Voltage Drop Calculator
Calculate the percentage voltage drop along a conductor run based on current, one-way distance, wire gauge, and system voltage. Use it to ensure circuits meet the NEC's 3% voltage-drop guideline.
Last updated: September 2026
Formula below · 2 sources (ieee.org, Wikipedia) · Updated Sep 2026
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About this calculator
Voltage drop occurs because every conductor has resistance, and current flowing through that resistance reduces the voltage available at the load. The formula used here is: VD = k × I × d × R / 1,000, where I is the load current in amps, d is the one-way distance in feet, R is the copper resistance in ohms per 1,000 ft from NEC Chapter 9 Table 8 (stranded, 75 °C: 14 AWG 3.14, 12 AWG 1.98, 10 AWG 1.24, 8 AWG 0.778, 6 AWG 0.491), and k is 2 for single-phase (out and back) or √3 ≈ 1.732 for a balanced three-phase circuit. Dividing by the system voltage gives the percentage drop. The NEC recommends keeping voltage drop below 3% on branch circuits and 5% total for feeders and branch circuits combined (informational notes to 210.19 and 215.2). Resistance only is used; for large conductors in steel conduit at AC, reactance adds a little more drop.
How to use
Example: single-phase circuit, 20 A load, 100 ft one-way run, 12 AWG copper wire, 120 V system. Step 1 — k = 2 for single-phase. Step 2 — R for 12 AWG = 1.98 ohm per 1,000 ft. Step 3 — VD = 2 × 20 × 100 × 1.98 / 1,000 = 7.92 V. Step 4 — 7.92 / 120 = 6.6%, so 12 AWG is too small for this run. 10 AWG gives 4.96 V (4.1%) and 8 AWG gives 3.11 V (2.6%), within the 3% target. On a three-phase circuit the 8 AWG drop would be 1.732 × 20 × 100 × 0.778 / 1,000 = 2.69 V.
Frequently asked questions
What is an acceptable voltage drop percentage for residential and commercial wiring?
The NEC recommends a maximum of 3% voltage drop on any individual branch circuit or feeder, and no more than 5% combined from the service entrance to the farthest outlet. Keeping within these limits ensures motors, sensitive electronics, and lighting operate within their rated voltage range. Some applications such as data centers and medical equipment specify even tighter limits — as low as 1–2% — to protect equipment and maintain performance.
How does wire gauge (AWG) affect voltage drop over a long run?
Lower AWG numbers indicate larger conductors with less resistance per foot, resulting in less voltage drop for the same current and distance. For example, upgrading from 12 AWG to 10 AWG roughly halves the resistance and the voltage drop. When runs exceed 50–75 feet at typical branch-circuit loads, it is often necessary to upsize the wire by one or two gauges beyond the minimum ampacity requirement to keep voltage drop within acceptable limits.
Why does a three-phase circuit have lower voltage drop than a single-phase circuit with the same wire?
In a balanced three-phase circuit, the line-to-line voltage drop is √3 × I × R × L instead of 2 × I × R × L for a single-phase two-wire circuit, because the three line currents are 120° apart and no current returns on a neutral. That is a 13% smaller multiplier (1.732 vs 2), and three-phase circuits also usually run at higher voltages, so the percentage drop is lower for the same power. This calculator applies k = 2 or k = 1.732 according to the system type you select.