How many amps can 12 gauge wire carry?
12 AWG copper is rated 25 A at 75°C in NEC Table 310.16 — but NEC 240.4(D) caps the breaker at 20 A for general use, so 20 A is the working answer.
The same small-conductor rule limits 14 AWG copper to 15 A and 10 AWG to 30 A, whatever the ampacity table says. Above 10 AWG the cap no longer applies and the table value governs.
Table 310.16 assumes no more than three current-carrying conductors in a raceway at 30°C ambient. Both change the answer downward: at 40°C a 75°C conductor derates to 88%, and four to six conductors in one raceway derate to 80%. Those factors multiply.
| Size | Cu 60°C | Cu 75°C | Cu 90°C | Al 75°C | Max breaker, 240.4(D) |
|---|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A | — | 15 A (Cu) |
| 12 AWG | 20 A | 25 A | 30 A | 20 A | 20 A (Cu) |
| 10 AWG | 30 A | 35 A | 40 A | 30 A | 30 A (Cu) |
| 8 AWG | 40 A | 50 A | 55 A | 40 A | — |
| 6 AWG | 55 A | 65 A | 75 A | 50 A | — |
| 4 AWG | 70 A | 85 A | 95 A | 65 A | — |
| 3 AWG | 85 A | 100 A | 115 A | 75 A | — |
| 2 AWG | 95 A | 115 A | 130 A | 90 A | — |
| 1 AWG | 110 A | 130 A | 145 A | 100 A | — |
| 1/0 AWG | 125 A | 150 A | 170 A | 120 A | — |
| 2/0 AWG | 145 A | 175 A | 195 A | 135 A | — |
| 3/0 AWG | 165 A | 200 A | 225 A | 155 A | — |
| 4/0 AWG | 195 A | 230 A | 260 A | 180 A | — |
| 250 kcmil | 215 A | 255 A | 290 A | 205 A | — |
| 300 kcmil | 240 A | 285 A | 320 A | 230 A | — |
| 350 kcmil | 260 A | 310 A | 350 A | 250 A | — |
| 500 kcmil | 320 A | 380 A | 430 A | 310 A | — |
Conditions this table assumes
Table 310.16 figures apply to not more than three current-carrying conductors in a raceway, cable or direct burial, at an ambient of 30°C (86°F). Change either and the numbers change:
- Ambient correction (Table 310.15(B)(1)): at 40°C a 75°C conductor derates to 88%, at 50°C to 75%. Conduit in an unvented attic or in direct sun is routinely well above 30°C.
- Bundling adjustment (Table 310.15(C)(1)): four to six current-carrying conductors derate to 80%, seven to nine to 70%, ten to twenty to 50%.
- They multiply. Six conductors in a 45°C attic take both factors: 0.80 × 0.82 = 0.66 of the table value.
Picking the right column
Use the column matching the lowest temperature rating anywhere in the circuit — conductor, breaker, lug or device. Most modern breakers and devices are rated 75°C. Some residential equipment, and anything marked 60°C only, pulls you to the 60°C column regardless of the wire.
THHN is a 90°C conductor, but you rarely get to use the 90°C column for sizing, because it almost always terminates on something rated lower. The 90°C column's main use is as the starting point for derating calculations, where you may derate from 90°C down and still land above the 75°C value.
Aluminium
Aluminium carries roughly 78% of copper's ampacity at the same size, so you typically go two sizes up to match: 4/0 aluminium is about equivalent to 2/0 copper. For large feeders and service entrances it is substantially cheaper and very widely used.
What matters with aluminium is the terminations. Use connectors and devices rated for it (marked AL or CU-AL), apply antioxidant compound where specified, and torque to the manufacturer's figure with a torque screwdriver — aluminium creeps under pressure, and a loose aluminium connection is the classic cause of an overheating lug.
Common questions
How many amps can 12 AWG wire carry?
25 A at 75C per Table 310.16, but NEC 240.4(D) limits the breaker to 20 A for general use, so 20 A is the practical answer.
How many amps can 6 AWG copper carry?
65 A at 75C, 55 A at 60C, 75 A at 90C. No small-conductor cap applies above 10 AWG.
Is aluminium wire safe?
Modern aluminium building wire in the larger sizes is safe and standard for feeders and services. It needs terminations rated for aluminium, correct torque, and antioxidant where specified. The historical problems were with small-gauge aluminium branch circuits and connectors not designed for it.
Does conduit in an attic change the ampacity?
Yes. Ampacity is rated at 30C ambient, and an attic can be far hotter. At 40C a 75C conductor derates to 88% of the table value, and at 50C to 75%.