
How do I calculate voltage drop?
Multiply 2 × current × resistance per 1,000 feet × one-way length, then divide by 1,000. For three-phase circuits use 1.732 instead of 2. The worked example above shows each step.
What is the three-phase voltage drop formula?
VD = 1.732 × I × R × L ÷ 1,000. For example, 100 A on 200 ft of 1/0 AWG aluminum at 480 V drops 6.96 V, or 1.45%.
How do I calculate DC voltage drop?
The same way as single-phase: the current runs out and back, so use 2 × I × R × L ÷ 1,000. Low-voltage circuits feel it most. 10 A on 25 ft of 10 AWG copper at 12 V drops 0.62 V, which is 5.2%, so batteries and solar runs usually need heavier wire than the current alone suggests.
What causes voltage drop?
The wire's resistance. Drop goes up with longer runs, thinner wire and more current. Aluminum resists about 1.6 times as much as copper of the same size, so it drops more.
Does this tell me what wire size I need?
Only for voltage drop. The wire also has to be rated for the current (NEC Table 310.16) and matched to the breaker, which often calls for a larger size. A wire size calculator is coming soon; until then, have a licensed electrician confirm the size.