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NEC 310.15 Ampacity Adjustment: Temperature and Bundling Combined

Conductor ampacity values in NEC Table 310.16 assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. When real-world conditions exceed either of these assumptions, you must apply correction factors that reduce the allowable ampacity. Both factors multiply together — in hot environments with bundled cables, the combined derating can be severe.

Adjusted Ampacity = Base Ampacity × Temperature Factor × Bundling Factor

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

When ambient temperature exceeds 30°C, conductor ampacity must be reduced because the insulation has less thermal headroom. The correction factor depends on the insulation's temperature rating — higher-rated insulation tolerates more heat:

Notice how 90°C insulation (THHN, THWN-2) retains much more capacity in hot environments. At 50°C ambient, 90°C insulation retains 87% of its base ampacity, while 60°C insulation drops to only 71%. This is a key reason electricians prefer THHN/THWN-2 wire — even when terminal ratings limit you to the 75°C column, you can use the 90°C column for derating calculations per NEC 110.14(C).

Bundling Adjustment Factors (NEC 310.15(C)(1))

When more than three current-carrying conductors share a raceway or cable, they generate mutual heat that reduces each conductor's ampacity:

Worked Example: 12 AWG THHN in a Hot Attic with 6 Conductors

Scenario: Running 12 AWG THHN (90°C insulation) through an attic in Phoenix, AZ where summer attic temperatures reach 50°C (122°F), with 6 current-carrying conductors in a single conduit.

• Base ampacity: 12 AWG at 90°C from NEC Table 310.16 = 30A
• Temperature factor at 50°C: 0.87 (from NEC Table 310.15(B)(1), 90°C column)
• Bundling factor for 6 conductors: 0.80 (from NEC 310.15(C)(1))
• Adjusted ampacity: 30A × 0.87 × 0.80 = 20.9A
• Total derating: 30.3% reduction from base ampacity

This means a 12 AWG conductor that would normally carry 30A is reduced to only 20.9A. For a 20A circuit protected by a 20A breaker, this is barely adequate. If you add one more conductor pair (8 total), the bundling factor drops to 0.70, giving only 18.3A — which is below the 20A breaker rating. You would need to upsize to 10 AWG wire.

The 90°C Wire Trick (NEC 110.14(C))

One of the most important NEC principles that electricians must understand: even though most terminals are rated for only 75°C connections (per NEC 110.14(C)), you can still install 90°C-rated wire and use the 90°C ampacity column solely for the purpose of temperature and bundling correction. After applying the derating factors, the resulting ampacity must not exceed what the 75°C column allows for that wire size.

Example: 6 AWG THHN has a 90°C ampacity of 75A (NEC Table 310.16). After temperature correction (0.87 at 50°C) and bundling (0.80 for 6 conductors), the adjusted ampacity is 75A × 0.87 × 0.80 = 52.2A. The 75°C column value for 6 AWG is 65A. Since 52.2A < 65A, the 6 AWG wire is code-compliant for a circuit loaded up to 52.2A at this derated condition.

Which Conductors Count for Bundling?

Only count current-carrying conductors per NEC 310.15(E):

• Phase conductors: Always count. A three-phase circuit has 3 current-carrying conductors.
• Neutral in balanced three-phase: Does NOT count per NEC 310.15(E)(1) — it carries only unbalanced current.
• Neutral with nonlinear loads (harmonics): DOES count per NEC 310.15(E)(2) — third harmonics add in the neutral, potentially making neutral current exceed phase current.
• Equipment grounding conductor: Never counts.
• Control circuit conductors: Count if they carry continuous current.

Common scenario: a conduit with two three-phase circuits (balanced loads) has 6 current-carrying conductors (3 phases × 2 circuits). The two neutrals and equipment ground don't count. Apply the 80% bundling factor for 4–6 conductors.