Voltage Drop Calculator
A long wire run drops voltage along the way. Calculate how much, whether it's within NEC's 3% recommendation, and what shows up at the device.
Drop
Voltage drop
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Drop percentage
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Voltage at load
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NEC compliance
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Why it matters
A long wire run has resistance. Resistance × current = voltage dropped along the wire. A motor designed for 120V running on 110V draws more current and runs hotter; a 12V landscape lighting circuit might show only 9V at the far fixture, making LEDs dim.
The 3% rule
NEC 210.19(A) Informational Note suggests <3% drop on branch circuits. Combined with feeder + branch, total <5% from service to load. This isn't strictly required by code, but inspectors and most installers treat it as the standard.
Fixing too much drop
Three options: (1) go up a wire size (each AWG step is ~25% less drop), (2) increase the source voltage if possible (240V vs 120V halves the drop), (3) move the load closer to the source. Copper drops less than aluminum at the same gauge — but aluminum is much cheaper per foot.