How this calc works

Voltage drop is the loss along a wire run from resistance — the longer the run and the smaller the conductor, the more voltage you lose before it reaches the load. This calc takes your conductor size, one-way distance, current, and system voltage and returns the drop, the figure that decides whether you can use the wire you planned or have to size up.

The number you are checking against is a percentage of system voltage. The common design target is keeping the branch-circuit drop at or under 3%, and the combined feeder-plus-branch drop at or under 5%. Cross those and equipment runs hot, motors run weak, and lights dim under load.

Common mistakes

The frequent error is using the one-way distance when the formula needs the full circuit length — current flows out and back, so a 100-foot run is 200 feet of conductor. Check which length the calc expects before you trust the result.

The second is treating the 3% target as code-mandatory. In the U.S. it is a recommendation in the NEC, not a hard rule for general circuits — but it is good engineering, and some local codes do enforce it. Verify locally.

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