NEC 310.15 holds the two factors that shrink a conductor's Table 310.16 ampacity: the ambient temperature correction factors of Table 310.15(B)(1), and the adjustment factors of Table 310.15(C)(1) for more than three current-carrying conductors in a raceway. Both are multipliers, and when both apply you multiply them together.
| Number of current-carrying conductors | Percent of Table 310.16 ampacity |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
| Ambient (°C) | 60°C conductors | 75°C conductors | 90°C conductors |
|---|---|---|---|
| 10 or less | 1.29 | 1.20 | 1.15 |
| 11–15 | 1.22 | 1.15 | 1.12 |
| 16–20 | 1.15 | 1.11 | 1.08 |
| 21–25 | 1.08 | 1.05 | 1.04 |
| 26–30 | 1.00 | 1.00 | 1.00 |
| 31–35 | 0.91 | 0.94 | 0.96 |
| 36–40 | 0.82 | 0.88 | 0.91 |
| 41–45 | 0.71 | 0.82 | 0.87 |
| 46–50 | 0.58 | 0.75 | 0.82 |
| 51–55 | 0.41 | 0.67 | 0.76 |
| 56–60 | — | 0.58 | 0.71 |
| 61–65 | — | 0.47 | 0.65 |
| 66–70 | — | 0.33 | 0.58 |
| 71–75 | — | — | 0.50 |
| 76–80 | — | — | 0.41 |
| 81–85 | — | — | 0.29 |
Nine 12 AWG THHN current-carrying conductors run through a raceway in a 40°C attic. Start from the 90°C column of Table 310.16, because THHN is a 90°C conductor and derating starts from the conductor rating: 30 A.
Adjustment for 7–9 conductors: 30 × 0.70 = 21 A. Correction for 36–40°C ambient on a 90°C conductor: 21 × 0.91 = 19.1 A. That is below the 20 A the circuit was designed for, so a 20 A breaker on 12 AWG no longer works in this raceway — you either pull 10 AWG, split the conductors across two raceways, or reduce the circuit to 15 A.
Note the order does not matter — multiplication is commutative — but the starting point does. Deriving from the 75°C column instead would have given 25 × 0.70 × 0.88 = 15.4 A and sent you to a larger conductor than the code requires.
These tables moved in the 2020 NEC and older documentation still uses the old numbers. Ambient temperature correction was Table 310.15(B)(2)(a) and is now Table 310.15(B)(1). The adjustment factors for more than three current-carrying conductors were Table 310.15(B)(3)(a) and are now Table 310.15(C)(1). The values themselves did not change.
Always confirm which NEC edition your jurisdiction has adopted — adoption lags publication by years and varies by state and city.
NEC 310.15(C)(1) lists exceptions worth knowing. Conductors in a nipple 24 inches or shorter between enclosures are exempt. Underground conductors entering or leaving an outdoor trench are exempt where they are protected by a raceway no longer than 10 feet and there are no more than four conductors. (The separate “10 feet or 10%, whichever is less” rule is in NEC 310.15(A): where a short section of a run has a lower ampacity, the higher ampacity may be used for the whole run.) And where the load diversity is such that not all conductors carry current simultaneously, the count only includes those that do.
NEC 310.15 contains two derating tables. Table 310.15(B)(1) corrects conductor ampacity for ambient temperatures other than 30°C. Table 310.15(C)(1) adjusts ampacity when more than three current-carrying conductors share a raceway or cable: 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20, 45% for 21–30, 40% for 31–40, and 35% for 41 or more.
Derating begins at the fourth current-carrying conductor in a raceway or cable bundled for more than 24 inches. Three or fewer current-carrying conductors use the Table 310.16 ampacity unchanged.
No. Equipment grounding conductors are never counted as current-carrying conductors for NEC 310.15(C)(1). They do still count toward conduit fill, which is a separate area calculation.
It depends on the circuit. The neutral of a 3-wire circuit from a single-phase system carries only the unbalanced current and does not count. The neutral of a 4-wire, 3-phase wye circuit does count when the major portion of the load is nonlinear, because harmonic currents add rather than cancel in the neutral.
Yes. Derating starts from the ampacity column matching the conductor insulation rating, so THHN and THWN-2 derate from the 90°C column. The final derated result is then limited by the termination temperature rating under NEC 110.14(C), normally 75°C.