Voltage Drop Calculator - Wire Sizing Tool

Every conductor is also a resistor. When current flows down a branch circuit, part of the source voltage is consumed inside the copper instead of reaching the load, and that consumed part is voltage drop. The arithmetic is simply Ohm's law applied to the wire itself. The two circuit multipliers used here exist because the current path is never one-way: a single-phase circuit sends current out on one conductor and back on another, so a 100 foot run is really 200 feet of copper, which is the factor of 2. A balanced three-phase circuit returns current through the other phase conductors rather than a neutral, and the line-to-line geometry yields a factor of sqrt(3), about 1.732. The resistance values behind each AWG option come from NEC Chapter 9 Table 8 for uncoated copper at 75 degrees C, the rating most branch-circuit insulation carries. Two consequences follow. Copper resistance climbs roughly 0.4 percent per degree C, so a conductor running hot inside a crowded conduit drops more than the table predicts. Aluminum carries about 1.6 times the resistance of copper at the same gauge, so scale the answer up for aluminum runs. The missing voltage does not vanish, it becomes heat at a rate of current squared times resistance, which is why an undersized long run both starves the load and warms the wall cavity. Motors suffer worst, because low terminal voltage raises current draw, increases slip and shortens winding life.

Georg Simon Ohm published the relationship that underpins this calculation in 1827, in a monograph on the galvanic circuit that much of the German academic establishment initially dismissed before the Royal Society vindicated him with the Copley Medal in 1841. Standardised wire sizing came next. The American Wire Gauge scale, still known as Brown and Sharpe gauge after the Providence toolmakers who fixed it in 1857, defined diameters as a geometric progression, which is why every three-step drop in gauge number roughly halves the resistance per foot and why the table in this calculator falls in such regular ratios. Voltage drop became an economic problem during the war of the currents in the 1880s. Edison's low-voltage direct-current distribution could not push usable power much beyond a mile from the generating station without unacceptable loss, whereas the Westinghouse alternating-current system used transformers to raise voltage, cut current for the same delivered power, and shrink drop across long lines. Codified guidance followed the technology. The National Electrical Code, first published in 1897 and maintained today by the National Fire Protection Association as NFPA 70, carries the familiar targets in informational notes: about 3 percent drop on a branch circuit and 5 percent combined with the feeder. Those figures are recommendations for efficient operation rather than enforceable rules, though many jurisdictions and design specifications adopt them as hard limits.