Understanding Temperature Derating
Wire ampacity ratings assume specific installation conditions. When actual conditions differ—particularly higher temperatures or bundled conductors—the wire's safe current capacity must be reduced (derated) to prevent overheating.
Why Derating is Necessary
- Insulation protection: High temperatures damage wire insulation
- Heat dissipation: Hot environments reduce cooling ability
- Bundled heat: Multiple wires in conduit compound heat buildup
- Code compliance: NEC requires derating for non-standard conditions
Ambient Temperature Correction
NEC Table 310.15(B)(1)(1) provides correction factors for ambient temperatures above 30°C (86°F):
| Ambient Temp (°C) | Ambient Temp (°F) | 60°C Wire | 75°C Wire | 90°C Wire |
|---|---|---|---|---|
| 26-30 | 78-86 | 1.00 | 1.00 | 1.00 |
| 31-35 | 87-95 | 0.91 | 0.94 | 0.96 |
| 36-40 | 96-104 | 0.82 | 0.88 | 0.91 |
| 41-45 | 105-113 | 0.71 | 0.82 | 0.87 |
| 46-50 | 114-122 | 0.58 | 0.75 | 0.82 |
| 51-55 | 123-131 | 0.41 | 0.67 | 0.76 |
Conductor Bundling Adjustment
NEC Table 310.15(C)(1) requires adjustment when more than 3 current-carrying conductors are in a raceway:
| Number of Conductors | Adjustment Factor |
|---|---|
| 1-3 | 1.00 (100%) |
| 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%) |
Combined Derating Calculation
When both conditions apply, multiply the factors together:
Derated Ampacity = Base Ampacity × Temp Factor × Bundling Factor
Apply both factors when high temperature AND bundling conditions exist
Example Calculation
Scenario
- Wire: 10 AWG THHN (90°C rating)
- Base ampacity: 40A at 30°C
- Ambient temperature: 45°C (113°F)
- Conductors in conduit: 6
Calculation:
- Temperature factor at 45°C for 90°C wire: 0.87
- Bundling factor for 6 conductors: 0.80
- Derated ampacity: 40A × 0.87 × 0.80 = 27.8A
Common High-Temperature Scenarios
Attic Installations
- Summer attic temps can exceed 130°F (55°C)
- Use 90°C rated wire (THHN) for better derating
- Consider routing around attic when possible
Rooftop Conduit
- Solar installations on dark roofs get extremely hot
- Conduit in direct sunlight: add 17°C to ambient
- Consider conduit color (white reflects heat)
Industrial Environments
- Near boilers, furnaces, or process equipment
- Measure actual ambient at wire location
- May need high-temperature insulation types
Wire Insulation Temperature Ratings
60°C Insulation (TW, UF)
- Lowest rating, most affected by temperature
- Rarely used in new construction
- Found in older installations
75°C Insulation (THW, THWN)
- Most common for general wiring
- Good balance of performance and cost
- Standard for most terminations
90°C Insulation (THHN, THWN-2, XHHW)
- Best temperature performance
- Use 75°C column for most terminations
- 90°C rating helps with derating calculations
Important Note
Even with 90°C rated wire, you must use the 75°C ampacity column if terminating at devices rated for 75°C (most common devices).
Best Practices
- Always measure or estimate actual ambient temperature
- Use 90°C rated wire for better derating options
- Separate circuits into multiple conduits when possible
- Consider larger wire size to compensate for derating
- Document calculations for inspections
Tools and Calculators
Use our tools to properly calculate derated ampacity:
- Ampacity Calculator - Calculate derated ampacity
- Wire Gauge Calculator - Size wire for your application
- AWG Reference Chart - Base ampacity values
Conclusion
Temperature derating is essential for safe electrical installations in non-standard conditions. By properly applying NEC correction and adjustment factors, you can ensure wire operates within safe limits even in hot environments or bundled installations. Always verify your calculations and consider upsizing wire when significant derating is required.