Voltage Drop / Wire Gauge Calculator
Enter your circuit details to check voltage drop against the NEC's recommended limits, and see the thinnest gauge that would still pass.
How Voltage Drop / Wire Gauge Calculator Works
Voltage drop is the loss in electrical potential that occurs as current travels through a wire's resistance over distance. Too much drop on a circuit means dimmer lights, motors running hotter and less efficiently, and voltage at the load that's noticeably lower than the panel voltage you started with. This calculator takes your wire gauge, conductor material, one-way run length, load current, and system voltage, and tells you the drop as a percentage against the NEC's recommended guidelines.
Formula & Method
Voltage drop is calculated as Vdrop = (k × L × R × I) / 1000, where L is the one-way wire length in feet, R is the conductor's resistance in ohms per 1000 feet (from a standard AWG resistance table for copper or aluminum), I is the load current in amps, and k is a multiplier that accounts for the fact current flows out and back: k = 2 for single-phase circuits, and k = √3 (about 1.732) for three-phase circuits. The result is divided by the system voltage to get a percentage, which is then checked against 3% (branch circuit) and 5% (combined feeder + branch) reference thresholds. The tool also works backward from your inputs to suggest the thinnest gauge that would keep drop at or under 3%.
Worked Example
For a 120V single-phase circuit carrying a 20A load over a 100 ft one-way run of 12 AWG copper wire (resistance 1.98 Ω per 1000 ft): voltage drop = (2 × 100 × 1.98 × 20) / 1000 = 7.92V, which is 6.6% of 120V — a fail against the 5% combined-limit guideline. The calculator's gauge suggestion for this same run shows that stepping up to 8 AWG copper (resistance 0.778 Ω per 1000 ft) would bring the drop down to about 2.59%, safely under the 3% branch-circuit guideline.
Frequently Asked Questions
- Is exceeding the NEC voltage drop percentage actually a code violation?
- No. The 3% and 5% figures are informational recommendations in the NEC for efficient operation, not enforceable safety limits in most jurisdictions. Exceeding them wastes energy and can cause dim lighting or poor motor performance, but it isn't automatically a code violation the way exceeding a wire's ampacity would be.
- Does this calculator also check if my wire is thick enough to be safe (ampacity)?
- No, this tool checks voltage drop only. Ampacity — the maximum current a wire can safely carry without overheating — is a separate calculation that depends on insulation type, ambient temperature, and how many current-carrying conductors share a conduit. Always verify ampacity separately and consult a licensed electrician for real installations.
- Why does three-phase wiring use a different multiplier than single-phase?
- In a single-phase circuit, current flows out on one conductor and back on another, so the resistance of both legs applies — hence a multiplier of 2. In a balanced three-phase circuit, the phase relationship between the three conductors means the effective voltage drop multiplier works out to the square root of 3 (about 1.732) instead of 2.
- Why does aluminum wire show a higher voltage drop than copper of the same gauge?
- Aluminum has higher electrical resistance than copper for a given cross-sectional area, so an aluminum conductor of the same AWG size will always show a larger voltage drop (and typically needs to be sized larger than an equivalent copper run to compensate).
The NEC's 3% (branch circuit) and 5% (combined feeder + branch) voltage drop figures are recommended guidelines for efficient operation (an informational note, not a strict safety requirement) — not a hard code violation if exceeded. This tool checks voltage drop only, not ampacity (safe current-carrying capacity), which depends on insulation rating, ambient temperature, and how many conductors share a conduit — verify that separately, and consult a licensed electrician for actual installations.