National Electrical Installation Standards

Standards as High as Your Own

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Tuesday, October 8, 2019

Question:

Hello, I’m trying to get clarification on section 310.15 about bundling MC. In my situation, I have many MC of different size (12-2,12-4, 12-6, 10-4) and I need to run them in 8 foot long emt chases across an exposed ceiling hallway. The only place they are run in a “bundle” will be in these 8’ chases (before and after the chase they are supported so they maintain spacing). My question is how many of these mc can I shove down a chase without having to derate. 20 current carrying conductors? Do I count the neutrals? Do I automatically have to derate since some of them will be 12-4 and bigger? Thanks in advance.

Nik Carson
A

Answer:

Hey Nik thanks for your question. Conductor ampacities can be determined using Table 310.15(B)(16) as stated in 310.15(A) and 310.15(B). As the wording in the Table title indicates the ampacities are based on not more than 3 current carrying conductors in raceways, cables, or earth. Where single conductors or multiconductor cables are installed, without maintaining a space between them for longer than 2 feet, and they are not in raceways then the adjustment factors (formerly derating) from Table 310.15(B)(2)(a) must be applied as stated in 310.15(B)(2)(a).

Neutral conductors that carry only the unbalanced current from other conductors of the same circuit do not need to be counted as current carrying as stated in 310.15(B)(5)(a). If you only have two of the phase conductors (of a 3-phase wye system) or if the major portion of the load is non linear then the neutral conductors are considered current carrying as stated in in 310.15(B)(5)(b) and 310.15(B)(5)(c). In your example the neutral conductors in the 12-2 cables are considered current carrying because they carry the same current as the ungrounded (phase) conductor. Likely the same for the other cables unless you meet the conditions in 310.15(B)(5)(b) and 310.15(B)(5)(c).

If the conductors in the MC cable have a 90 degree C insulation rating (likely) you can use the 30 ampere value in Table 310.15(B)(16) to apply the adjustment factors from Table 310.15(B)(2)(a). So if you don't have more than 9 current carrying conductors (such as four 12-2 cables [=8 conductors]) the factor is 70% which results in an ampacity or 21. You are limited to the 20 ampacity for 12 AWG anyway because of the terminations as stated in 110.14(C) and 20 ampere overcurrent protection as stated in 240.4(D)(5).

Another option might be to use a metal wireway(s) but make sure the AHJ agrees this is acceptable because it does not seem to be specifically allowed or prohibited. Up to 30 current carrying conductors (at a 20% fill from 376.22(A)) can be installed in metal wireways without applying adjustment factors as stated in 376.22(B). Many other raceway articles allow cables if the cable article does not prohibit that use in the XXX.22 Section. Article 376 for metal wireways do not have that wording but the word "cable" is used in the definition of Metal Wireways in 376.2 and Article 330 does not specifically prohibit that use.

Or maybe you could install the MC cables on the underside of the 8-foot ceiling, maintain spacing between them, and have that area covered by some other material that is not unsightly and is compatible with the ceiling material.

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