NEC Derating
A practical walkthrough of Table 310.15(B)(1), building on the general ambient derating concept with NEC-specific structure.
Building on the general concept covered in the "Ambient Temperature Correction — Table 310.15(B)(1)" article, here's the practical, row-by-row structure of how to actually read this table.
The table is organized by ambient temperature ranges (in both °C and °F) — find the row matching your installation's actual expected ambient temperature, not the temperature at the moment you happen to be installing.
Separate correction factor columns exist for each insulation temperature rating (60°C, 75°C, 90°C) — matching the same column structure from the base ampacity table. Use the column matching whichever insulation rating column you originally pulled the base ampacity from.
A 90°C-rated conductor's base ampacity, installed in an environment with a 40°C ambient temperature, gets multiplied by the correction factor from the 90°C column at the 40°C ambient row — not the 75°C column, even if the actual terminations elsewhere in the circuit are 75°C-rated (that's a separate consideration, covered in the Table 310.15(B)(16) article).
OhmWorks applies this lookup automatically in the NEC Cable Sizing calculator.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks