Voltage Drop Calculator
Calculate voltage drop across any wire run. Enter wire size, length, current, and material to ensure your system meets NEC requirements.
Formula & How It Works
VD = (2 × K × I × D) / CM (single phase) | VD = (√3 × K × I × D) / CM (three phase)
K is the resistivity constant: 12.9 for copper and 21.2 for aluminum. I is current in amps, D is one-way distance in feet, and CM is the circular mil area of the wire. The NEC recommends a maximum 3% voltage drop on branch circuits and 5% total from service to load.
Wire Size & Resistance Reference (Copper)
| AWG | Circular Mils | Ohms / 1000 ft | Max Amps (75°C) |
|---|---|---|---|
| 14 AWG | 4,107 | 3.077 | 15A |
| 12 AWG | 6,530 | 1.932 | 20A |
| 10 AWG | 10,380 | 1.215 | 30A |
| 8 AWG | 16,510 | 0.764 | 50A |
| 6 AWG | 26,240 | 0.491 | 65A |
| 4 AWG | 41,740 | 0.308 | 85A |
| 2 AWG | 66,360 | 0.194 | 115A |
| 1/0 AWG | 105,600 | 0.122 | 150A |
| 2/0 AWG | 133,100 | 0.0967 | 175A |
| 4/0 AWG | 211,600 | 0.0608 | 230A |
Voltage Drop Best Practices
- NEC recommendation: Keep voltage drop under 3% on branch circuits, 5% total feeder + branch
- Upsize for long runs: Wire one size larger for runs over 50 feet to minimize losses
- Higher voltage helps: 240V circuits have half the voltage drop percentage of 120V at same current
- Copper vs aluminum: Aluminum has 61% more resistance; upsize by 2 AWG for equivalent performance
- Solar DC runs: Keep array-to-controller runs short or use high-voltage arrays (600V) to reduce wire losses
Frequently Asked Questions
What is an acceptable voltage drop?
The NEC recommends a maximum of 3% voltage drop on branch circuits and 5% total from the service entrance to the load. Excessive voltage drop causes motors to run hotter, lights to dim, and electronics to malfunction. For sensitive equipment, aim for under 2%.
How do I reduce voltage drop on long wire runs?
You can reduce voltage drop by using a larger wire gauge, shortening the run, increasing the system voltage, or using copper instead of aluminum. For solar DC runs, consider using higher-voltage arrays or combiner boxes to reduce current and wire losses.
What is the K value in voltage drop calculations?
K is the resistivity constant of the conductor material at 75°C. For copper, K = 12.9 ohms per circular mil-foot. For aluminum, K = 21.2. These values are used in the standard voltage drop formula: VD = (2 × K × I × D) / CM.
Does voltage drop affect solar panel systems?
Yes. Voltage drop on DC wiring between solar panels and the charge controller or inverter wastes energy and reduces charging efficiency. For solar systems, keep voltage drop under 2% on DC runs and under 1.5% on AC output runs. Use larger wire or higher-voltage series strings for long runs.
Single phase vs three phase voltage drop - what is the difference?
Single-phase voltage drop uses a factor of 2 (current travels out and back). Three-phase uses a factor of √3 (approximately 1.732) because the three phases partially cancel each other. Three-phase systems have roughly 13% less voltage drop than single-phase at the same current and distance.