Wire Derating for Temperature and Conduit Fill
Published ampacity tables give you a starting number, not a final answer. Two real-world conditions can force that number down significantly: the ambient temperature around the conductors and the number of current-carrying conductors sharing a raceway. Ignoring either factor leads to undersized wire that runs hot, degrades insulation, and can cause failures over time.
Why Ampacity Tables Have Limits
The ampacity figures in NEC Table 310.16 (and equivalent tables in other codes) are based on specific assumed conditions: 60°F (16°C) or 75°F (24°C) ambient air temperature, and no more than three current-carrying conductors in a raceway or cable. Both assumptions get violated routinely in real installations.
Attics in summer climates regularly hit 130°F+. Conduits feeding industrial panels often contain six, eight, or more circuits bundled together. Each of those situations calls for a correction factor applied to the base ampacity before you size the conductor.
Understanding both corrections, and how they interact, is the core of proper derating. Always verify your calculations against the current edition of the NEC and consult a licensed electrician before finalizing any installation.
Temperature Correction Factors
Heat affects how well a conductor sheds the thermal energy generated by current flow. The hotter the surrounding environment, the less heat the wire can dissipate, so the lower the safe current limit.
NEC Table 310.15(B)(1) provides correction factors based on ambient temperature for 60°C, 75°C, and 90°C insulation ratings. A few representative values for 75°C-rated conductors:
| Ambient Temp (°F) | Ambient Temp (°C) | Correction Factor |
|---|---|---|
| 70 or below | 21 or below | 1.08 |
| 71–77 | 22–25 | 1.00 |
| 78–86 | 26–30 | 0.94 |
| 87–95 | 31–35 | 0.88 |
| 96–104 | 36–40 | 0.82 |
| 105–113 | 41–45 | 0.75 |
| 114–122 | 46–50 | 0.67 |
A factor above 1.00 means the environment is cooler than the baseline assumption, so you actually get a little more capacity. Factors below 1.00 reduce ampacity. At 104°F, a conductor rated for 100A under baseline conditions is only good for 82A.
If you are using 90°C-rated insulation (THHN, XHHW-2), the correction factors are slightly less aggressive because the insulation tolerates more heat. However, termination temperature ratings often cap the usable ampacity at the 75°C column anyway, so check both ends of the circuit.
For more on reading these tables correctly, see how to read NEC ampacity tables.
Conduit Fill Adjustment Factors
The second derating scenario applies when four or more current-carrying conductors share a raceway, cable tray, or cable. Bundled conductors trap each other's heat. The more conductors, the worse the thermal stacking.
NEC Table 310.15(C)(1) gives adjustment factors based on conductor count:
| Current-Carrying Conductors | Adjustment Factor |
|---|---|
| 1–3 | 1.00 (no change) |
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
| 21–30 | 0.45 |
| 31–40 | 0.40 |
| 41+ | 0.35 |
A few important clarifications on what counts. Neutral conductors carrying only unbalanced load from a 3-wire or 4-wire system are not counted. Equipment grounding conductors are never counted. A 20A circuit with one hot and one neutral, run in conduit with five other similar circuits, gives you 12 current-carrying conductors total, not six.
For specifics on counting conductors before you pull wire, the conduit fill guide covers the rules in detail.
Applying Both Factors Together
When both conditions are present, you multiply the two correction factors together, then apply that combined factor to the base ampacity. The math is straightforward:
Derated ampacity = Base ampacity × Temperature factor × Conduit fill factor
The order does not matter mathematically, but tracking each factor separately before multiplying helps catch errors.
One nuance: if you are using 90°C-rated conductors (like THHN), you may be able to start from the 90°C ampacity column to get a higher base number before applying corrections, provided that the derated result does not exceed the 75°C ampacity of the same conductor and the termination ratings support it. This is a common technique to recover some headroom lost to aggressive derating.
Worked Derating Example
Here is a concrete scenario to tie it together.
Setup: Six 12 AWG THHN conductors in a single conduit, installed in an unconditioned mechanical room that reaches 104°F (40°C) in summer. The circuit load is 14A.
Step 1, Base ampacity. From NEC Table 310.16, 12 AWG THHN at 90°C is rated 30A. However, since terminations are typically rated 75°C, the usable starting point is the 75°C column: 20A.
Step 2, Temperature correction. At 104°F (40°C), the correction factor for 75°C conductors is 0.82.
20A × 0.82 = 16.4A
Step 3, Conduit fill adjustment. Six current-carrying conductors fall in the 4–6 range, so the adjustment factor is 0.80.
16.4A × 0.80 = 13.1A
Step 4, Compare to load. The derated ampacity is 13.1A. The load is 14A. That means 12 AWG is undersized for this installation.
Moving up to 10 AWG THHN (75°C base ampacity: 30A) recalculates to 30 × 0.82 × 0.80 = 19.7A, which clears the 14A load with margin. A properly sized breaker (20A max for 10 AWG in most applications) also fits within that derated value.
The full step-by-step sizing process, including breaker selection, is covered in how to size a cable step by step.
Common Mistakes to Avoid
Pulling from the wrong column is the most frequent error. Using the 90°C ampacity without checking termination ratings inflates your starting number artificially.
Forgetting to count conductors from ALL circuits in a shared conduit catches people off guard. A homerun conduit carrying three separate 2-pole 240V circuits contains six current-carrying conductors, not two.
Assuming the ambient temperature is always the standard 77°F (25°C) baseline creates hidden safety margins that do not exist in attics, rooftop installations, or industrial spaces near heat-generating equipment. Measure or estimate the worst-case temperature, not the comfortable-day average.
Frequently Asked Questions
Does a ground wire count as a current-carrying conductor for derating purposes?
No. Equipment grounding conductors (the green wire or bare copper) are never counted when determining conduit fill adjustment factors. They carry current only during fault conditions, not continuously.
Can I use the 90°C ampacity column to offset derating?
Sometimes. If your conductors have 90°C-rated insulation (THHN, XHHW-2 are common examples), you can start from the 90°C column, apply both correction factors, and use the result as long as it does not exceed the 75°C ampacity of the same wire size. The termination temperature rating at both the panel and the load end must also support this approach. See ampacity explained for how the columns relate to each other.
What if my conduit passes through different temperature zones?
Use the highest ambient temperature the conductors will see along their entire run. A conduit that spends 90% of its length in conditioned space but passes through a 120°F attic for six feet still needs to be derated for that attic temperature across the whole circuit.
Do these rules apply to cable assemblies like NM-B (Romex) as well?
Yes, similar bundling rules apply to cables run together. NEC 310.15(A)(2) covers bundled cables, and the adjustment factors are the same. Romex is also limited to the 60°C ampacity column in most residential applications, which makes derating even more critical to account for early in the design.