Wire Size Calculator

Conductor sizing is governed by two independent limits, and the larger wins. Ampacity is the thermal limit: current dissipates I-squared-R heat that must not push the insulation past its rated temperature. NEC Table 310.16 gives 25 A for 12 AWG copper, 35 A for 10 AWG, and 230 A for 4/0 copper in the 75 degrees C column, though NEC 240.4(D) caps overcurrent protection for those small conductors at 20 A and 30 A. Voltage drop is the second limit, an efficiency and equipment-performance constraint. Drop equals k x I x R x L, where k is 2 for single-phase because current traverses both the hot and the return conductor, and 1.732, the square root of 3, for three-phase, where the 120-degree phase displacement partially cancels the drop. Rearranging that relation for the largest resistance per 1,000 feet meeting a target drop percentage converts a voltage-drop budget into a gauge. Copper carries roughly 61 percent more current than aluminum of equal cross-section, so aluminum runs typically need about two AWG sizes more. The NEC treats 3 percent drop on a branch circuit and 5 percent combined as recommended practice in informational notes, not enforceable limits, though many jurisdictions adopt them as mandatory. Corrections outside this calculation matter: NEC 110.14(C) usually forces you back to the 75 degrees C column even with 90 degrees C THHN, because terminals are rated 75 degrees C; ambient above 30 degrees C requires correction factors; and bundling more than three current-carrying conductors derates ampacity to 80 percent at four to six conductors and 50 percent at ten to twenty. Tabulated resistances are DC values, so AC reactance matters above roughly 1/0.

The gauge numbers come from the American Wire Gauge system, devised in 1857 by J. R. Brown of Brown and Sharpe in Providence, Rhode Island, and long known as the Brown and Sharpe gauge. Its apparently arbitrary numbering is a geometric series: 39 equal steps separate 4/0, at 0.4600 inch diameter, from 36 AWG at 0.0050 inch, so each step multiplies diameter by the 39th root of 92, about 1.1229. That construction produces the field rules electricians still use, since six gauge steps roughly halve the diameter and three steps roughly halve the area. Earlier wire sizes used inconsistent drawing-plate numbers that varied between manufacturers. The ampacity side has a regulatory origin. The first National Electrical Code appeared in 1897, assembled from five competing insurance and industry codes to address fires caused by early arc and incandescent wiring. The National Fire Protection Association took over sponsorship in 1911 and has published it since as NFPA 70, revising it every three years; ampacity tables have been present from the earliest editions and repeatedly renumbered, most recently as Table 310.16. Voltage drop entered as advisory language rather than a rule, reflecting that excessive drop wastes energy without directly causing fire. Aluminum branch-circuit wiring installed during the copper price spike of the 1960s produced enough overheated connections that the Consumer Product Safety Commission investigated in the 1970s, and the code subsequently required AA-8000 series alloy conductors, which is why aluminum now appears mainly in larger feeders and service conductors.