2 Gauge Wire Size

2 gauge is AWG 2: 6.544 mm across, 33.63 mm2 of copper

Written as a wire size, 2 gauge means 2 AWG. ASTM B258, the standard that fixes the American Wire Gauge diameters, lists a nominal diameter of 257.6 mils, which converts to 0.2576 inches or 6.544 mm across the bare copper. The cross section works out to 33.63 mm2, printed as 33.6 mm2 in the ASTM table (ASTM B258-18).

The weight rows come from density rather than a table: 33.63 mm2 is 0.3363 cm2, and a kilometer of it is 33,630 cm3. Copper weighs 8.89 g/cm3, so the run is 299 kg, and 1000 ft (304.8 m) of it is 91 kg, or 201 lb. Wire this size needs two hands and a real puller, which is why electricians plan the route before it leaves the spool.

The formula behind the number: d = 0.127 x 92^((36-n)/39)

AWG is a numbered scale, not a set of measurements. ASTM B258 defines the diameter of size n in millimeters with one formula: d = 0.127 x 92^((36-n)/39). Size 36 is exactly 0.127 mm, size 4/0 is 92 times fatter, and the exponent spreads the 39 steps between them.

For 2 AWG the arithmetic runs like this: (36 - 2) / 39 = 34/39 = 0.8718, then 92^0.8718 = 51.53, and 0.127 x 51.53 = 6.544 mm. That single division is the whole standard; every published table row is this formula rounded to four significant digits. The scale also carries two built-in habits: every 6 sizes halves the diameter, and every 3 sizes halves the area, so 2 AWG carries twice the copper of 5 AWG and four times that of 8 AWG.

One area check: 33.63 / 26.67 = 1.26, and 42.41 / 33.63 = 1.26, so one size multiplies the metal by about a quarter again. Ten sizes in the other direction, down to 12 AWG at 3.31 mm2, multiplies it by 10.2. Circular mils are the trade unit on the inch side: 66,360 cmil means a circle 257.6 mils across, and the unrounded formula squares to about 66,370, a rounding gap smaller than any jacket printing.

Copper resistance at 20 degrees C: 0.5127 ohm per kilometer

Resistance is what turns a wire into a heater. The annealed copper standard puts 100 percent IACS at 1/58 ohm per meter per square millimeter at 20 degrees C, the figure NIST Handbook 100 (Copper Wire Tables) has carried since it fixed the American Wire Gauge constants. For 33.63 mm2: 1000 / (58 x 33.63) = 0.5127 ohm per kilometer, or 0.1563 ohm per 1000 ft (NIST Handbook 100).

The factor column is 1 + 0.00393 x (T - 20), the temperature coefficient for annealed copper from the same NIST handbook, so a conductor running at its 75 degree rating carries 22 percent more resistance than the spool showed on a cool morning. Most 2 AWG building wire is stranded, and class B stranding adds about 2 percent: published cable data for 2 AWG THHN lands near 0.194 ohm per 1000 ft at 75 degrees C, and 0.6235 x 1.02 x 0.3048 = 0.1938 puts the estimate in the same place.

Keep the two shapes of the number apart. At 20 degrees C a solid 2 AWG copper conductor reads 0.1563 ohm per 1000 ft; at its working temperature the same wire reads closer to 0.19. Sizing from the cold number quietly understates every voltage drop downstream.

Ampacity from NEC Table 310.16: 95, 115 or 130 amperes

Ampacity is a code table, not a formula. NEC 2023 Table 310.16, for up to three current-carrying conductors in a raceway at 30 degrees C ambient, gives 2 AWG copper 95 amperes at the 60 degree C column, 115 at 75, and 130 at 90. Aluminum or copper-clad aluminum of the same gauge gets 75, 90 and 100 (NFPA 70, NEC 2023, Table 310.16).

Which column rules depends on the ends as much as the wire. Section 110.14(C) holds terminations to 60 degrees C for circuits rated under 100 A unless the equipment is marked otherwise, and most modern panels and breakers carry a 75 degree C marking, which is why 115 A is the usual design number for 2 AWG copper. The 90 degree C value exists mainly as the starting point for derating, not as a target to load to.

Two house rules follow from the table. NM-B cable (Romex) is held to the 60 degree C column by 334.80, so 2 AWG NM-B is a 95 A cable, the classic residential 100 A service entrance paired with the next-size-up breaker rule in 240.4(B). And aluminum stops lower: 90 A at 75 degrees C, so a 100 A service in aluminum steps up to 1 AWG, listed at 100 A on the same column.

Aluminum at the same gauge: 1.64 times the resistance

Same gauge number, less metal per ampere. Aluminum conducts about 61 percent as well as the IACS copper standard, so a 2 AWG aluminum conductor runs 1000 / (0.61 x 58 x 33.63) = 0.8404 ohm per kilometer at 20 degrees C, and 0.8404 / 0.5127 = 1.64 times the resistance of copper.

The aluminum weight row uses 2.70 g/cm3 against copper's 8.89: a kilometer of 2 AWG is 33,630 cm3, so aluminum weighs 90.8 kg (200 lb) per kilometer and 27.7 kg (61 lb) per 1000 ft. That third of the weight is why utilities feed whole neighborhoods with aluminum, and the resistance row is why the same house gets a bigger gauge number: a 100 A service uses 2 AWG copper or 1 AWG aluminum, because 2 AWG aluminum tops out at 90 A on the 75 degree C column.

For a fixed current and length, aluminum also doubles the heating on paper: a 100 A run that drops 3.8 V on copper drops 1.64 x 3.8 = 6.2 V on aluminum. Connections matter more than either number, since aluminum creepback at terminals, not ampacity, is what fails in practice, and terminations must be rated AL/CU.

Voltage drop on a real run: 100 A over 100 ft

Voltage drop follows straight from resistance: VD = 2 x I x R x L / 1000, with the factor of 2 counting the path out and back. Take 100 A over 100 ft of 2 AWG copper, sized from the 75 degree C figure: 0.6235 ohm per km is 0.19004 ohm per 1000 ft, so VD = 2 x 100 x 0.19004 x 0.1 = 3.80 V. On a 240 V circuit that is 1.6 percent; on 120 V it is 3.2 percent.

NEC 210.19(A) carries an informational note suggesting branch-circuit drop stay at or under 3 percent and feeder plus branch together at or under 5 percent. Those are recommendations, not mandatory limits, but they explain a pattern: 2 AWG copper handles 100 A comfortably to about 150 ft at 240 V, and past 250 ft the percentage alone starts to argue for 1 AWG or 1/0.

Low-voltage systems shrink the same math. A 50 A inverter feed of 6 ft on 2 AWG copper at 75 degrees C drops 2 x 50 x (0.6235 x 0.0018288) = 0.114 V, which is 0.95 percent of a 12 V bank. Short is what saves it; stretch that run to 20 ft and the same current loses 0.38 V, pushing 3.2 percent of a 12 V system into the cables.

Where 2 AWG shows up in real installs

2 AWG sits at the heavy end of residential work. Its usual jobs: service entrance conductors for 100 A panels in copper, subpanel feeders, long runs to well pumps or shops where drop matters as much as ampacity, inverter and battery-bank cables at 12 or 24 V, and welding leads, where flexibility matters and the duty cycle lets a smaller cross section run hot safely.

Aluminum takes the same jobs one size larger, and the economics are the reason it gets the chance: copper at 201 lb per 1000 ft is priced by a heavy metal market, while 1 AWG aluminum weighs less per foot than 2 AWG copper and usually costs less for the same ampacity rating. The trade is terminations, conduit fill and a habit of re-torquing to the marking on the lug.

Three mix-ups around 2 gauge

First: 2 is not 2/0. Two-aught (2/0 AWG) is 0.3648 inches, 9.266 mm, and 67.4 mm2 of copper, twice the 33.63 mm2 of 2 AWG. Order "two" when the plan says "two-aught" and the feeder is half the size it should be; the written forms 2 AWG and 2/0 AWG exist because speech blurs them.

Second: gauge numbers are not one ruler. A 2 AWG copper conductor is 6.544 mm of wire; "2 gauge" sheet steel is a different standard entirely, and a 2 gauge shotgun barrel is neither. Wire, sheet metal and shotguns each run their own gauge series, so the number needs its context before it means a size (the gauge to mm converter on this site keeps wire and sheet separate).

Third: ampacity is not one number either. The 95, 115 and 130 A ladder assumes three or fewer current-carrying conductors at 30 degrees C; run it hotter under 310.15(B)(1) or bundle four or more under 310.15(C)(1) and the 90 degree C figure of 130 A is the starting point that shrinks, never the 75 degree C figure. And a conductor at 115 A of ampacity still ends at terminals whose own rating caps the circuit, which is why the finished install, not the table, sets the last number. The widget above runs the gauge math: pick a size, set the material and column, add amperes, feet and system voltage, and it returns diameter, area, resistance, ampacity and voltage drop in one pass.