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
Derating protects wire from damage in non-standard conditions:
- 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):
- 31-35°C: 0.91-0.96 factor depending on wire rating
- 36-40°C: 0.82-0.91 factor depending on wire rating
- 41-45°C: 0.71-0.87 factor depending on wire rating
Conductor Bundling Adjustment
NEC Table 310.15(C)(1) requires adjustment when more than 3 current-carrying conductors are in a raceway.
Derating Formula
Derated Ampacity = Base Ampacity × Temp Factor × Bundling Factor. Apply both factors when high temperature AND bundling conditions exist.
Common High-Temperature Scenarios
Watch for these common situations requiring derating:
- Attic installations: Summer temps can exceed 130°F (55°C)
- Rooftop conduit: Add 17°C to ambient for direct sunlight
- Industrial environments: Near boilers, furnaces, or process equipment
- Multiple circuits in conduit: Apply bundling factors
Best Practices
Always measure or estimate actual ambient temperature. Use 90°C rated wire for better derating options.
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).
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.
Use our Ampacity Calculator to calculate derated ampacity for your installation.