Conduit fill limits
Chapter 9, Table 1 limits how much of a conduit's cross section conductors may fill: 53% for one conductor, 31% for two, and 40% for more than two. Nipples 24 in. or shorter between enclosures may be filled to 60%.
What the code requires
Table 1 in Chapter 9 of the NEC sets the maximum percentage of a conduit's or tubing's cross-sectional area that conductors and cables may take up. One conductor or cable may fill 53%, two may fill 31%, and more than two may fill 40%.
The notes to the tables explain how to apply it. The limits apply to complete conduit systems, not to sections of conduit or tubing used only to protect exposed wiring and cable from physical damage. Equipment grounding and bonding conductors count, at their actual size. A multiconductor cable or cord counts as one conductor, and an oval cable is figured by its larger diameter.
A nipple of 24 in. or less between enclosures may be filled to 60%, and the 310.15(C)(1) adjustment factors do not apply to it. When figuring how many conductors of the same size fit, a result with a decimal of 0.8 or more is rounded up to the next whole number.
An informational note warns that conductors can jam during a three-wire pull when the raceway-to-conductor diameter ratio is between 2.8 and 3.2.
On the job
- Equipment grounding and bonding conductors must be counted in the fill calculation, at their actual size.
- The 60% allowance applies only to nipples 24 in. or shorter; longer runs use the normal limits.
Exceptions that matter
- Nipples 24 in. or shorter between enclosures may be filled to 60%.
- Dimensions for bare conductors may be taken from Chapter 9, Table 8.
In California
The Building Standards Commission, the State Fire Marshal and HCD adopt Chapter 9 without amendment. None of the three adopts Informative Annex C, which holds the NEC's ready-made conduit fill tables.
Related sections
- NEC Chapter 9, Table 4
- NEC Chapter 9, Table 5
- NEC Chapter 9, Table 8: Conductor sizes, areas and resistance
- NEC 310.15(C)(1)
- NEC 300.17
Where this comes from
- NFPA: 2026 NEC (A2025) CMP-8 First Draft public input report, reprinting 2023 Chapter 9 Table 1 and Notes
- California Building Standards Commission: 01/23 Final Express Terms, Part 3
- California State Fire Marshal: 01/23 Final Express Terms, Part 3
- California HCD: 01/23 Final Express Terms, Part 3 (2025 CEC adoption)
- Read the official NEC free at NFPA
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Kyle Robert Anderson KBC Electric · CSLB #1073066 · C-10 Electrical · Licensed since 2021