Voltage Drop Calculator
Calculate voltage drop for any lighting or electrical circuit. Checks NEC compliance, recommends correct wire size, and flags unsafe drops before you pull wire.
Voltage drop explained
Every wire has resistance, and pushing current through it loses a little voltage along the way — turned into heat in the conductor. On short runs this is negligible, but over distance it adds up: lights dim, motors run hot, and electronics misbehave. Voltage drop is that loss, usually expressed as a percentage of the source voltage. This calculator takes the wire you actually plan to use — gauge, material, length, current — and tells you the drop in volts and percent, the voltage left at the load, and whether it passes the common limits.
Drop = (2 × K × length × amps) ÷ circular mils (single-phase)
K is the resistivity of the metal — about 12.9 for copper, 21.2 for aluminum. The 2 accounts for current flowing out to the load and back. Three-phase uses 1.732 (the square root of 3) in place of the 2, which is why this tool asks whether the circuit is single- or three-phase.
Acceptable voltage drop
The widely used guideline (and NEC recommendation) keeps drop modest so equipment gets the voltage it expects:
| Circuit | Recommended max drop |
|---|---|
| Branch circuit | 3% |
| Feeder | 3% |
| Feeder + branch (total) | 5% |
| Sensitive electronics | 2% or less |
These are performance guidelines, not hard code limits for conductor safety — but staying within them keeps lights bright and motors happy. If the drop is too high, the fix is a larger conductor, a shorter run, or a higher system voltage.
Single-phase, three-phase, AC and DC
Single-phase AC (most homes) and DC both use the round-trip factor of 2. Three-phase drops less for the same load because the math uses 1.732, not 2 — one reason large loads are run at three-phase where it is available. DC behaves like single-phase for this estimate since there is no power factor or reactance to account for in a simple run.
Frequently asked questions
How much voltage drop is acceptable?
Aim for 3% on a branch circuit or feeder, and no more than 5% total. Keep it under 2% for sensitive electronics.
How do I fix excessive voltage drop?
Use a larger wire gauge, shorten the run, split the load, or raise the system voltage. A bigger conductor is the usual answer.
Does length use one-way or round-trip?
Enter the one-way run. The formula’s factor of 2 (or 1.732 for three-phase) already accounts for the full current path.
Why does three-phase drop less?
The three-phase formula uses 1.732 instead of 2, so for the same load and wire the calculated drop is lower.
Is voltage drop the same as a code violation?
Not directly — the percentages are recommendations for performance, while conductor ampacity and overcurrent protection are the safety requirements.
What wire size do I need to limit drop?
Size up until the drop falls under your target. The Wire Gauge Calculator solves for that size directly.
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Estimates use standard resistivity values for planning and education only. Actual drop depends on conductor temperature, power factor, bundling and terminations. Voltage-drop percentages are performance guidelines; conductor ampacity and overcurrent protection are the safety requirements. Have wiring designed and installed to local code by a qualified electrician.