Voltage Drop Calculator
Calculate voltage drop in electrical wire based on wire gauge (AWG), length, current, and source voltage.
How to Calculate Voltage Drop
Where L = one-way length in feet, I = current in amps, and R = resistance per 1,000 feet for the wire gauge. For three-phase circuits, multiply by √3 instead of 2.
NEC Voltage Drop Recommendations
The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% total for feeder + branch combined. Excessive voltage drop causes equipment to run inefficiently.
12 AWG wire, 100 ft, 20A, 120V single phase:
Resistance of 12 AWG = 1.588 Ω/1000ft
VD = (2 × 100 × 20 × 1.588) / 1000 = 6.35 V (5.3%)
FAQ
What causes voltage drop?
All conductors have resistance. As current flows through wire, some voltage is lost to heat due to this resistance. Longer runs and smaller wire gauges increase voltage drop.
How do I reduce voltage drop?
Use a larger wire gauge, shorten the wire run, or reduce the load current. For long runs, consider upsizing wire by one or two gauges.
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Last updated: 2026-08-08