NM cable ampacity and bundling
NM cable's current rating may not exceed the 60°C rating for its wire size. When more than two cables are bundled through a sealed framing hole or packed together in insulation, the rating must be adjusted using the code's bundling table.
What the code requires
The ampacity of NM and NMC cable is set under 310.14 but may not exceed the conductors' 60°C (140°F) rating. The 90°C (194°F) rating may be used as the starting point for adjustment and correction, as long as the final result does not exceed the 60°C value.
When more than two NM cables, each with two or more current-carrying conductors, pass without spacing through the same opening in wood framing that will be sealed with thermal insulation, caulk or sealing foam, their ampacity must be adjusted using Table 310.15(C)(1). The exception in 310.14(A)(2) does not apply.
The same adjustment applies when more than two such cables are installed in contact with thermal insulation without spacing between them.
On the job
- Bundles of home runs pushed through one fire-caulked or foamed top-plate hole are a common problem. The cables must be counted and the ampacity adjusted.
- Cables stacked tightly together against insulation are treated the same way.
Related sections
- NEC 310.14
- NEC 310.15(C)(1)
- NEC 240.4(D): Breaker limits for small wires: 14, 12 and 10 AWG
- NEC 334.112
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Kyle Robert Anderson KBC Electric · CSLB #1073066 · C-10 Electrical · Licensed since 2021