Why Derating is Critical
Wire ampacity tables in the NEC assume ideal conditions: 30°C (86°F) ambient temperature with no more than three current-carrying conductors in a raceway. Real-world installations rarely meet these conditions, requiring derating calculations to prevent overheating and ensure safety.
Understanding Wire Heat Generation
When current flows through wire, electrical resistance generates heat. Wire insulation is rated for maximum temperature it can withstand continuously. Exceeding this temperature causes:
- Insulation degradation and breakdown
- Increased resistance and voltage drop
- Fire hazards from overheating
- Shortened wire lifespan
- Equipment damage
Wire Temperature Ratings
| Insulation Type | Max Temp | Common Applications |
|---|---|---|
| TW, UF | 60°C (140°F) | Older installations, underground |
| THWN, THHN, NM-B | 75°C (167°F) | Most residential/commercial |
| THHN (dry), XHHW | 90°C (194°F) | High-temperature environments |
Ambient Temperature Correction
NEC Table 310.15(B)(1) provides correction factors when ambient temperature differs from 30°C (86°F).
Temperature Correction Factors
| Ambient Temp | 60°C Wire | 75°C Wire | 90°C Wire |
|---|---|---|---|
| 10°C (50°F) | 1.29 | 1.20 | 1.15 |
| 20°C (68°F) | 1.15 | 1.11 | 1.08 |
| 30°C (86°F) | 1.00 | 1.00 | 1.00 |
| 40°C (104°F) | 0.82 | 0.88 | 0.91 |
| 50°C (122°F) | 0.58 | 0.75 | 0.82 |
| 60°C (140°F) | — | 0.58 | 0.71 |
| 70°C (158°F) | — | — | 0.58 |
Common High-Temperature Locations
- Attics: Can reach 60-70°C (140-158°F) in summer
- Rooftops: Direct sun exposure, 50-65°C (122-149°F)
- Near furnaces/boilers: 40-50°C (104-122°F)
- Engine rooms: 45-55°C (113-131°F)
- Industrial ovens: Can exceed 70°C (158°F)
Conductor Bundling Adjustment Factors
When multiple current-carrying conductors are bundled together in a raceway, cable, or conduit, they generate cumulative heat. NEC Table 310.15(C)(1) provides adjustment factors.
| Number of Conductors | Adjustment Factor | % of Base Ampacity |
|---|---|---|
| 1-3 | 1.00 | 100% (no adjustment) |
| 4-6 | 0.80 | 80% |
| 7-9 | 0.70 | 70% |
| 10-20 | 0.50 | 50% |
| 21-30 | 0.45 | 45% |
| 31-40 | 0.40 | 40% |
| 41+ | 0.35 | 35% |
Important Notes on Conductor Counting
- Count only current-carrying conductors (exclude grounds and neutrals in balanced loads)
- Equipment grounding conductors are never counted
- Neutral wires count only if they carry unbalanced current or harmonics
- Spare conductors don't count unless energized
Applying Multiple Derating Factors
When both ambient temperature and conductor bundling require derating, multiply both factors together:
Adjusted Ampacity = Base Ampacity × Temperature Factor × Bundling Factor
Example Calculation:
- Wire: 12 AWG copper (75°C insulation)
- Base ampacity: 25A
- Ambient temp: 50°C (122°F) → Factor = 0.75
- Conductors in conduit: 6 → Factor = 0.80
- Adjusted ampacity = 25A × 0.75 × 0.80 = 15A
Practical Application Examples
Example 1: Attic Installation
Scenario:
- Location: Attic with 60°C ambient temperature
- Conductors: 3 current-carrying (one circuit)
- Wire: 14 AWG copper, THHN (90°C rating)
Calculation:
- Base ampacity (90°C column): 30A
- Temperature correction (60°C): 0.71
- Bundling factor (3 conductors): 1.00
- Adjusted ampacity = 30A × 0.71 = 21.3A
- Conclusion: Still suitable for 15A circuit, but verify with 15A breaker terminal rating (usually 75°C)
Example 2: Multiple Circuits in Conduit
Scenario:
- Location: Indoor at 30°C ambient
- Conductors: 9 current-carrying (three 3-wire circuits)
- Wire: 12 AWG copper, THWN (75°C rating)
Calculation:
- Base ampacity (75°C column): 25A
- Temperature correction (30°C): 1.00
- Bundling factor (7-9 conductors): 0.70
- Adjusted ampacity = 25A × 1.00 × 0.70 = 17.5A
- Problem: Not suitable for 20A circuit!
- Solution: Upsize to 10 AWG (35A × 0.70 = 24.5A)
Example 3: Commercial Rooftop Installation
Scenario:
- Location: Rooftop conduit, 55°C ambient in summer
- Conductors: 12 current-carrying (four 3-wire circuits)
- Required load: 30A continuous
Calculation:
- Continuous load factor: 30A × 1.25 = 37.5A minimum
- Try 8 AWG copper, 90°C: Base = 55A
- Temperature correction (55°C): 0.76
- Bundling factor (10-20 conductors): 0.50
- Adjusted = 55A × 0.76 × 0.50 = 20.9A
- Not sufficient! Try larger size...
- Try 4 AWG copper, 90°C: Base = 95A
- Adjusted = 95A × 0.76 × 0.50 = 36.1A
- Still not enough for 37.5A requirement!
- Try 3 AWG copper, 90°C: Base = 110A
- Adjusted = 110A × 0.76 × 0.50 = 41.8A
- ✓ Meets 37.5A requirement
Special Considerations
Terminal Temperature Ratings
Even if wire is rated 90°C, breakers and devices often have 75°C terminals. You must:
- Use 90°C rating for derating calculations
- Use 75°C rating for final ampacity check
- Verify device terminal ratings before sizing
Continuous Loads
Don't forget the 125% continuous load factor applies AFTER derating:
⚠️ Correct Order of Operations
- Start with base ampacity from NEC tables
- Apply temperature correction factor
- Apply bundling adjustment factor
- Result must be ≥ 125% of continuous load
Sunlight and Direct Heating
Conduit exposed to direct sunlight can be 15-30°C hotter than air temperature. Use conservative estimates.
Common Derating Mistakes
1. Using Wrong Base Ampacity
Always use the ampacity from the column matching your wire's insulation temperature rating.
2. Forgetting to Derate
Assuming table values work in all conditions is dangerous and violates code.
3. Incorrect Conductor Counting
Don't count equipment grounds. Be careful with neutrals in 3-phase systems.
4. Applying Factors in Wrong Order
Must apply continuous load factor LAST, after all derating calculations.
Tools and Resources
Use our Wire Ampacity Calculator to automatically apply derating factors and determine correct wire sizes for your specific installation conditions.
Conclusion
Temperature derating is not optional—it's required by code and essential for safety. Failing to properly apply correction and adjustment factors can result in dangerous overheating, insulation failure, and fire hazards. Always account for both ambient temperature and conductor bundling when sizing wire, and when in doubt, size up to the next larger gauge.