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Calculate voltage drop in electrical wiring based on wire gauge, length, and current.
Voltage Drop Formula:
V_drop = 2 x L x R_per_ft x I
The factor of 2 accounts for the round-trip length (hot and neutral conductors). L is one-way wire length in feet, I is current in amperes, R is resistance per foot of the selected AWG wire.
NEC (National Electrical Code) recommends a maximum voltage drop of 3% for branch circuits and 5% for feeder and branch circuits combined. High voltage drop causes heat, inefficiency, and equipment damage.
Wire has resistance per unit length. Longer runs have more total resistance, causing more voltage to be lost as heat before reaching the load.
American Wire Gauge (AWG) is the US standard for wire diameter. Lower AWG numbers indicate thicker wire with less resistance. Common household circuits use 12 or 14 AWG.
Current must travel from the source to the load (hot conductor) and back (neutral conductor). The total wire length carrying current is therefore twice the one-way distance.
For a 20A load over 100 feet, 10 AWG copper wire is typically recommended to keep voltage drop within 3%. Use this calculator to verify with your specific voltage and load.