How to use it
- Enter the one-way wire length in feet and the load current.
- Pick the wire size (AWG), conductor and system.
- Enter the voltage and the maximum drop you allow.
- Read the drop, the voltage at the load and the recommended size.
Why voltage drop matters
Every wire has resistance, so some voltage is lost along the run. Too much drop means motors run hot, lights dim and electronics misbehave. Long runs to a detached garage, a well pump or landscape lighting are the usual trouble spots.
The calculator estimates the drop in volts and percent, the voltage left at the load, the resistance of one conductor and the power lost as heat. It also finds the smallest AWG size that keeps the drop under your limit. Everything is calculated in your browser.
Formulas
Single-phase: VD = 2 × L × I × ρ / A × cos φ
Three-phase: VD = √3 × L × I × ρ / A × cos φ
- L = one-way length, I = current in amps
- ρ = resistivity: about 0.0175 Ω·mm²/m for copper and 0.0286 for aluminum
- A = conductor cross-section in mm² (the calculator converts AWG for you)
- cos φ = power factor, 1 for resistive loads such as heaters
The factor 2 accounts for current flowing out and back on two conductors in a single-phase circuit.
Worked example
A 15 A load at the end of a 100 ft run on a 120 V single-phase circuit:
| Wire | Drop | Percent |
|---|---|---|
| 12 AWG copper | about 4.8 V | about 4.0% |
| 10 AWG copper | about 3.0 V | about 2.5% |
If you want to stay at or below 3%, 12 AWG is not enough for this run, and the calculator suggests 10 AWG. Note that conductors must also meet ampacity and overcurrent protection rules, which are separate from voltage drop.
Choosing 120 V or 240 V
For the same power, a 240 V circuit carries half the current of a 120 V circuit, so the voltage drop in volts is halved and, as a percentage of the higher voltage, it is only a quarter as much. That is why well pumps, EV chargers and subpanels in outbuildings often run on 240 V. Enter both voltages with the matching current to compare.
AWG wire sizes
| AWG | Area (mm²) |
|---|---|
| 14 | 2.08 |
| 12 | 3.31 |
| 10 | 5.26 |
| 8 | 8.37 |
| 6 | 13.3 |
| 4 | 21.2 |
| 2 | 33.6 |
| 1/0 | 53.5 |
| 4/0 | 107.2 |
Smaller AWG numbers mean thicker wire. Each step of three gauges roughly doubles the cross-section and halves the resistance.
Ohm's law
The second tab solves V = I × R. Enter any two of voltage, current and resistance, leave the third blank, and the calculator finds it, plus power (P = V × I).
Example: a 60 Ω heating element on 120 V draws 120 / 60 = 2 A and uses 120 × 2 = 240 W.
Limits of this estimate
The National Electrical Code includes informational notes suggesting about 3% maximum drop on a branch circuit and 5% on the feeder and branch combined; these are recommendations rather than requirements, and local rules may differ. The calculation uses resistivity at about 20 °C and ignores wire reactance and heating, so real drops in hot conduit are somewhat higher.
This tool is for estimates. Wire sizing, ampacity and protection must follow the NEC and your local code; have a licensed electrician design and inspect the installation.
Frequently asked questions
What is an acceptable voltage drop?
NEC informational notes suggest about 3% on a branch circuit and 5% total, but follow your local code and the equipment's requirements.
Do I enter the one-way or round-trip length?
The one-way length from the panel to the load. The formula accounts for the return conductor.
How do I reduce voltage drop?
Use a larger wire (smaller AWG number), shorten the run, or use a higher voltage such as 240 V.
Is aluminum wire different?
Aluminum has higher resistivity than copper, so it needs a larger size for the same drop.
How do I use Ohm's law here?
Switch to the Ohm's law tab, enter two of voltage, current and resistance, and leave the third empty.
Last updated: November 12, 2026