The short answer
Start from the conductor's ampacity in NEC Table 310.16 for its size, material and insulation temperature rating, multiply by the adjustment factor for more than three current-carrying conductors and the correction factor for an ambient above 30 °C, then apply the small-conductor limits of 240.4(D): 15 A for 14 AWG copper, 20 A for 12 AWG and 30 A for 10 AWG.
Step by step
- Find the conductor's ampacity in Table 310.16 for its size, copper or aluminum, and the 60 °C, 75 °C or 90 °C column of its insulation.
- Multiply by the Table 310.15(C)(1) adjustment factor when more than three current-carrying conductors share a raceway or cable.
- Multiply by the Table 310.15(B)(1) correction factor when the ambient temperature is not 30 °C.
- For 14, 12 and 10 AWG copper and 12 and 10 AWG aluminum, cap the overcurrent device at the 240.4(D) limit.
- Pick the largest standard overcurrent device rating (240.6(A)) that does not exceed the result.
Worked example
10 AWG copper THHN, 90 °C column: 40 A. Six current-carrying conductors in one conduit: × 0.8. A 40 °C ambient: × 0.91. The adjusted ampacity is 29.12 A, under the 30 A limit of 240.4(D), so the largest standard breaker that does not exceed it is 25 A; 240.4(B) can allow the next standard size up where its conditions are met.
| Conductor | 60 °C Cu | 75 °C Cu | 90 °C Cu | 75 °C Al |
|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | — |
| 12 AWG | 20 | 25 | 30 | 20 |
| 10 AWG | 30 | 35 | 40 | 30 |
| 8 AWG | 40 | 50 | 55 | 40 |
| 6 AWG | 55 | 65 | 75 | 50 |
| 4 AWG | 70 | 85 | 95 | 65 |
| 3 AWG | 85 | 100 | 115 | 75 |
| 2 AWG | 95 | 115 | 130 | 90 |
| 1 AWG | 110 | 130 | 145 | 100 |
| 1/0 AWG | 125 | 150 | 170 | 120 |
| 2/0 AWG | 145 | 175 | 195 | 135 |
| 3/0 AWG | 165 | 200 | 225 | 155 |
| 4/0 AWG | 195 | 230 | 260 | 180 |
Adjustment for more than three conductors (Table 310.15(C)(1))
| Current-carrying conductors | Factor |
|---|---|
| up to 3 | 100% |
| 4 to 6 | 80% |
| 7 to 9 | 70% |
| 10 to 20 | 50% |
| 21 to 30 | 45% |
| 31 to 40 | 40% |
| 41 or more | 35% |
Common mistakes
- Sizing from the 90 °C column without checking terminations: most breakers and lugs are rated 60 °C or 75 °C (110.14(C)), so the 90 °C ampacity is a starting point for adjustment and correction, not the final rating.
- Counting the neutral and ground as current-carrying: an equipment grounding conductor never counts, and a neutral that carries only the unbalanced current of the same circuit usually does not.
- Skipping the 240.4(D) limits: 12 AWG copper stays on a 20 A device even though Table 310.16 lists more.
Code numbers and editions
Section numbers here are NEC 2023's, which most states enforce.
Code adoption cycles change. Always confirm the edition and amendments in force with your Authority Having Jurisdiction (AHJ) before permitting work.