Ampacity is the current a conductor can carry continuously without exceeding its temperature rating. The everyday table, NEC Table 310.16, is written for one specific installation: not more than three current-carrying conductors, in a raceway, cable, or direct burial, with 30°C (86°F) air around them. Use a different column and those assumptions no longer hold.
How to read the columns
Pick the column for the insulation on the conductor, not the column you wish you had. TW and UF are 60°C. THW, THWN, XHHW, and USE are 75°C. THHN, THWN-2, XHHW-2, and USE-2 are 90°C. NM-B cable is limited to the 60°C column even when the conductors inside are rated hotter. The full list of type letters lives in the code’s insulation table; the names above are the ones that show up on most reels.
The terminal on the breaker or the panel has its own limit. For equipment rated 100 A or less, or marked for 14 AWG through 1 AWG, unmarked terminals are treated as 60°C. Most modern breakers are marked 75°C, and then the 75°C column is the cap. Conductors over 100 A, or terminals marked for conductors larger than 1 AWG, are 75°C unless marked otherwise. A 90°C insulation is still useful: you may start the ambient and bundling math in the 90°C column, and you may not finish above the terminal column.
| Size | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 60°C | Aluminum 75°C | Aluminum 90°C | 240.4(D) max breaker |
|---|---|---|---|---|---|---|---|
| 18 AWG | — | — | 14 | — | — | — | — |
| 16 AWG | — | — | 18 | — | — | — | — |
| 14 AWG | 15 | 20 | 25 | — | — | — | 15 Cu |
| 12 AWG | 20 | 25 | 30 | 15 | 20 | 25 | 20 Cu · 15 Al |
| 10 AWG | 30 | 35 | 40 | 25 | 30 | 35 | 30 Cu · 25 Al |
| 8 AWG | 40 | 50 | 55 | 35 | 40 | 45 | — |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 55 | — |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 | — |
| 3 AWG | 85 | 100 | 115 | 65 | 75 | 85 | — |
| 2 AWG | 95 | 115 | 130 | 75 | 90 | 100 | — |
| 1 AWG | 110 | 130 | 145 | 85 | 100 | 115 | — |
| 1/0 AWG | 125 | 150 | 170 | 100 | 120 | 135 | — |
| 2/0 AWG | 145 | 175 | 195 | 115 | 135 | 150 | — |
| 3/0 AWG | 165 | 200 | 225 | 130 | 155 | 175 | — |
| 4/0 AWG | 195 | 230 | 260 | 150 | 180 | 205 | — |
| 250 kcmil | 215 | 255 | 290 | 170 | 205 | 230 | — |
| 300 kcmil | 240 | 285 | 320 | 195 | 230 | 260 | — |
| 350 kcmil | 260 | 310 | 350 | 210 | 250 | 280 | — |
| 400 kcmil | 280 | 335 | 380 | 225 | 270 | 305 | — |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 | — |
| 600 kcmil | 350 | 420 | 475 | 285 | 340 | 385 | — |
| 700 kcmil | 385 | 460 | 520 | 315 | 375 | 425 | — |
| 750 kcmil | 400 | 475 | 535 | 320 | 385 | 435 | — |
| 800 kcmil | 410 | 490 | 555 | 330 | 395 | 445 | — |
| 900 kcmil | 435 | 520 | 585 | 355 | 425 | 480 | — |
| 1000 kcmil | 455 | 545 | 615 | 375 | 445 | 500 | — |
| 1250 kcmil | 495 | 590 | 665 | 405 | 485 | 545 | — |
| 1500 kcmil | 525 | 625 | 705 | 435 | 520 | 585 | — |
| 1750 kcmil | 545 | 650 | 735 | 455 | 545 | 615 | — |
| 2000 kcmil | 555 | 665 | 750 | 470 | 560 | 630 | — |
Eighteen and 16 AWG appear in the 90°C copper column and are not general building-wire sizes. The calculator does not offer them for power circuits. Fourteen AWG aluminum is not listed. A dash in the chart means the same thing: no value, not zero.
Ambient temperature
Table 310.16 was calculated at 30°C. Hotter air leaves the conductor less room to shed heat, so the ampacity is multiplied by a factor that depends on the insulation rating. A 90°C conductor keeps more of its ampacity in a hot attic than a 60°C conductor does, which is why derating is allowed to start in the insulation column. The factors below are NEC Table 310.15(B)(1). A dash means that insulation is not listed at that ambient.
| Ambient | 60°C | 75°C | 90°C |
|---|---|---|---|
| 10°C or less | 1.29 | 1.20 | 1.15 |
| 11–15°C | 1.22 | 1.15 | 1.12 |
| 16–20°C | 1.15 | 1.11 | 1.08 |
| 21–25°C | 1.08 | 1.05 | 1.04 |
| 26–30°C | 1.00 | 1.00 | 1.00 |
| 31–35°C | 0.91 | 0.94 | 0.96 |
| 36–40°C | 0.82 | 0.88 | 0.91 |
| 41–45°C | 0.71 | 0.82 | 0.87 |
| 46–50°C | 0.58 | 0.75 | 0.82 |
| 51–55°C | 0.41 | 0.67 | 0.76 |
| 56–60°C | — | 0.58 | 0.71 |
| 61–65°C | — | 0.47 | 0.65 |
| 66–70°C | — | 0.33 | 0.58 |
| 71–75°C | — | — | 0.50 |
| 76–80°C | — | — | 0.41 |
| 81–85°C | — | — | 0.29 |
More than three current-carrying conductors
Conductors bundled for more than 24 inches, without spacing, heat each other. Table 310.15(C)(1) gives the adjustment. Equipment grounding conductors are not counted. A neutral that carries only the unbalanced current of a linear single-phase multiwire circuit is not counted either. A neutral that carries the full current, including the neutral of a circuit with harmonic loads, is counted. The count is the total in the raceway or cable, including spares that are connected, not the number of circuits you remember pulling.
| Current-carrying conductors | Percent of ampacity |
|---|---|
| 1–3 | 100% |
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
The two factors multiply. Nine 12 AWG THHN conductors in a 40°C attic start at 30 A in the 90°C column. The bundling factor for seven to nine conductors is 0.70, and the 36–40°C factor for 90°C insulation is 0.91. That is 30 × 0.70 × 0.91 = 19.11 A, which is then capped by the terminal column if the cap is lower. Nineteen amperes does not support a 20 A circuit, so the raceway needs a larger conductor, fewer conductors, or a cooler path. Open that attic run in the calculator.
Small-conductor breaker limits
NEC 240.4(D) limits the overcurrent device, which is a different number from the table ampacity. Copper is limited to 15 A on 14 AWG, 20 A on 12 AWG, and 30 A on 10 AWG. Aluminum is limited to 15 A on 12 AWG and 25 A on 10 AWG. Twelve AWG copper at 75°C is a 25 A conductor in the ampacity table, and it still cannot be the only wire on a continuous 20 A load, because that load needs a 25 A device and 240.4(D) stops at 20 A. The code has exceptions for specific equipment. The calculator applies the general rule, and the checkbox on the form turns it off when you are working under one of those exceptions.
The chart is a planning reference, not a copy of the code book and not a substitute for it. Editions and local amendments differ. When the installation is not the ordinary raceway at 30°C, read the section that applies before you buy the wire. The wire size calculator applies this chart together with voltage drop.