How WireGauge calculates

Wire diameter and area

AWG diameters follow ASTM B258: the diameter of gauge n is 0.005 inch × 92^((36 − n) ÷ 39). 36 AWG is exactly 0.005 inch and 4/0 (written as n = −3) exactly 0.4600 inch. Area in circular mils is the diameter in mils squared; kcmil is thousands of circular mils. Area in mm² is π × (diameter in mm)² ÷ 4.

Resistance

Resistance is resistivity × length ÷ area for a solid conductor. Copper uses the International Annealed Copper Standard, 1.7241 µΩ·cm at 20 °C (NIST Handbook 100, Copper Wire Tables), with a temperature coefficient of 0.00393 per °C. Aluminum uses 2.8264 µΩ·cm at 20 °C (EC/1350-H19, 61% IACS; NBS Handbook 109) with 0.00403 per °C.

The chart shows resistance at 20 °C. Voltage drop uses resistance at 75 °C, which for 12 AWG solid copper is 1.93 Ω per 1000 ft, the value NEC Chapter 9, Table 8 lists for solid uncoated copper. Stranded wire of the same gauge is a few percent higher (1.98 Ω for 12 AWG stranded).

Ampacity

Current ratings are the allowable ampacities of NEC Table 310.16 (NFPA 70, National Electrical Code, 2026 edition; the values are the same in the 2023 and 2020 editions, which many areas still enforce, so check which edition your inspector uses) for copper and aluminum at 60, 75 and 90 °C. They assume a surrounding temperature of 30 °C (86 °F) and no more than three current-carrying conductors in a raceway, cable or the earth. More conductors together, or higher temperatures, require the adjustment and correction factors of NEC 310.15, which can reduce the rating considerably.

For small conductors, NEC 240.4(D) limits the overcurrent protection: copper 14 AWG 15 A, 12 AWG 20 A and 10 AWG 30 A; aluminum 12 AWG 15 A and 10 AWG 25 A. The wire size calculator applies these limits; the chart marks them with an asterisk. NM-B cable is limited to the 60 °C column (NEC 334.80(A); 334.80 in 2023).

WireGauge reproduces the numeric values only, with the table cited; it doesn't reproduce the code's text. The values are checked against the edition named on each page.

Voltage drop and sizing

Voltage drop for single-phase and DC circuits is 2 × current × resistance per foot × one-way length; for three phase it is √3 × current × resistance per foot × length. Load current from watts is watts ÷ volts, or watts ÷ (√3 × volts) for three phase, assuming a power factor of 1.

The calculator finds the smallest size whose allowable current (with the 240.4(D) limits) is at least the load, sized at 125% for continuous loads (NEC 210.19(A)(1) for branch circuits and 215.4(A)(1) for feeders, 215.2(A)(1) in 2023), and the smallest size whose voltage drop is within your limit. It recommends the larger of the two. The default 3% limit follows the informational notes to NEC 210.19 and 215.4(A)(2) (215.2(A)(2) in 2023), which suggest 3% for a branch circuit or feeder and 5% in total; they are recommendations, not requirements.

Metric sizes

Metric sizes are the nominal cross-sections of IEC 60228: 0.5, 0.75, 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 and 300 mm². Equivalent AWG for an area uses the ASTM B258 formula in reverse.

Rounding

Calculations keep full precision. Diameters are shown to four decimal places in inches and three in millimeters; areas and resistances to four significant digits; voltage drop to two decimal places.