What is NEC Table 310.15?

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.

How many conductors before derating starts?

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.

Does the ground wire count for derating?

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.

Does the neutral count as a current-carrying conductor?

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.

Do I derate from the 90°C column?

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.

NEC Table 310.15 — Correction and Adjustment Factors

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.

Table 310.15(C)(1) — More Than Three Current-Carrying Conductors

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%

Applies when conductors are bundled longer than 24 inches without maintaining spacing. Equipment grounding conductors never count. A neutral counts only when it carries current — the neutral of a 3-wire circuit from a single-phase system does not count, but the neutral of a 4-wire wye circuit feeding nonlinear loads does.

Table 310.15(B)(1) — Ambient Temperature Correction (30°C Based Tables)

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

For ambients other than 30°C, multiply the Table 310.16 ampacity by the factor in the column matching the conductor insulation rating. Rooftop raceways in direct sunlight, attics, boiler rooms, and outdoor equipment are the usual triggers.

Worked example: both factors at once

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.

The 2017 to 2020 renumbering

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.

When derating does not apply

NEC 310.15(C)(1) lists exceptions worth knowing. Conductors in a nipple 24 inches or shorter between enclosures are exempt. Cables and conductors installed in raceways underground and entering directly from the earth are exempt for the first 10 feet or 10% of the run, whichever is less. And where the load diversity is such that not all conductors carry current simultaneously, the count only includes those that do.

NEC Table 310.16 — conductor ampacity →Conduit fill charts →Conduit fill calculator →Ampacity tables →Wire size calculator →