Current Carrying??

[synchro]

OP has raceway to a pull box above the ceiling then likely has smaller raceways with lesser derating requirements leaving that pull box. 10 ft or 10% of circuit length that was mentioned could apply, but you basically need a 100 foot long circuit before you can apply this allowance

The code 310.15(A)(2) that Rob cited says the allowance can be taken if:

"the total portion(s) of the circuit with lower ampacity does not exceed the lesser of 3.0 m (10 ft) or 10 percent of the total circuit."
So lower ampacity is allowed for up to 10% of the total circuit, but only up to a maximum of 10 ft in actual length. Therefore if the total circuit is 100 ft or more then the lower ampacity section cannot exceed 10ft.
But why they don't say it that way in plain terms is sometimes frustrating.

 

[kwired]

The code 310.15(A)(2) that Rob cited says the allowance can be taken if:

"the total portion(s) of the circuit with lower ampacity does not exceed the lesser of 3.0 m (10 ft) or 10 percent of the total circuit."
So lower ampacity is allowed for up to 10% of the total circuit, but only up to a maximum of 10 ft in actual length. Therefore if the total circuit is 100 ft or more then the lower ampacity section cannot exceed 10ft.
But why they don't say it that way in plain terms is sometimes frustrating.

I caught that afterwards and thought about editing or replying but then got distracted by something else, but yes maximum of 10 feet.

On the 10% end of that - if your circuit is only 20 feet long 10% is the 2 foot nipple that you don't have to derate for in the first place but a three foot nipple is more than 10%, so circuits with length of less than 100 feet you do need to look at carefully to make sure you do comply, which that fact was in my head from past experiences just wasn't putting it together as to why at the moment.