12 AWG Wire — Amp Rating, Ampacity & Diameter

The amp rating for 12 AWG wire depends on the insulation temperature rating, not on the conductor alone. Copper ampacity is about 20 A on a 60 °C conductor, 24 A at 75 °C and 27 A at 90 °C, all at 30 °C ambient with no more than three current-carrying conductors in the raceway. The carrying capacity drops from there in hot spaces or crowded conduit, and terminations rated 75 °C cap what you can actually use.

Geometrically, 12 AWG is 0.0808 in (2.05 mm) in bare diameter, which is 6,530 circular mils. The cross sectional area works out to 3.31 mm², so the nearest IEC 60228 metric conductor is the next standard size at or above that figure — AWG sizes and mm² sizes never line up exactly, so round up rather than to the closest value.

American Wire Gauge (AWG) is a standard specification for the sizes of solid round wires used as electrical conductors, and the AWG number counts the drawing operations used to reach the diameter. That is why larger gauges are thinner wire and why standardized wire steps follow a fixed ratio: one AWG step changes diameter by about 12.2% and area by about 26%. Above 1 AWG the American standard continues 1/0 to 4/0, then switches to kcmil for anything larger.

A cable marked 12/2 contains two insulated 12 AWG conductors — one ungrounded, one grounded — plus a bare equipment grounding conductor that is not counted in the name. 12/3 adds a third insulated conductor, typically red, for switch legs, travellers or a second hot leg. The ampacity is identical in both cases, because it is a property of the conductor size and insulation, not the conductor count: still 20 A on the 60 °C basis that non-metallic sheathed cable is held to.

Read the higher-temperature figures as insulation and calculation properties, not as usable circuit ratings. The insulation used in NM-B is a 90 °C material, and the same copper conductor with 90 °C insulation installed in a raceway computes to a higher ampacity — but a non-metallic sheathed cable assembly must be applied at the 60 °C ampacity of 20 A. On top of that, 12 AWG copper is limited to a 20 A overcurrent device by the small-conductor rule, which applies to 14, 12 and 10 AWG copper regardless of insulation, with only narrow listed exceptions such as certain motor and A/C equipment circuits. So the 27 A in the 90 °C column above is a calculated value for a 90 °C-rated conductor, used as the starting point for derating — never as a load limit for 12/2 NM-B. In practice a standard residential 12/2 NM-B circuit is protected at no more than 20 A, and the load on it must stay within that.

That is also why 12/2 NM-B and 12 AWG THHN in conduit are not interchangeable on paper: THHN computes to 24 A where every termination in the circuit is listed for 75 °C, while the NM cable stays at 20 A regardless of the breaker it lands on. Termination temperature is a ceiling in both cases — 90 °C insulation on equipment with 75 °C lugs is still read from the 75 °C column. Separately, where more than three current-carrying conductors run together in one raceway or cable, or cables are bundled together for more than 24 inches so that more than three current-carrying conductors share the bundle, ampacity adjustment applies to all of them — which lowers these numbers further, never raises them.

Solid versus stranded does not change ampacity either. Solid 12 AWG is standard in residential branch circuits; stranded is used where the conductor must flex or be pulled around tight bends. Both have the same 6,530 circular mils of copper, though stranded has a slightly larger overall diameter, which matters for conduit fill and for termination lugs.

12 AWG is a gauge designation, not a measurement. The number counts the drawing dies historically used to pull the wire down to size, so it runs backwards: a higher number is a thinner conductor. Each single step changes diameter by about 12.2% and area by about 26%, which is why moving from 12 AWG to the next size up is a meaningful jump in both cost and capacity rather than a rounding change.

Written forms you will see for the same conductor include 12 AWG, 12 ga, 12 gauge and #12. They are identical. What is not identical is the cable assembly the conductor ships in, the insulation rated on it, or the termination temperature at the breaker — all three can lower the usable ampacity below the 24 A that 12 AWG copper reaches at 75 °C.

12 AWG is the workhorse of North American branch-circuit wiring: protected at 20 A in ordinary building wiring by the same small-conductor rule that caps 14 AWG at 15 A, and used for every circuit type where 15 A is not enough. Kitchen and dining small-appliance circuits, laundry receptacles, bathroom receptacle circuits, garage and workshop outlets, and nearly all commercial receptacle work land here.

The reason it is the default upsize is voltage drop, not current. A 20 A load on 12 AWG copper holds 3% drop far enough to cover most residential home runs, and stepping from 14 to 12 buys roughly 60% more circular mils for a marginal increase in material cost. Many electricians pull 12 AWG throughout and set the device rating per circuit rather than stocking two cable sizes.

12 AWG is also where the 60/75/90 °C column distinction starts to bite in a way installers notice. Inside 12/2 or 12/3 NM-B the conductor is read at the 60 °C column, while the same copper in THHN in conduit at 40 °C ambient derates from the 90 °C base — two very different numbers for the same wire, and the source of most argument about what 12 AWG 'is' rated for.

Continuous loads are the other trap. A load running three hours or more is designed at 125%, so a 20 A device on 12 AWG supports only 16 A of continuous current. That is exactly why a 16 A EV charger, a commercial lighting run, or a long-cycle heater lands on 12 AWG at 20 A while a nominally identical 20 A intermittent load does not need the same margin.