Wire Adjustment Factors

NEC 2020 • 2023 • 2026 COMPLIANT

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Mandatory derating factors for raceways containing more than three current-carrying conductors.

Number of Conductors	Percent of Values in Tables
4 - 6	80%
7 - 9	70%
10 - 20	50%
21 - 30	45%
31 - 40	40%
41 and above	35%

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Related NEC Tables

Table 310.16: Allowable Ampacity Table 310.15(B)(1): Temp. Correction

Professional Usage Disclaimer

This data is provided for informational purposes only. Always verify with the latest edition of the National Electrical Code (NFPA 70) and consult with a licensed electrical professional or AHJ (Authority Having Jurisdiction) before performing any electrical work. Zing² is a reference tool, not a substitute for qualified engineering.

Professional Field Notes & Gotchas

▸Current-Carrying Conductors (CCC): Only count wires that actually carry load. Grounds (EGC) and neutrals in balanced 3-phase systems do NOT count toward this table.
▸The 24-Inch Rule: Derating is only required if the conductors are in a raceway longer than 24 inches. Short nipples do not require derating.
▸Diversity Rule: If you have a large number of conductors but only a small percentage are energized at once, check 310.15(C)(1) exceptions for potential relief.
▸Heating Up: Putting 10 current-carrying wires in one pipe cuts your safe capacity by 50%. It's often cheaper to run two pipes than one huge, derated pipe.

NEC 2023 vs 2026: What changed?

NEC 2023 vs 2026: Language was added to reinforce that 'bundling' applies to cables (like NM-B) just as much as wires in conduit when they are run through common holes in framing for more than 24 inches.

Technical FAQs

How many wires can go in one pipe?

While 'Box Fill' limits volume, the 'Adjustment Factor' limits heat. Once you exceed 3 current-carrying conductors, you must reduce the ampacity of the wires per Table 310.15(C)(1).

Do I count the ground wire for derating?

No. Per NEC 310.15(E)(1), grounding and bonding conductors are only used for faults and do not count as current-carrying conductors for derating.

What happens if I have 9 wires in one EMT?

Per the table, you must apply a 70% multiplier to the wire's ampacity. A #12 THHN (30A base) would be limited to 21A, just barely safe on a 20A breaker.