Voltage Drop Calculator
Calculate voltage drop and percent drop on a copper circuit from voltage, current, run length, and wire gauge. Flags runs over the 3% recommended limit.
How this calculator works
Voltage drop is the voltage lost to the resistance of the wire between the panel and the load. It grows with current and distance, and shrinks with a thicker conductor. The calculator uses the standard circular-mil formula for a single-phase copper circuit.
Formula: Voltage drop = (2 × 12.9 × length ft × amps) ÷ circular mils of the wire. Percent drop = drop ÷ voltage × 100.
Worked example
A 120V circuit, 20A load, 100-foot one-way run, 12 AWG copper (6,530 circular mils):
- Voltage drop: (2 × 12.9 × 100 × 20) ÷ 6,530 ≈ 7.90 V
- Percent drop: 7.90 ÷ 120 ≈ 6.58%
- Voltage at the load: 120 − 7.90 ≈ 112.10 V
- Over the 3% limit → upsize the wire
Reference (120V, 20A copper)
| Wire | 50 ft | 100 ft |
|---|---|---|
| 14 AWG | 5.2% | 10.5% |
| 12 AWG | 3.3% | 6.6% |
| 10 AWG | 2.1% | 4.1% |
| 8 AWG | 1.3% | 2.6% |
How to use
Measure the one-way run length, note the circuit voltage and load current, and enter your wire gauge. If the drop exceeds 3%, use the Wire Size Calculator to find the gauge that keeps it in range.
Circuit sizing is code-governed and safety-critical. Confirm conductor sizing with a licensed electrician and your local code.
Frequently asked questions
What is an acceptable voltage drop?
The NEC recommends no more than 3% drop on a branch circuit, and no more than 5% total from the service to the outlet. Beyond that, lights dim, motors run hot, and electronics misbehave. This calculator flags whether your run stays under 3%.
Why does a long run drop more voltage?
Voltage drop is proportional to distance: the further electricity travels through wire, the more it loses to the wire's resistance. Doubling the run length doubles the drop. That's why a gauge that's fine at 25 feet can fail at 100 feet.
Do I enter one-way or round-trip length?
Enter the one-way distance from the panel to the load. The formula already doubles it to account for the return conductor, so you don't add the return trip yourself.
Is this for copper or aluminum wire?
This calculator uses copper, the most common branch-circuit conductor. Aluminum has higher resistance (about 60% more), so an aluminum run of the same size and length drops more voltage — size up if you're using aluminum.
How do I fix excessive voltage drop?
Use a thicker wire (lower AWG number), shorten the run, or raise the voltage (240V drops half the percent of 120V for the same load). The Wire Size Calculator finds the minimum gauge that keeps you under your limit.
Estimates only. Verify quantities with your supplier before purchasing.