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Voltage Drop Calculator

Checks the voltage drop for a specific wire gauge and run, and flags whether it's within NEC-recommended limits.

Single-phase vs three-phase, at a glance

If you're not sure which one applies to your panel, the single-phase vs three-phase wiring guide covers how to tell — most single-family homes are single-phase.

How to use this calculator

Pick the wire gauge already installed (or planned) for the run, the conductor material, the load current, the source voltage, the one-way distance, and whether it's a single-phase or three-phase circuit. The calculator returns the actual voltage drop in both volts and percent, and flags it against the NEC-recommended limits — 3% for a branch circuit on its own, 5% for the combined feeder-plus-branch total.

What voltage drop is and why it matters

Every conductor has some resistance, and current flowing through that resistance loses a small amount of voltage over distance — the longer the run and the higher the current, the more voltage the load actually receives falls short of what the source is putting out. A little drop is normal and code-legal. Too much drop means motors run hotter and less efficiently, electronics can misbehave, and lighting can visibly dim or flicker under load. The NEC doesn't hard-mandate a specific drop percentage as a safety violation the way it does ampacity, but 3%/5% is the long-standing recommended design target, and most inspectors and manufacturers treat it as the practical ceiling.

Worked example

Take a 20A load on 10 AWG copper, 240V single-phase, but this time run 150 feet one-way instead of a short run — long enough that even a properly-ampacity-rated wire starts to matter for voltage drop:

This is the exact case where a wire that's "safe" by the ampacity table can still be the wrong choice for the run — the fix here would be stepping up to 8 AWG, or shortening the run if that's an option.

Frequently asked questions

What's the formula for voltage drop?

For single-phase: VD = (2 × K × I × D) / CM. For three-phase: VD = (1.732 × K × I × D) / CM. Where K is the conductor's resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amps, D is one-way distance in feet, and CM is the conductor's circular-mil area. Percent drop is just VD / source voltage × 100. This is the same formula the calculator above runs — the worked example shows it applied to real numbers.

What's an acceptable voltage drop percentage?

3% or less for a branch circuit on its own is the standard recommended target; 5% is the ceiling for a branch circuit combined with its feeder. Above that, expect real-world symptoms — dimming, motor inefficiency, or electronics behaving oddly under load — even if the wire is otherwise sized correctly for ampacity.

How is three-phase voltage drop different from single-phase?

Three-phase voltage drop is lower than single-phase for the same wire, current, and distance — the three-phase formula uses a √3 (≈1.732) multiplier instead of the single-phase 2, which works out to roughly 13.4% less drop for identical conditions. That's one reason three-phase distribution is preferred for longer industrial and commercial runs.

Does a longer wire run always mean more voltage drop?

Yes, drop scales linearly with one-way distance in the formula above — double the distance and, all else equal, you double the voltage drop. This is exactly why a wire gauge that's fine for a short run can fail the drop target at 150 feet, as in the worked example, even though its ampacity rating never changes.

Based on NEC Chapter 9 Table 8 (circular mils) and Table 310.16 (75°C ampacity, ≤3 current-carrying conductors). Reference only — verify against the current NEC and local code, and consult a licensed electrician before installing.