Ampacity derating: two factors, two different tables
Start from the 90°C ampacity of the insulation you actually installed. Multiply by the ambient temperature correction factor, then by the conductor count adjustment factor. Compare that result to your load. Separately, compare the unadjusted ampacity in your termination's temperature column to the same load. The conductor has to pass both checks, and the lower of the two governs.
Two things reduce how much current a conductor can carry: heat from the air around it, and heat from the other current-carrying conductors bundled with it. They are handled by two different tables and they multiply — applying only the worse of the two is a common and unsafe shortcut.
Step 1 — ambient temperature correction
The ampacity table is built on a 30°C ambient. Anything hotter costs you capacity; anything cooler gives some back. Pick the row for your ambient, then the column for the insulation rating you are starting from.
| Ambient °C | 60°C | 75°C | 90°C |
|---|---|---|---|
| 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 |
| 61–70 | — | 0.47 / 0.33 | 0.65 / 0.58 |
| 71–85 | — | — | 0.50 / 0.41 / 0.29 |
Step 2 — more than three current-carrying conductors
Count only the conductors that actually carry current. A neutral carrying only the imbalance of a balanced multiwire circuit does not count; a neutral on a circuit with significant harmonic content does. Equipment grounding conductors never count.
| Conductors | Factor |
|---|---|
| 1–3 | 1.00 (no adjustment) |
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
| 21–30 | 0.45 |
| 31–40 | 0.40 |
| 41 and up | 0.35 |
This is Table 310.15(C)(1) in the 2020 through 2026, and Table 310.15(B)(3)(a) in the 2017. Note how steep the first step is — going from three conductors to four costs you 20 percent, which is often the difference between a 12 AWG and a 10 AWG home run.
The mistake: derating from the wrong column
Derate from the 90°C column if that is what your insulation is rated for, even when your breaker and lugs are only rated 75°C. The termination rating is a separate, parallel limit under 110.14(C) — it is not the starting point for the multiplication. Starting from the 75°C number throws away capacity you paid for.
- 1
Worked example
Six current-carrying 12 AWG THHN copper conductors in one raceway, 40°C ambient, 75°C terminations.
- 2
Start at 90°C
12 AWG copper is 30 A in the 90°C column.
- 3
Apply both factors
30 × 0.91 (36–40°C at 90°C) × 0.80 (4–6 conductors) = 21.8 A.
- 4
Check the termination limit
12 AWG copper is 25 A in the 75°C column, unadjusted. 21.8 A is lower, so 21.8 A governs.
- 5
Then apply 240.4(D)
12 AWG copper is still capped at a 20 A overcurrent device regardless — the small-conductor rule is a third, independent limit.
The 2026 change worth knowing
The rule requiring an equipment grounding conductor to be enlarged in proportion when the ungrounded conductors are enlarged moved from 250.122(B) in the 2020 and 2023 to 250.122(D) in the 2026. The 2026 wording also adds an exclusion: conductors enlarged because of these derating factors do not trigger the proportional EGC increase. Under the earlier editions, read the section as adopted by your AHJ before assuming the same.
Where this goes wrong on real jobs
- Applying only the larger of the two factors instead of multiplying both.
- Counting equipment grounding conductors toward the conductor count.
- Derating from the termination column instead of the insulation column.
- Forgetting 240.4(D) — derating can never raise a 14, 12 or 10 AWG copper conductor above its small-conductor overcurrent cap.
- Using a 2023 table number on a 2017 or 2020 job. The factors match; the citation does not.
Related guides
- What size wire do I need? The whole run, in orderSizing a conductor is six checks, not one lookup — continuous load, ampacity, termination temperature, the small-conductor cap, the equipment ground, and voltage drop. Do them in this order and the answer only moves one direction.
- What actually changed in the NEC 2026A working electrician's list of the NEC 2026 changes that move numbers on the job: Table 310.16 gained rows, 250.122 letters shifted, and one renumbered article now points somewhere else entirely.
Run the numbers
AmpRule is not affiliated with or endorsed by NFPA. NEC and National Electrical Code are trademarks of the National Fire Protection Association. Always verify against the edition adopted by your AHJ.