A 12 AWG THWN copper conductor carries 25 A in the 75°C column and 30 A in the 90°C column, yet most branch circuits using it still take a 20 A breaker.
Three numbers get stapled to one spool of wire, and the smallest usually decides your breaker size. 12 AWG THWN wire ampacity comes down to two questions: how much current the copper survives, and how much the overcurrent rules allow.
The chart below shows 12 AWG beside comparable gauges; the sections after explain why the breaker number lands lower than the wire’s rating.
12 Gauge THWN Wire Ampacity: Which Temperature Column Applies
The insulation’s temperature rating decides the column. THWN is rated 75°C, placing 12 AWG copper at 25 A; THWN-2 carries a 90°C rating, placing it at 30 A. Because every jacket is marked, the answer is printed on the wire — 12 AWG THWN reads in the 75°C column, 12 AWG THWN-2 in the 90°C column. The “-2” signals the higher temperature rating, not a thicker conductor. Wire makers such as Southwire print those designations on their THHN/THWN copper spec pages.
A higher column pays off only when everything touching the conductor is rated for that heat. Most breakers, receptacles, and switches are 75°C, so a 90°C conductor is usually still read in the 75°C column. Ratings exist because heat ages insulation. Cerrowire’s published ampacity charts list these values by gauge and temperature column.
| Copper Conductor | 75°C Ampacity (THWN) | 90°C Ampacity (THWN-2) |
|---|---|---|
| 14 AWG | 20 A | 25 A |
| 12 AWG | 25 A | 30 A |
| 10 AWG | 35 A | 40 A |
| 8 AWG | 50 A | 55 A |
| 6 AWG | 65 A | 75 A |
Why Is 12 Gauge Copper Limited To 20 Amps?
The small-conductor rule in the National Electrical Code caps overcurrent protection for 12 AWG copper at 20 A on general branch circuits, whatever the ampacity table shows. A breaker protects the wire as much as the load. The same rule holds 14 AWG at 15 A and allows 10 AWG up to 30 A — why household circuits come in those steps instead of matching the chart.
Two mix-ups cause most confusion. The first treats conductor ampacity and maximum breaker size as one figure; they answer different questions, and the smaller governs. The second reads the wrong column — a 12 AWG THWN run is not a 30 A circuit just because a 90°C column lists 30 A.
Installation Conditions That Change The Allowed Current
Ambient heat, conduit fill, and termination ratings can pull usable current below the chart value; the lowest number in the chain counts.
- High ambient temperature. Tables assume 30°C (86°F), so a hot attic or boiler room needs a correction factor that drops allowable current.
- Conduit fill. More than three current-carrying conductors in a raceway adjusts the value downward; four to six typically land at 80% of the table figure.
- Termination ratings. A 90°C conductor tied to a 75°C breaker is read in the 75°C column, so its headline number never gets used.
- Wet locations. THWN’s “W” means listed for wet conditions, covering outdoor conduit and damp basements — check the jacket marking before assuming a spool qualifies.
Shopping is where gauges and ratings get mixed up fastest. When the circuit needs 20 A of protection, a tested roundup of 12 gauge THWN wire beats comparing spool listings one at a time, because the listings worth buying print the full temperature marking on the jacket.
For a standard 20 A branch circuit: read the jacket marking, use the 75°C column value for THWN, check ambient and conduit-fill conditions, and keep the breaker at 20 A. A load genuinely needing more current calls for 10 AWG copper — raising the breaker on 12 AWG is not permitted.
FAQs
Can 12 AWG THWN handle 25 amps?
A 12 AWG THWN conductor is listed at 25 A in the 75°C column, and 12 AWG THWN-2 reaches 30 A in the 90°C column. Those are the conductor’s own limits under ideal conditions. On a general branch circuit, the breaker stays capped at 20 A, so the wire rarely runs near that chart value.
What does the -2 mean on THWN-2?
The -2 marks a 90°C insulation rating instead of the 75°C rating plain THWN carries. That matters only when every part of the circuit, including breaker terminals, splices, and devices, is also rated 90°C. Since most home terminations are 75°C, both wire types often end up read in the same column.
Can I put a 30 amp breaker on 12 AWG copper?
No. The small-conductor overcurrent rule caps 12 AWG copper at 20 A on general branch circuits, and a bigger breaker does not raise the wire’s permitted protection level. A 30 A circuit calls for 10 AWG copper. When a 12 AWG run keeps tripping, the load usually needs to change.
References & Sources
- Cerrowire. “Ampacity Charts | Wire Gauge Chart.” Lists copper ampacity by gauge across the 75°C and 90°C columns.
- Southwire. “THHN/THWN Copper Silicone-Free.” Confirms the THWN and THWN-2 insulation temperature designations.
- Ugly’s Electrical References. “Residential Wiring 2017 Errata, Page 2.” Supports the 20 A overcurrent limit applied to 12 AWG copper branch circuits.
