Wire Ampacity & Breaker Size
The 60, 75, and 90 degree columns, the termination cap, conductor-count and ambient derating, and the maximum breaker the code lets you put on that wire.
A 12 AWG THHN is 30 A in the 90 column, 25 A in the 75 column, and 20 A on a breaker, because 240.4(D) caps small conductors. Attic runs and 4-wire circuits eat the rest.
How the math works
- Table 310.16 copper, not more than 3 conductors, 30°C: 14 = 20/25/25, 12 = 25/30/30, 10 = 35/40/40, 8 = 40/50/55, 6 = 50/65/75, 4 = 65/85/95, 2 = 95/115/130, 4/0 = 230/255/290, in the 60 / 75 / 90°C columns.
- Termination cap (110.14(C)): size to the 75°C column when terminations are rated 75°C (nearly all modern breakers and lugs), and to the 60°C column for equipment rated 60°C, which includes breakers and lugs 100 A and smaller on many older panels.
- Ampacity adjustment (310.15(C)(1)): 4 to 6 current-carrying conductors = 80%, 7 to 9 = 50%, 10 to 20 = 45% of the table value. Neutrals carrying harmonic current count. In a 4-wire 3-phase circuit all 4 conductors are current-carrying, so a 12 AWG 3-wire plus neutral 20 A branch derates to 24 A and lands under the 20 A breaker.
- Ambient correction (310.15(B)): 40°C = 0.91, 45°C = 0.87, 50°C = 0.82, 60°C = 0.71 on 90°C insulation; the 75°C column factors are 0.88 / 0.82 / 0.75 / 0.58. Attic and roof-space wiring is the usual reason a conductor is derated.
- Breaker (240.4): next standard size not above the design ampacity, with the small-conductor caps of 240.4(D): 14 = 15 A, 12 = 20 A, 10 = 30 A. A 20 A breaker on 12 AWG is code even though the wire is rated 30 A; a 30 A breaker on 12 AWG is not.
What breaker does 12 gauge wire support?
20 A. The 90°C column says 30 A and the 75°C column says 30 A, and 240.4(D) caps 12 AWG copper at 20 A, so 20 A is the maximum breaker, and 15 A on a 15 A circuit is fine.
How many 12 gauge wires can I run in one conduit?
Up to 9 current-carrying 12 AWG conductors fit physically in 1 inch EMT, and 310.15(C)(1) derates 7 to 9 conductors to 50% of ampacity, so 12 AWG lands at 12.5 A: not a usable 20 A branch. Keep 12 AWG branch circuits at 4 wires or fewer per raceway.
Does THHN wire in a hot attic need upsizing?
Yes. At 40°C a 12 AWG THHN derates to 0.91 of 30 A = 27 A, then the 75°C termination cap of 25 A binds, and 240.4(D) caps it at 20 A. At 50°C the derated value is 24.6 A, so 10 AWG is the safe answer for a 20 A attic branch.