Wire Voltage Drop Calculator｜Check NEC 3% and 5% Limits
Size a circuit before you pull wire: enter the source voltage, load current, one-way run length, and AWG gauge, and the voltage drop, drop percentage, voltage at the load, and power loss in the conductor all appear together. Thirteen copper and aluminum sizes from AWG 18 to 4/0 are covered, plus single- and three-phase modes, and a status line tells you whether the result clears the National Electrical Code's 3% recommended and 5% maximum thresholds.
💡 About this tool
Voltage drop is the voltage "lost" along a conductor because copper and aluminum both have real resistance. The longer the run and the smaller the wire, the more of your source voltage gets eaten before it reaches the load — which is why a motor at the end of a long feeder can hum instead of start, or LED strips fade toward the far end. The math is Vdrop = I × R, where R = ρ × L / A: resistivity times conductor length divided by cross-sectional area.
This calculator handles the parts people get wrong by hand. For single-phase it doubles the run length automatically, because current travels out and back, so a 30 m one-way run is 60 m of conductor. For three-phase it applies the √3 line-to-line factor instead. It uses copper resistivity of 0.01724 Ω·mm²/m and aluminum of 0.02826 Ω·mm²/m at 20 °C, so swapping materials makes it clear why aluminum needs to be roughly one or two gauges larger for the same drop.
The NEC status line is the practical payoff. Article 210.19 and 215.2 carry informational notes recommending no more than 3% drop on a branch circuit or feeder, and 5% on the combined run. The tool flags green under 3%, amber between 3% and 5%, and red above 5%, so "is AWG 14 good enough for this run?" becomes a single glance instead of a lookup table.
🧐 Frequently Asked Questions
Do I enter one-way or round-trip distance? One-way — the distance from the source to the load. The tool adds the return path itself: ×2 for single-phase, and the √3 factor for three-phase. Entering a doubled length would overstate the drop.
Why does aluminum show a bigger drop than copper at the same gauge? Aluminum's resistivity is about 64% higher than copper's, so for the same cross-section it drops more voltage. That's the everyday reason aluminum feeders are sized up a step or two compared with a copper run carrying the same current.
Is the 3% an NEC rule I must follow? It's a recommendation in an informational note, not a hard code requirement, but inspectors and good practice both lean on it. The 5% combined figure is the commonly cited ceiling for acceptable performance.
Does this size the wire for ampacity too? No. This tool answers the voltage-drop question only. Ampacity — whether a gauge can safely carry the current without overheating — is a separate table in NEC 310.16 and should be checked alongside drop.
Why AWG and mm²? The dropdown lists American Wire Gauge sizes with their metric cross-section in mm², so the same picker works whether your local convention is AWG (North America) or metric area (most of the rest of the world).
📚 Fun Facts
The AWG numbering feels backwards — bigger number, thinner wire — because it counts how many times the wire was drawn through progressively smaller dies during manufacturing. More draws means a thinner strand, so AWG 18 has been pulled more times than AWG 4/0. The scale is geometric: every six gauge steps roughly halves the cross-sectional area and doubles the resistance, which is a handy mental shortcut when you're deciding whether to jump two sizes or four. That step-by-step relationship between gauge and area was standardized in the 1850s, and electricians still recite "drop two gauges to halve the drop" on job sites today.