AWG wire gauge chart

Wire gauge chart, 4/0 to 40 AWG

Solid conductors, computed from d = 0.005 in × 92^((36 − n) / 39). Resistance from K = 10.371 (copper) and 17.002 Ω·cmil/ft (aluminum) at 20 °C; stranded conductors are about 2% higher.

How AWG numbering works

American Wire Gauge, also known as the Brown & Sharpe gauge, fixes two end points and spaces the sizes between them geometrically. 36 AWG is defined as 0.005 in in diameter and 4/0 AWG (written 0000) as 0.4600 in, with 39 equal steps between them, so each step changes the diameter by a factor of 92^(1/39), about 1.1229. The diameter of gauge n is therefore d = 0.005 in × 92^((36 − n) / 39), where 1/0, 2/0, 3/0 and 4/0 count as 0, −1, −2 and −3.

The numbers run backward because they originally counted the passes through drawing dies: a finer wire went through more dies and earned a higher number. A larger gauge number is a smaller wire.

Three rules of thumb follow from the 1.1229 ratio. Six gauge steps halve the diameter (1.1229^6 ≈ 2.005). Three steps halve the area and so double the resistance (1.1229^6 is also the area ratio for three steps). Ten steps change the area, and the resistance, by a factor of about ten: 12 AWG has 6.53 kcmil and 2 AWG about ten times that. The rules explain why 10 AWG (10.38 kcmil) carries noticeably more than 12 AWG while 6 AWG (26.25 kcmil) is a much heavier conductor.

Circular mils and kcmil above 4/0

Conductor area in US practice is measured in circular mils (cmil). One circular mil is the area of a circle 1 mil (0.001 in) in diameter, so the area of a round wire in circular mils is simply its diameter in mils squared, with no π involved. A 12 AWG conductor is 80.8 mils across, which gives about 6,530 cmil.

The gauge numbers stop at 4/0 (211,600 cmil). Larger building wire is named directly by its area in thousands of circular mils: 250 kcmil, 300 kcmil, 350 kcmil, 500 kcmil and so on up to 2,000 kcmil. Older drawings and catalogs write the same sizes as MCM (thousand circular mils); 250 MCM and 250 kcmil are the same conductor.

Solid and stranded wire

A gauge number defines the conductor's cross-sectional area, not how the area is made up. A stranded 12 AWG conductor has about the same copper area as a solid one, divided among 7 or 19 smaller strands, so its overall diameter is larger by the gaps between the strands. The diameters in the chart are for solid wire; stranded conductors and insulated wire are larger, and conduit fill uses the manufacturer's or the NEC's insulated dimensions, not the bare diameter.

Stranding also adds a little resistance, because each strand spirals and is slightly longer than the conductor. The usual allowance is about 2%. The chart gives solid-conductor resistance; the voltage drop and wire size calculators apply the 2% for stranded wire. Building wire is normally solid up to 10 AWG and stranded from 8 AWG up, since stranded wire is easier to pull and bend.

Resistance at 20 °C and 75 °C

Resistance per 1,000 ft is K × 1,000 ÷ cmil, where K is the conductor's resistivity in Ω·cmil/ft: 10.371 for annealed copper and 17.002 for 1350 aluminum at 20 °C. Resistance rises with temperature, by 0.393% per °C for copper and 0.403% for aluminum, so the same conductor has about 22% more resistance at 75 °C, the temperature normally assumed for loaded building wire and voltage drop calculations.

Aluminum has about 61% of the conductivity of copper, which is why an aluminum conductor is generally two gauge sizes larger than the copper conductor it replaces.

AWG and mm²

Most of the world sizes conductors in square millimeters under IEC 60228: 1.5, 2.5, 4, 6, 10, 16, 25 mm² and up. The metric series does not line up with AWG. 14 AWG is 2.08 mm², between 1.5 and 2.5 mm²; 12 AWG is 3.31 mm², between 2.5 and 4 mm²; 10 AWG is 5.26 mm², a little under 6 mm². The mm² column gives the exact area of each gauge for comparison, but a metric conductor is not a direct substitute for an AWG size in an installation under the NEC, which is written around AWG and kcmil.

Why the chart has no ampacity column

The current a wire may carry is not a property of its gauge alone. It depends on the insulation temperature rating, the terminations, the wiring method, the ambient temperature and the number of conductors bundled together, and it is set by the NEC ampacity tables rather than by geometry. A single amps column beside a gauge chart hides all of those conditions. The wire size calculator applies them for a specific circuit, and the ampacity calculator applies correction and adjustment factors.

Frequently asked questions