Voltage Drop Calculator
Calculate voltage drop across a wire run.
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How It Works
Calculates voltage drop in a wire run using: Vdrop = 2 × I × L × R_per_unit. R varies by wire gauge (AWG). Higher current, longer runs, or thinner wire = more voltage drop.
Formula
Vdrop = 2 x I x R x LQuick Reference
Common calculations — results you can verify instantly.
10A, 50ft 12AWG
2×10×50×1.588/1000
1.59V drop
20A, 100ft 10AWG
2×20×100×1.0/1000
4.0V drop
Max 3% drop: 120V system
3% of 120V
3.6V max drop
Tips & Shortcuts
NEC recommends max 3% voltage drop for branch circuits, 5% total.
For 120V system: max 3% = 3.6V drop. For 240V: max 3% = 7.2V drop.
Larger wire gauge (lower AWG number) reduces voltage drop.
Common Mistakes
Forgetting to double the wire length for round trip
Current travels out AND back. Total wire length = 2 × one-way distance.
Ignoring voltage drop for short runs
Short runs are fine, but in long runs (100+ ft), even small voltage drops affect device performance.
Frequently Asked Questions
NEC recommends no more than 3% for branch circuits and 5% total (source to load). Excessive drop causes dimming, overheating, and equipment failure.
Use larger wire (lower AWG), reduce current (use higher voltage), or shorten the run. Upgrading from 14 AWG to 12 AWG significantly reduces drop.
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