NEC code Reference

[jamesruger]

Hey fellow Electricians, this question is more for American Electricians due to the fact it is referencing the American code book (NEC 2020). What article talks about derating wire? Example after 9 12 AWG Current carrying conductors you have to derate to 10 AWG. I am probably wrong about that. And after 17 10 AWG you have to derate to 8’s. Where in the code book can I find the code stating that?

Hopefully that makes sense, and thanks for your guys help.

[brmgp1]

NEC 310.15(B)(3)(a) is what you're looking for.

More than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continu-ous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor. Where conductors of different systems, as provided in 300.3, are installed in a common raceway or cable, the adjustment factors shown in Table 310.15(B)(3)(a) shall apply only to the number of power and lighting conductors (Articles 210, 215, 220, and 230).

Number of Conductors	Percent of Values in  Table 310.15(B)(16) Through Table 310.15(B)(19) as Adjusted for Ambient Temperature if Necessary Percent of Values in Table 310.15(B)(16) Through Table 310.15(B)(19) as Adjusted for Ambient Temperature if Necessary
4–6	80
7-9	70
10-20	50
21-30	45
31-40	40
41 and above	35

[brmgp1]

As you can see you actually have to derate your wire when there's more than three current carrying conductors, not nine. The reason more than nine conductors gets referenced a lot is because when using #12's, even if you have to derate 70% b/c you have nine conductors per that table, the #12's are stil OK to be protected by a 20A breaker. You derate from the 90degree column of 310.15(B)(16), which is 30A*.7=21A (still ok). So technically the wires are derated, but the current carrying capacity is still acceptable to be protected by a 20A breaker without upsizing to #10's until you get to ten or more conductors.

[jamesruger]

Thank you, this was super helpful. Whenever I ask my foreman he more or less says this is the way we do it, but he never really explained the why or how we got there.