Overcurrent Protection for Derated Conductors - Table 310.15(B)(2)(a)

[KLM]

When applying the adjustment factors for more than three current carrying conductors in a raceway, should the overcurrent protective device trip rating be sized based on the adjusted ampacity? See the example below:

Example: (41) - #10 AWG (90 degree C, THHN) copper conductors are installed in a raceway. Based on Table 310.15(B)(2)(a) an adjustment factor of 0.35 shall be applied. The #10 AWG conductors have an ampacity of 40A based on Table 310.16. Therefore, the adjusted ampacity is 40A x 0.35 = 14A. Article 240.4(B) permits the use of the next highest standard overcurrent protective device. Therefore, a maximum 15A circuit breaker would be required to protect the adjusted conductors.

Are my assumptions correct?

 

[charlie b]

Yes, you have it right. The calculated load, however, must be no higher than 14 amps, or you could not use the conductors as you have described the installation.

Welcome to the forum.

 

[crazedengineer]

You're the Man

You're the Man

I absolutely agree.

NEC 240.4(B) says(2):
The ampacity of a conductor, after ampacity adjustment and/or correction, doesn't correspond with the standard rating of a fuse or circuit breaker in 240.6(A) the is is permitted to move up to the next higher standard rating over-current device. In your case it is a 15A.

:bye:

-CT, P.E. State of Florida

 

[augie47]

I concur, also, but note the stipulations (limits) in 240.4(B)

 

[infinity]

As Augie stated there are limitations as to what types of loads you can use the next size up rule with, so without that information you really cannot definitively answer the question.

 

[KLM]

Thanks for the input! It appears that 240.4(B) would require the overcurrent protection to match the adjusted conductor ampacity for conductors supplying multi-outlet branch circuits serving receptacles? More simply stated "the overcurrent protective device could not be increased to the next higher standard size".