NEC Table 310.16 gives the allowable ampacity of insulated conductors rated up to 2000 volts, at not more than three current-carrying conductors in a raceway, cable, or earth, in a 30°C (86°F) ambient. It is the table every branch circuit and feeder in a building starts from. The headline numbers most people need: 14 AWG copper = 20 A, 12 AWG = 25 A, 10 AWG = 35 A at 75°C — but NEC 240.4(D) caps those same conductors at 15 A, 20 A and 30 A of overcurrent protection.
| Size (AWG/kcmil) | 60°C (TW, UF) | 75°C (THW, THWN, XHHW) | 90°C (THHN, THWN-2, XHHW-2) |
|---|---|---|---|
| 14 | 15 A | 20 A | 25 A |
| 12 | 20 A | 25 A | 30 A |
| 10 | 30 A | 35 A | 40 A |
| 8 | 40 A | 50 A | 55 A |
| 6 | 55 A | 65 A | 75 A |
| 4 | 70 A | 85 A | 95 A |
| 3 | 85 A | 100 A | 115 A |
| 2 | 95 A | 115 A | 130 A |
| 1 | 110 A | 130 A | 145 A |
| 1/0 | 125 A | 150 A | 170 A |
| 2/0 | 145 A | 175 A | 195 A |
| 3/0 | 165 A | 200 A | 225 A |
| 4/0 | 195 A | 230 A | 260 A |
| 250 kcmil | 215 A | 255 A | 290 A |
| 300 kcmil | 240 A | 285 A | 320 A |
| 350 kcmil | 260 A | 310 A | 350 A |
| 400 kcmil | 280 A | 335 A | 380 A |
| 500 kcmil | 320 A | 380 A | 430 A |
| Size (AWG/kcmil) | 60°C (TW, UF) | 75°C (THW, THWN, XHHW) | 90°C (THHN, THWN-2, XHHW-2) |
|---|---|---|---|
| 12 | 15 A | 20 A | 25 A |
| 10 | 25 A | 30 A | 35 A |
| 8 | 35 A | 40 A | 45 A |
| 6 | 40 A | 50 A | 55 A |
| 4 | 55 A | 65 A | 75 A |
| 3 | 65 A | 75 A | 85 A |
| 2 | 75 A | 90 A | 100 A |
| 1 | 85 A | 100 A | 115 A |
| 1/0 | 100 A | 120 A | 135 A |
| 2/0 | 115 A | 135 A | 150 A |
| 3/0 | 130 A | 155 A | 175 A |
| 4/0 | 150 A | 180 A | 205 A |
| 250 kcmil | 170 A | 205 A | 230 A |
| 300 kcmil | 195 A | 230 A | 260 A |
| 350 kcmil | 210 A | 250 A | 280 A |
| 400 kcmil | 225 A | 270 A | 305 A |
| 500 kcmil | 260 A | 310 A | 350 A |
This is where most Table 310.16 mistakes happen. You may use the 90°C column as the starting point for derating calculations, but the final ampacity is limited by the temperature rating of the terminations at both ends of the circuit. NEC 110.14(C) puts most equipment at 60°C for circuits up to 100 A and 75°C above that, and virtually every breaker and lug in a residential or light commercial panel is listed 75°C.
In practice: 12 AWG THHN is a 90°C conductor, so you start derating from 30 A. But you land on a 75°C-rated breaker, so the answer can never exceed the 25 A in the 75°C column — and NEC 240.4(D) then caps the breaker at 20 A regardless. The 90°C column buys you derating headroom, not a bigger breaker.
For 14, 12 and 10 AWG copper and 12 and 10 AWG aluminum, NEC 240.4(D) sets a hard limit on the overcurrent device that is lower than the Table 310.16 ampacity. This is not a suggestion or a rounding rule — it is the reason a 12 AWG circuit is a 20 A circuit even though the table says 25 A.
14 AWG copper: 15 A. 12 AWG copper: 20 A. 10 AWG copper: 30 A. 12 AWG aluminum: 15 A. 10 AWG aluminum: 25 A. 16 AWG copper: 10 A. 18 AWG copper: 7 A. Above 10 AWG the rule stops applying and Table 310.16 governs directly. NEC 240.4(E) and 240.4(G) list the exceptions — tap conductors, motor circuits, welders and similar loads sized under their own articles.
Our wire size calculator applies the 240.4(D) cap automatically.
NEC 310.12 permits smaller conductors for a single-phase dwelling service or feeder carrying the entire load: the service conductors may be sized at 83% of the service rating. That is why a 200 A residential service is commonly wired with 2/0 copper or 4/0 aluminum rather than the 4/0 copper that Table 310.16 alone would demand. The 83% rule applies only to the main service or a feeder supplying the whole dwelling, never to ordinary branch circuits.