NEC Cable Derating per 310.15(B)
The Cable Derating Calculator applies correction factors for ambient temperature and conductor bundling to prevent overheating.
Derating Steps
- Start with Table 310.16 ampacity (75°C column)
- Apply temperature correction factor
- Apply conductor adjustment factor (4+ conductors)
- Adjusted Ampacity = Base × Temp Factor × Bundle Factor
Temperature Correction Factors (90°C)
| 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 |
Bundling Adjustment Factors
| Conductors | Factor |
|---|---|
| 1–3 | 1.00 |
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
| 21–30 | 0.45 |
Examples
- #10 THHN, 30A base, 40°C ambient, 5 conductors → 21.2A
- #12 NM-B, 20A base, 30°C, 2 conductors → 20A
Best Practices
- Use 90°C column for derating, 75°C for terminations
- Count only current-carrying conductors
- Derate before applying 80% rule for continuous loads
Conclusion
Always derate for safety. Use with Wire Size and Conduit Fill tools.