NEC Table 310.15(B)(1): Temperature Correction Factors

NEC Table 310.15(B)(1) provides ambient temperature correction factors for adjusting conductor ampacity when the ambient temperature differs from the standard 30°C (86°F) reference used in Table 310.16. This is one of the two mandatory adjustments in NEC conductor sizing (the other being conduit fill adjustment per Table 310.15(C)(1)).

The table gives correction factors for three insulation temperature ratings:

• 60°C column: TW, UF insulation types
• 75°C column: RHW, THHW, THW, THWN, XHHW, USE, ZW insulation types
• 90°C column: THHN, THHW, THW-2, THWN-2, RHH, RHW-2, USE-2, XHHW, XHHW-2, XHH, ZW-2 insulation types

When the ambient temperature exceeds 30°C, the correction factor is less than 1.0 (reducing the ampacity). When the ambient is below 30°C, the factor exceeds 1.0 (increasing the ampacity).

The following table shows the correction factors from NEC Table 310.15(B)(1) for the most commonly used temperature ratings:

Reading the table: The NEC uses temperature ranges rather than single values. All temperatures within a range use the same correction factor. For example, any ambient temperature from 36–40°C uses a correction factor of 0.91 for 90°C insulation.

The corrected ampacity is calculated by multiplying the Table 310.16 base ampacity by the temperature correction factor:

Corrected Ampacity = Table 310.16 Ampacity × Correction Factor

If both temperature correction and conduit fill adjustment apply, both factors are multiplied together:

Adjusted Ampacity = Table 310.16 Ampacity × Temp Factor × Conduit Fill Factor

Worked example:

Conductor: 6 AWG THHN copper (90°C rated)
Ambient temperature: 43°C (110°F)
Location: Rooftop conduit in Arizona

Step 1: Base ampacity from Table 310.16
  6 AWG at 90°C = 75 A

Step 2: Temperature correction from Table 310.15(B)(1)
  43°C falls in the 41–45°C range
  90°C column factor = 0.87

Step 3: Corrected ampacity
  75 × 0.87 = 65.25 A

Step 4: Terminal temperature check
  75°C column for 6 AWG = 65 A
  65.25 A ≤ 65 A?  Marginal — use 65 A as the limit.

Result: 6 AWG THHN in 43°C ambient can carry 65 A.

A key NEC design technique is using the 90°C column for derating, then capping at the terminal temperature rating. This is explicitly permitted by Section 310.14 and provides a significant advantage in high-temperature environments:

Example: 8 AWG conductor at 40°C ambient

Approach 1 — Using 75°C column directly:
  Base ampacity: 50 A
  Temp factor (75°C column): 0.88
  Result: 50 × 0.88 = 44 A

Approach 2 — Using 90°C for derating, cap at 75°C:
  Base ampacity: 55 A (90°C column)
  Temp factor (90°C column): 0.91
  Derated: 55 × 0.91 = 50.05 A
  Cap: min(50.05, 50) = 50 A  (75°C column value)

Approach 2 yields 50 A vs 44 A — a 14% improvement!

This strategy is particularly valuable when ambient temperature correction and/or conduit fill adjustment would otherwise require an increase in conductor size. By starting with the 90°C ampacity, the derated value often stays above the 75°C limit, avoiding a cable upsizing.

Temperature derating is particularly important in these US installation scenarios:

NEC 310.15(B)(3) addendum: Section 310.15(B)(3)(c) requires an additional temperature adder of 33°C (60°F) for conductors in conduit or cable exposed to direct sunlight on or above rooftops. This means rooftop conduit at 35°C air temperature is treated as 68°C ambient — a dramatic derating that often requires significantly larger conductors.