Derating: Bundling Adjustment and Ambient Correction
Table 310.16 ampacities assume ideal conditions (30°C ambient, ≤3 current-carrying conductors). Real installs are often hotter or more crowded, so you "derate" — reduce the allowable ampacity. Two factors do this: bundling adjustment and ambient correction.
1. Bundling adjustment — Table 310.15(C)(1)
When more than three current-carrying conductors run bundled together for more than 24 inches, reduce each conductor's ampacity:
| Number of current-carrying conductors | Adjustment |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
What counts as "current-carrying":
- EGC / grounding conductors: never counted. (CEC 310.15(F))
- Neutral: not counted when it carries only the unbalanced current (e.g., a 3-wire single-phase circuit). It is counted when it carries significant harmonic current from nonlinear loads. (CEC 310.15(E)(1))