Free Voltage Drop Calculator - NEC Wire Run Check (2026)

Voltage drop is the loss of voltage as electrical current flows through a conductor over distance. Every wire has resistance, and that resistance converts some energy to heat instead of delivering it to the load. Excessive voltage drop causes motors to overheat, lights to dim, and equipment to malfunction. The NEC recommends keeping voltage drop within limits to ensure equipment operates safely and efficiently. On long runs to outbuildings, shops, or well pumps, voltage drop is often the factor that determines wire size, not ampacity.

NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2 recommend a maximum of 3% voltage drop for branch circuits and 5% total for the combined feeder and branch circuit. These are recommendations, not hard code requirements. However, most inspectors and engineers treat 3% as the practical limit for branch circuits. Some jurisdictions enforce stricter limits. Always check with your local authority having jurisdiction (AHJ).

The formula for single-phase voltage drop is: Vdrop = (2 x L x I x R) / 1000, where L is the one-way distance in feet, I is the current in amps, and R is the wire resistance in ohms per 1,000 feet from NEC Table 8. For three-phase circuits, replace the 2 with 1.732 (square root of 3). The "2" in single-phase accounts for the current traveling out on the hot conductor and returning on the neutral. Divide the voltage drop by the source voltage and multiply by 100 to get the percentage.

Aluminum wire has roughly 1.6x the resistance of copper wire at the same gauge. That means aluminum produces more voltage drop per foot. To compensate, you typically need to go up two wire sizes when switching from copper to aluminum. For example, if a circuit needs 6 AWG copper, you would use 4 AWG aluminum to achieve similar voltage drop performance. Aluminum is cheaper per foot but requires more material, so the cost difference depends on the run length and current prices.

Voltage drop becomes the controlling factor on long wire runs, typically over 100 feet. A 12 AWG copper wire is rated for 20 amps, but at 200 feet on a 120V circuit, the voltage drop hits 6.6%, well over the 3% recommendation. You would need to upsize to 8 AWG or even 6 AWG to keep voltage drop within limits, even though 12 AWG handles the ampacity just fine. This is common with detached garages, outbuildings, well pumps, and landscape lighting circuits.

Yes. The single-phase formula (Vdrop = 2 x L x I x R / 1000) works for DC circuits as well, since DC current flows out on one conductor and returns on another. This is especially relevant for solar panel installations, battery systems, and low-voltage landscape lighting where long runs at 12V or 24V can produce significant percentage drops even with relatively low resistance wire.