What is voltage drop?
Voltage drop is the reduction in electrical voltage as current travels through a wire, caused by the wire's resistance to current flow.
When electricity travels through wiring, the resistance of the wire itself causes the voltage to decrease by the time it reaches the end of the run. This loss is voltage drop. The longer the wire run or the smaller the wire gauge, the greater the drop.
Voltage drop matters in real installations because equipment may not operate properly at reduced voltage. Motors run hotter and less efficiently, lights dim, and controls respond sluggishly. Long circuits from a breaker panel to a distant outlet, a shed, a well pump, or a generator hookup are prime spots where voltage drop becomes a problem.
The National Electrical Code (NEC) sets limits on acceptable voltage drop. Branch circuits should not exceed 3 percent drop, and the combination of feeder and branch circuit should stay under 5 percent total. A qualified residential electrician will verify voltage drop calculations during design and installation to ensure wiring is properly sized for the distance and load.