Voltage drop is the quiet performance killer of electrical runs: the wire is safe, the breaker holds, yet motors run hot, LED drivers buzz, and tools lose power at the end of a long cable. This guide explains how voltage drop is calculated, why long runs need bigger wire, the difference between copper and aluminum, and how voltage drop differs from the ampacity sizing your local code actually requires.
Why long runs need bigger wire
Every conductor has resistance, and resistance grows with length. Voltage drop is simply Ohm's law applied to that resistance: the further the current travels, the more voltage is lost as heat in the wire. Because the current has to flow out to the load and back, a single-phase or DC circuit uses twice the one-way distance in the formula.
The practical consequence is that a wire perfectly sized for the current can still drop too much voltage over distance. Doubling the run roughly doubles the drop, so a gauge that is fine at 25 feet can fail badly at 100 feet. The cure is a larger conductor (more circular mils means less resistance) or, where possible, a higher system voltage.
The 3% and 5% rule
The National Electrical Code does not mandate a maximum voltage drop, but an informational note to NEC 210.19 (branch circuits) and 215 (feeders) recommends keeping drop at or below 3% on a branch circuit and 5% across the feeder plus branch combined. Staying under these figures keeps equipment operating at its rated voltage, protects motor windings, and avoids dim lighting and nuisance equipment faults.
This calculator flags anything over 3% as worth upsizing and anything over 5% as out of spec. Treat the percentages as design targets: critical loads such as motors and sensitive electronics benefit from staying well below 3%.
Copper versus aluminum
Copper conducts better than aluminum, so for the same gauge it drops less voltage. The K-factor captures this: 12.9 for copper versus 21.2 for aluminum, meaning aluminum drops about 64% more voltage at the same size. Aluminum is lighter and cheaper, which is why it dominates utility feeders and large service entrances, but you generally size it one to two AWG steps larger than copper to match both ampacity and voltage drop. Aluminum also requires antioxidant compound and rated terminations.
Voltage drop is not ampacity
It is easy to confuse the two. Ampacity is the maximum current a conductor can carry without overheating, and it is a hard safety requirement from NEC Table 310.16. Voltage drop is a performance metric about how much voltage survives the trip. A conductor can satisfy ampacity yet still drop too much voltage on a long run, which is exactly when you upsize beyond the ampacity-required gauge. Always size for ampacity first using a wire-size calculator, then check voltage drop and upsize if needed.