Voltage drop over long wire runs can cause dim lights, weak motor performance, or code violations. This calculator estimates the drop for a copper conductor run based on current, distance, and wire gauge.
How to Use This Calculator
Enter the circuit’s current in amps, the one-way length of the wire run in feet, the source voltage, and select the wire gauge, then click Calculate.
Formula
Voltage Drop = 2 × Current × Length × (Resistance per foot), where resistance is looked up per 1,000 ft for standard copper AWG sizes and the factor of 2 accounts for the round-trip (out and back) length of the circuit.
This is a simplified planning estimate. For code-compliant electrical work, always verify sizing with a licensed electrician against the latest NEC or local code.
Frequently Asked Questions
What voltage drop percentage is acceptable?
The NEC recommends keeping voltage drop under 3% for branch circuits and under 5% total from source to the farthest outlet, though always confirm against your local code.
Why is the length multiplied by 2?
Current travels out to the load and back to the source, so the wire resistance applies to both directions of the circuit.
Does this calculator account for aluminum wire?
No — this version uses standard copper wire resistance values. Aluminum wire has different resistance and would need a separate table.