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There is an existing ~20"-long 1/2" EMT conduit run through a concrete floor between floors (old work). It would be convenient to run NM through the existing conduit instead of switching wiring to THHN and back to NM on either side, but the NM would exceed 40% fill.

Can I exceed 40% fill if the conduit is short enough, ie, just as a damage shield and not as a true end-to-end EMT conduit?

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Yes, you can

The conduit fill rules only apply to end-to-end conduit systems, not short lengths used to shield cables from physical damage. This is most simply explained in NEC Chapter 9, Note 2:

(2) Table 1 applies only to complete conduit or tubing systems and is not intended to apply to sections of conduit or tubing used to protect exposed wiring from physical damage.

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  • Out of curiosity, is the length of the damage shield tube still relevant for derating if >24"?
    – KJ7LNW
    Oct 8, 2021 at 20:11
  • @KJ7LNW yes, best I can tell, it is Oct 9, 2021 at 2:52

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