NEC 310.15
Ampacity Derating Factors (NEC 310.15)
The two adjustments that reduce conductor ampacity: ambient temperature correction (310.15(B)(1)) and conductor bundling (310.15(C)(1)). Multiply both against the base ampacity.
Temperature Correction Factors (NEC 310.15(B)(1))
| Ambient °C | Ambient °F | 60°C wire | 75°C wire | 90°C wire |
|---|---|---|---|---|
| 11-15 | 50–59 | 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 (base) | 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 that insulation cannot be used at that ambient (the corrected ampacity would be zero or negative). This is why 60°C wire is rarely used in hot environments.
Conductor Bundling Adjustment (NEC 310.15(C)(1))
| Current-carrying conductors | Adjustment factor |
|---|---|
| 1-3 | 1.00 |
| 4-6 | 0.80 |
| 7-9 | 0.70 |
| 10-20 | 0.50 |
| 21-30 | 0.45 |
| 31-40 | 0.40 |
| 41+ | 0.35 |
Count only current-carrying conductors. The equipment grounding conductor and a neutral that carries only unbalanced current do not count (NEC 310.15(E)).
How Derating Works
Derating is multiplication. Corrected ampacity = base ampacity × temperature factor × bundling factor. Start from the 90°C column of NEC Table 310.16 (for 90°C-rated insulation like THHN), apply the ambient correction from 310.15(B)(1) and the conductor count adjustment from 310.15(C)(1), then confirm the result does not exceed the terminal temperature rating of the equipment, which is 75°C for most breakers and lugs per NEC 110.14(C). Starting from the higher 90°C column is what gives you derating headroom, but the final answer can never beat the 75°C value.
Worked Example
#6 THHN, 108°F attic, 6 current-carrying conductors
Base: #6 THHN at 90°C = 75A (NEC 310.16)
Temp: 108°F (41–45°C) = 0.87
Bundling: 6 conductors = 0.80
75 × 0.87 × 0.80 = 52.2A
Result: the derated ampacity is 52A, well under the 65A that #6 copper is rated at the 75°C terminal column, so 52A governs. This is why a #6 that looks good for 65A can only carry a 50A load once it is in a hot, crowded raceway.
Frequently Asked Questions
What is the temperature correction factor at 100 degrees F?
At 100 degrees F ambient (the 36 to 40 C range), the NEC 310.15(B)(1) correction factor is 0.91 for 90 C conductors, 0.88 for 75 C, and 0.82 for 60 C. Multiply the conductor ampacity by this factor.
How do I derate ampacity for more than 3 conductors?
Per NEC 310.15(C)(1), for 4 to 6 current-carrying conductors multiply ampacity by 0.80, for 7 to 9 by 0.70, for 10 to 20 by 0.50, for 21 to 30 by 0.45, for 31 to 40 by 0.40, and for 41 or more by 0.35. The adjustment applies when conductors are bundled longer than 24 inches.
Do you apply temperature and conductor-count derating together?
Yes, you multiply both factors against the ampacity. A #6 THHN rated 75A at 90 C in a 100 F attic (0.91) with six current-carrying conductors (0.80) is 75 x 0.91 x 0.80 = 54.6A of usable ampacity.
What ambient temperature is NEC ampacity based on?
NEC Table 310.16 ampacities are based on a 30 C (86 F) ambient. The 310.15(B)(1) correction factors adjust up when it is cooler and down when it is hotter than that baseline.
Does the neutral count as a current-carrying conductor?
Usually no. Per NEC 310.15(E), a grounded neutral that carries only the unbalanced current of the other conductors is not counted. It does count on a 3-wire circuit from a 4-wire wye system or where harmonic currents are present. The equipment grounding conductor never counts.
Which ampacity column do you start derating from?
Start from the 90 C column of NEC Table 310.16 for THHN, THWN-2, and XHHW-2, apply the correction and adjustment factors, then confirm the derated result does not exceed the 60 C or 75 C terminal rating of the equipment per NEC 110.14(C).
Related Calculators & Charts
Wire Size Calculator
Applies these exact derating factors automatically for your ambient and conductor count.
Wire Ampacity Chart (310.16)
The base ampacities you start from before derating, copper and aluminum at 60/75/90°C.
Conduit Fill Chart
More than three conductors in a raceway triggers both fill limits and bundling derate.
Let the calculator apply both factors.
Derating errors fail inspections and overheat conductors. The wire size calculator pulls the base ampacity, applies temperature and bundling factors, and enforces the terminal rating in one pass.