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Tuesday, July 9, 2019

Question:

I with others appreciate being able to get the brain working every morning with thought provoking questions. When using 12/2/2NM cable in a residence is derating still required? It would be considered a cable but 2 individual circuits. I can agree with temperature limitations as with any NM cable in an attic but the question has been raised that there are more than 3 current carrying conductors therefore derating is required. Hot topic for awhile here in the local area of Indiana (still on 2008 NEC requirements) Thanks for any feedback!

James Jett
A

Answer:

Hey James thanks for your question. Yes, the wording has changed a little but all current carrying conductors count for adjustment purposes (formerly derating) as stated in 310.15(B)(3)(a). The title of Table 310.15(B)(16) also uses the phrase "not more than 3 current carrying conductors in a cable". In your example there are 4 so the 80% adjustment factor from Table 310.15(B)(3)(a) must be applied to the values in Table 310.15(B)(16).

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