Ampacity Correction & Adjustment Factors

The ampacity tables assume 30°C air and no more than three current-carrying conductors together. Real installations get hot attics and packed raceways, and these two multipliers are how the code accounts for both: the ambient temperature correction of Table 310.15(B)(1) and the conductor-bundling adjustment of Table 310.15(C)(1). Multiply them against the base ampacity — starting from the column matching the conductor's insulation rating.

Ambient temperature correction

NEC 310.15(B)(1)

Correction factor by ambient temperature and conductor insulation rating, based on the ampacity tables' 30°C (86°F) ambient (values per NEC Table 310.15(B)(1)). Multiply the Table 310.16 ampacity by the factor in the column matching the insulation's temperature rating.
Ambient (°C)	Ambient (°F)	60°C rated	75°C rated	90°C rated
10 or less	50 or less	1.29	1.20	1.15
11–15	51–59	1.22	1.15	1.12
16–20	60–68	1.15	1.11	1.08
21–25	69–77	1.08	1.05	1.04
26–30	78–86	1.00	1.00	1.00
31–35	87–95	0.91	0.94	0.96
36–40	96–104	0.82	0.88	0.91
41–45	105–113	0.71	0.82	0.87
46–50	114–122	0.58	0.75	0.82
51–55	123–131	0.41	0.67	0.76
56–60	132–140	—	0.58	0.71
61–65	141–149	—	0.47	0.65
66–70	150–158	—	0.33	0.58
71–75	159–167	—	—	0.50
76–80	168–176	—	—	0.41
81–85	177–185	—	—	0.29

A dash means the ambient is beyond that insulation's rating — the conductor can't be used there at any ampacity. The 90°C column is highlighted because derating starts from it whenever the wire is 90°C-rated (THHN, XHHW-2), even though terminations usually cap the final answer at the 75°C value.

More than three current-carrying conductors

NEC 310.15(C)(1)

Adjustment factor by the number of current-carrying conductors bundled in the same raceway or cable, or stacked without spacing longer than 24 inches (values per NEC Table 310.15(C)(1), as percent of the base ampacity). The 1–3 row is the ampacity tables' baseline.
Current-carrying conductors	Multiplier	% of base ampacity
1–3	1.00	100%
4–6	0.80	80%
7–9	0.70	70%
10–20	0.50	50%
21–30	0.45	45%
31–40	0.40	40%
41 and above	0.35	35%

Applies on top of any ambient correction — the factors multiply. Nipples of 24 inches or less are exempt, per 310.15(C)(1) itself.

Running a derate, start to finish

Take the base ampacity from the wire ampacity chart in the column matching the insulation — 90°C for THHN — then multiply by both factors: ambient first, bundling second (order doesn't matter; they multiply). The result must clear the load, must not exceed the termination-rated ampacity (usually the 75°C column), and small conductors keep their 240.4(D) breaker caps. The ampacity & derating calculator runs this exact chain — same factors, same tables — and shows its work.

One non-obvious consequence: derating is why the 90°C column exists for building wire at all. You can rarely terminate at 90°C, but starting the multiplication from the higher number often saves a wire size — 30 A × 0.91 × 0.70 from the 90°C column survives where the same math from the 60°C column fails.

Where the numbers come from

The correction factors aren't arbitrary: each one is the code's own equation — the square root of (conductor rating minus ambient) over (rating minus 30°C) — evaluated at the top of each band, which is why the columns fall off a cliff as the ambient approaches the insulation rating and why a 60°C conductor simply exits the table above 55°C. The bundling factors account for conductors heating each other when they can't shed heat independently; they kick in hard at ten conductors (50%), which is the point where stuffing one more circuit into an existing raceway usually stops being worth it.

Common questions

How do I apply both factors together?

Multiply, starting from the ampacity column matching the conductor insulation — for THHN that is the 90°C column. Example: 12 AWG THHN (30 A at 90°C) with eight current-carrying conductors in a conduit through a 38°C space is 30 × 0.91 × 0.70 = 19.1 A. The final answer must also not exceed what the terminations allow (usually the 75°C ampacity), and the small-conductor breaker caps of 240.4(D) still apply.

Does the neutral count as a current-carrying conductor?

Usually not, sometimes yes. A neutral that carries only the unbalanced current of a multiwire circuit is not counted (NEC 310.15(E)) — but it IS counted when it serves two of the three phases of a wye system, or when the load is mostly nonlinear (drives, electronic ballasts, IT equipment) and harmonic current keeps it loaded. Equipment grounding conductors never count.

What ambient do I use for conduit in the sun on a rooftop?

In the 2020 and later editions, NEC 310.15(B)(2) requires adding 33°C (60°F) to the outdoor ambient for raceways or cables in direct sunlight less than 7/8 inch above the roof — which routinely pushes the corrected ambient into the bands at the bottom of this table. Raise the raceway above 7/8 inch and the adder disappears. Earlier editions used a graduated height table; check the edition your jurisdiction enforces.

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