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How Voltage Drop Is Calculated
Voltage drop equals current times resistance of the wire. For a two-wire circuit, the total wire length is twice the one-way distance. Wire resistance depends on gauge, material (copper), and length, measured in ohms per 1,000 feet.
NEC Voltage Drop Guidelines
The National Electrical Code recommends no more than 3% voltage drop for branch circuits and 5% total for feeder plus branch circuits. Excessive voltage drop causes equipment to operate inefficiently, lights to dim, and motors to overheat.
Wire Gauge and Resistance
Smaller AWG numbers indicate thicker wire with lower resistance. Common residential gauges are 14 AWG (15A circuits), 12 AWG (20A circuits), and 10 AWG (30A circuits). For long runs, use a larger gauge to reduce voltage drop.
Solutions for Excessive Voltage Drop
If voltage drop exceeds 3%, increase the wire gauge (lower AWG number), shorten the wire run, or reduce the load. For long outdoor runs to sheds or garages, using a heavier gauge wire is the most practical solution.
Frequently Asked Questions
The NEC recommends a maximum of 3% voltage drop for branch circuits (3.6V on a 120V circuit). Total voltage drop from the panel to the outlet should not exceed 5% (6V on a 120V circuit). These are recommendations, not code requirements.
Thicker wire (lower AWG number) has lower resistance and less voltage drop. Going from 14 AWG to 12 AWG reduces resistance by about 37%. Going from 12 AWG to 10 AWG reduces it by another 37%.
Yes. The calculator takes the one-way distance and doubles it internally because current must flow out to the load and back. A 100-foot one-way run means 200 feet of total wire that contributes to voltage drop.
Voltage drop itself is not a fire hazard, but it causes inefficiency. Motors may draw more current to compensate, potentially overheating. Lights may flicker or dim. In extreme cases, sensitive electronics may malfunction.