Temp Correction (30°C)
Multipliers for ampacity based on ambient temperature variations.
Professional Usage Disclaimer
This data is provided for informational purposes only. Always verify with the latest edition of the National Electrical Code (NFPA 70) and consult with a licensed electrical professional or AHJ (Authority Having Jurisdiction) before performing any electrical work. Zing² is a reference tool, not a substitute for qualified engineering.
Professional Field Notes & Gotchas
- ▸Standard Ambient: All Table 310.16 values are based on an ambient temperature of 30°C (86°F). Anything hotter requires the multipliers from this table.
- ▸Rooftop Rule (Deleted): Note that the old 'Rooftop Distance' temperature adder (RAC) was removed in recent NEC cycles. You now use the actual ambient temperature unless specific local codes apply.
- ▸Compounding Factors: If you have a rooftop run with high ambient heat AND more than 3 wires, you must multiply the factors from and 310.15(C)(1) together.
- ▸The Cold Factor: Use these values to INCREASE ampacity in cold storage or freezer applications, though you can never exceed the terminal rating.
NEC 2023 vs 2026: What changed?
NEC 2023 vs 2026: The table remains mathematically identical, but the cross-references to 310.16 were updated for clarity. The 2026 code reinforces using localized weather data for more accurate ambient calculations.
Technical FAQs
What is the temperature correction for 100°F?
At 100°F (38°C), 90°C wire (THHN) has a multiplier of 0.91. This means your 100A wire is now only safe for 91A.
Do I need to derate for attic heat?
Absolutely. Attics often exceed 114°F, which requires a multiplier of 0.82 for 90°C conductors. High attic temperatures are a leading cause of nuisance breaker tripping.
Which column do I use for THHN?
For THHN wire, utilize the 90°C column for the correction factor. However, ensure the final adjusted ampacity doesn't exceed the 75°C terminal rating.