7.2 Temperature Correction & Adjustment Factors (310.14/310.15)

Ambient Temperature Correction — Table 310.15(B)(1)

Table 310.16 is built on a 30°C (86°F) ambient. When the surrounding air, earth, or conduit environment is hotter or colder than 30°C, you must apply a correction factor from Table 310.15(B)(1). The factor is read from the column that matches the conductor's insulation temperature rating (60°C, 75°C, or 90°C) — not the termination rating.

Excerpt of Table 310.15(B)(1) Correction Factors

If ambient is below 30°C the factor is greater than 1.00, so ampacity increases — but the increase is capped by the termination temperature limit (you cannot uprate past the 60°C or 75°C termination ampacity). If ambient is above 30°C the factor is less than 1.00, so the conductor's ampacity drops.

Which Column?

The correction factor uses the insulation rating column, not the termination column. THHN/THWN-2 (90°C insulation) is always corrected from the 90°C column, even when the terminations force you to use the 60°C or 75°C ampacity for final sizing. This is why the 90°C column is called the "derating starting point."

Conductor-Count Adjustment — Table 310.15(C)(1)

When more than three current-carrying conductors are installed in a raceway or cable longer than 600 mm (24 in.), the Table 310.16 ampacity must be multiplied by an adjustment factor from Table 310.15(C)(1) (renumbered in NEC 2020 from the 2017 edition's 310.15(B)(3)(a)):

Counting Current-Carrying Conductors

The Informational Note in 310.15(C)(1) and the rules in 310.15(C)(1) explain what to count:

• Ungrounded (hot) conductors — always count.
• Neutral on a 3-wire circuit from a 4-wire, 3-phase wye system — count as a current-carrying conductor because harmonic current adds to the neutral.
• Neutral of a balanced 3-wire single-phase circuit (e.g., a 120/240 V residential circuit) does not count — the neutral carries only the unbalanced load.
• Neutral feeding nonlinear (harmonic) loads — count it.
• Equipment grounding/bonding conductors — never count.
• Spare conductors that are energized in the future — count.
• Conductors not energized simultaneously (e.g., 3-way switch travelers, only one of which carries current at a time) — do not count.

The Full Derating Sequence

The correct sizing procedure combines both factors and the termination rule:

1. Look up the 90°C column ampacity from Table 310.16 (the derating starting point).
2. Multiply by the ambient correction factor from Table 310.15(B)(1) using the 90°C column factor.
3. Multiply by the adjustment factor from Table 310.15(C)(1).
4. Compare the derated result to the termination-limited ampacity (60°C or 75°C column from 110.14(C)).
5. The smaller value is the final allowable ampacity.

Worked Example: Six Current-Carrying #6 THHN Cu in 40°C Ambient

A conduit contains six #6 AWG copper THHN conductors (three two-wire circuits, neutral counted on each because the loads are nonlinear), in a boiler room with 40°C ambient. Terminations are on a 60 A breaker rated 75°C.

Step 1 — 90°C base ampacity. #6 Cu THHN = 75 A (90°C column).

Step 2 — Ambient correction. 40°C ambient, 90°C column factor = 0.91. 75 × 0.91 = 68.25 A.

Step 3 — Bundling adjustment. Six current-carrying conductors = 80%. 68.25 × 0.80 = 54.6 A.

Step 4 — Termination limit. Breaker rated 75°C → use 75°C column = 65 A for #6 Cu.

Step 5 — Compare. 54.6 A (derated) vs. 65 A (termination) → the smaller value governs → 54.6 A final ampacity.

The 60 A breaker is not allowed because the derated ampacity (54.6 A) is less than 60 A. You would need to either reduce the load, use a larger conductor (#4 Cu gives more headroom), or split the conductors into two raceways. Note that 240.4(B) next-size-up cannot help here because the derated ampacity (54.6 A) does not equal a standard breaker size and is below the 60 A rating.

Common Traps

• Applying correction to the termination ampacity. The correction factor always multiplies the insulation-column (90°C) ampacity, not the 60°C or 75°C termination ampacity.
• Forgetting to re-check the termination limit. After derating, the smaller of the derated ampacity and the termination ampacity wins — many candidates stop at the derated value.
• Miscounting the neutral. A 3-wire circuit from a 4-wire wye counts the neutral; a 120/240 V single-phase circuit does not.