Derating wire for heat and crowded conduits

What the code requires

The ampacity tables, such as Table 310.16, assume 30 °C (86 °F). In hotter locations the table ampacity is multiplied by a correction factor. For 60 / 75 / 90 °C conductors: 87–95 °F, 0.91 / 0.94 / 0.96; 96–104 °F, 0.82 / 0.88 / 0.91; 105–113 °F, 0.71 / 0.82 / 0.87; 114–122 °F, 0.58 / 0.75 / 0.82.

Raceways or cables in direct sunlight less than ¾ in. (19 mm) above a roof must add 33 °C (60 °F) to the outdoor temperature. Type XHHW-2 conductors are exempt from this adder.

With more than 3 current-carrying conductors in a raceway, or cables bundled without spacing for more than 24 in., the ampacity is reduced: 4–6 conductors, 80%; 7–9, 70%; 10–20, 50%; 21–30, 45%; 31–40, 40%; 41 and up, 35%. A neutral carrying only unbalanced current is not counted, and neither are grounding or bonding conductors. A neutral of a 3-wire circuit from a 4-wire wye, or a wye neutral serving mostly nonlinear loads, is counted.

Corrections and adjustments may start from the conductor's own temperature-rating ampacity, such as the 90 °C column, but the final result may not exceed what the terminations allow under 110.14(C).

On the job

• Conduit in direct sunlight on rooftop supports that sits less than ¾ in. above the roof gets the 60 °F adder.
• Forgetting to derate when more than 3 current-carrying conductors share one conduit is a common correction.

Exceptions that matter

• The adjustment for more than 3 conductors does not apply to raceways 24 in. long or less.
• It also does not apply to conductors entering or leaving an outdoor trench in RMC, IMC, PVC or RTRC up to 10 ft long with no more than 4 conductors.
• Unjacketed AC or MC cables, each with no more than 3 current-carrying 12 AWG copper conductors, need no adjustment if no more than 20 current-carrying conductors are bundled together. Above 20 current-carrying conductors bundled for more than 24 in., a 60% factor applies.

What changed in 2023

In 2023 the rooftop adder threshold dropped from ⅞ in. (23 mm) to ¾ in. (19 mm) above the roof.

Related sections

• NEC 110.14(C)
• NEC 310.12: Sizing service wires for a house: the 83% rule
• NEC 310.14
• NEC 310.16: Wire ampacity table and the 60/75 °C terminal rule
• NEC 334.80: NM cable ampacity and bundling

Where this comes from

• NFPA: 2023-cycle CMP-6 First Draft ballot (FR 8336)
• NFPA: 2026-cycle CMP-6 First Draft ballot (2023 baseline text)
• Read the official NEC free at NFPA