Voltage Drop Calculator
Figure out how much voltage you lose across a wire run before it reaches your load. Enter the wire gauge, length, current, and material to get the voltage drop, percentage, and NEC compliance status. Useful for sizing wire in residential and commercial electrical work.
How voltage drop works: When current flows through a wire, the wire's resistance causes a voltage loss between the source and the load. Longer runs and thinner wire mean more resistance and a bigger drop.
The formulas:
- Single phase: Vdrop = 2 x I x R x L/1000
- Three phase: Vdrop = sqrt(3) x I x R x L/1000
Where I is current in amps, R is resistance per 1000 feet for the wire gauge, and L is the one-way wire length in feet. The factor of 2 for single phase accounts for both the hot and neutral conductors.
Wire material matters: Aluminum wire has about 1.6 times the resistance of copper. It is lighter and less expensive, but you typically need to go up a gauge or two to match copper's performance on a given run.
NEC recommendations: The National Electrical Code recommends no more than 3% voltage drop for branch circuits and no more than 5% total (feeder plus branch). These are recommendations, not hard limits, but staying within them helps avoid flickering lights, overheating motors, and other problems.
Practical example: A 20-amp circuit on 12 AWG copper wire running 100 feet will drop about 7.92V on a 120V single-phase circuit, which is 6.6%. That exceeds NEC limits, so you would want to upsize to 10 AWG or shorten the run.